INSECTS. THE INSECTA, or INSECTS, form the largest class of that division of the animal kingdom for formerly called Articu-lata, but for which the more expressive term Arthropoda (joint-footed) is now more generally employed. This term includes, besides Insecta, the classes Crustacea, Arachnida, and Myriopoda.
The chief diagnostic characters of an Insect, as com-binedly distinguishing it from a Crustacean, an Arachnid, or a Myriopod, are as follows:Legs usually (never more than) six in number; two antennae; ordinarily two pairs of more or less membranous wings ; head, thorax, and abdomen distinctly separated ; respiration effected by means of internal tracheae, which communicate with the air by lateral openings termed spiracles or stigmata, or by external plates or filaments (these ordinarily only in the preparatory conditions of aquatic forms), which absorb air and convey it to the tracheae. A reference to the articles on the other classes of Arthropoda will indicate in what way these diagnostic points are modified in them.
As in all organized beings, the limits of the class are not strongly defined, for, although it is not difficult to indicate an insect, speaking broadly, there are certain small groups that do not satisfactorily fall into the class as limited by strongly marked lines of demarcation. These will be especially alluded to hereafter.
Number qf Species of Insects.At the head of this article it is stated that the Insecta form the largest group of the Arthropoda ; it might probably be said with justice that they far outnumber all the other members of the animal kingdom combined. It is certain that at the present time 80,000 presumably distinct species of beetles have been described, and it is safe to assume that the number of known species of other orders is greater, thus giving a total of about 200,000. And yet we are only on the threshold of a knowledge of the forms that actually exist in nature, many enormous groups of minute forms being still only very partially studied. In fact, it may be confidently anticipated that some day the number of known forms will not fall far short of 1,000,000.
Antiquity of InsectsFossil indications have been dis-covered in the Devonian series, and in the Carboniferous they become rather more numerous ; but, with few exceptions, these all belong to those orders in which the metamorphoses are incomplete, and there is no evidence that any anthophilous insects (such as Lepidoptera or Hymenoptera) were then in existence. Ascending the geological scale to the Mesozoic age, the representatives of the older groups become very numerous, and often of gigantic size. Coleoptera are fairly well indicated; and the flower-loving Lepidoptera and Hymenoptera make their appearance, but in very small numbers. In the Tertiary rocks remains become sometimes very abundant, and of all orders ; and in the post-Tertiary or Quaternary period these remains consist largely of those of species now existing. One of the most interesting features in fossil entomology is the well-known occurrence of myriads of insects entombed in the fossil resin known as amber, preserved in the most beautiful manner, and belonging for the most part to genera now existing, but differing specifically. In alluding to this it is well to mention that the insects found in gum copal and other recent resins are, on the contrary, of existing species. As in other animals, and also plants, the fossil remains prove that the distribution of heat and cold on the earth was once very different from what it now is : a fossil beetle of rather large size was discovered by our last Arctic Expedition almost at the highest northern point attained.
Geographical Distribution.It may be asserted that no part of the earths surface is without insects. They have been discovered in the Arctic and Antarctic regions at the highest point reached, and even showy butterflies of several species enliven the dreary solitudes of almost everlasting ice, as was abundantly proved by the naturalists of the "Alert" and "Discovery," who found them almost up to 83º N. lat. But, as a rule, the larger and more brilliant forms occur within the tropics. Yet it must not be assumed, as is sometimes erroneously done, that the majority of tropical insects are large and brilliant, and the smaller and more obscure forms comparatively less numerous. Recent investigations by competent observers show that the latter are at least as abundant in the tropics as in temperate regions, and that it is the wealth of large forms that has caused the others to be overlooked.
The attempts at subdivision of the globe into zoological regions, so successful with regard to mammals, and in a smaller degree with birds, have not been so entirely satisfactory with regard to insects, more especially as concerns, the separation of the Palaearctic and Nearctic regions (see DISTRIBUTION) ; still there is often a very marked localization in particular groups, which divide themselves specifically to an infinite extent within very circumscribed areas, and are found nowhere else. The results obtained from minute investigation of insular faunae have derived much of their value from insects, and have occasioned much valuable philosophical speculation on the origin both of the islands themselves and of their faunae and florae. Space will not permit of detailed allusion to the apparent affinity shown by the insect inhabitants of regions now very widely separated, such, for instance, as that of Western Europe with Western (rather than Eastern) North America, of Australia and New Zealand with Chili, of Chili and the southern extremity of South America with the Palaearctic region, &c.
As special points of distribution may be mentioned the occurrence of insects in hot springs, in brine, in the deepest caves (these are usually blind), below low-water mark, and even on the surface of the ocean (the genus Halobates in the Hemiptera) very far from land.
The power of many insects to acclimatize themselves rapidly when accidentally introduced into new regions is very marked, and adds to the difficulty often experienced in considering what species are really endemic and what introduced, especially in islands. Some of the common and noxious British species thrive enormously when introduced into Australia and New Zealand; and there is every reason to believe that the grape-vine pest (Phylloxera) was origin-ally an importation from America.
Duration of Life.The maximum duration of the life of a perfect insect is probably attained in bees and ants, the females or queens of which are known to live at least seven years; the minimum is found in some species of May-flies (Ephemeridae), in which twenty-four hours is perhaps the limit. But the length of life of a perfect insect is sometimes in direct opposition to that of the same insect in its preparatory stages, and some of the Ephemeridae that live at most but a few days in their aerial form have taken three years to complete their growth in their sub--aquatic stages. Temperature also has a marked effect on some species. The common house-fly, for instance, will complete its whole life cycle from embryo to fly in a few days in the heat of summer, but requires very much longet in cold weather.
Economic Entomology.Within the limits of an encyclopaedia article it is impossible to give even a sketch of this subject. Those who desire an exhaustive résumé cannot do better than consult Kirby and Spences delightful Introduction, even although it may be now somewhat out of date. Some especially noxious speciessuch, for instance, as the grape-vine pest, the Colorado beetle, and the Rocky Mountain locusthad not then been alluded to as occasion-ing damage, or were even altogether unknown. As concerns American species, Rileys Reports on the Noxious, &c., Insects of Missouri are mines of information. Amongst insects that are of direct benefit to man the hive-bee and the silk-worm moth stand pre-eminent, and the cochineal and lac insects are scarcely of less importance. No sub-stitutes for silk, boney, and beeswax have been or are likely to be discovered; but, on the other hand, chemical discoveries have now occasioned the disuse of some insect products articles of commerce, and in this category nothing is more remarkable than the manner in which the oak-gall of commerce has given way to inorganic substances in the manufacture of ink. As food for man, insects play a very unimportant part, and they can scarcely be said now to form part of the diet of the more highly civilized races, notwithstanding an attempt lately made in America so to utilize the masses of the destructive Rocky Mountain locust. Yet locusts themselves (with other large insects) are eaten raw or cooked by the inhabi-tants of more than one part of the globe, and the large fleshy grubs or larvae of beetles and other insects are as much esteemed as delicacies by the natives of some countries as the Cossus (the precise identity of which appears involved in some uncertainty) was by the luxurious Romans. The aborigines of Australia make a cake of the pounded bodies of a night-flying moth (Noctua spini), termed the Bugong moth; the natives of the Lake region of Central Africa make a kind of bread of the multitudes of small dead insects (chiefly Ephemeridae and Diptera) that collect on the shores; in Central America the eggs of a large water bug supply materials for a kind of bread.
Noxious insects are legion, and cannot here be alluded to even in the most general manner. The number of those that cause injury to man by direct attacks is comparatively small ; it is by their attacks on the produce of our fields and gardens that insects assert their importance. But it should not be overlooked that the especial province of insects is to act as scavengers, and very frequently they are not the initiating cause of damage, which is rather to be sought in a previously unhealthy condition of the trees or plants; they simply step in to complete the work of destruction commenced by disease or by a low state of the vital functions.
Insects and the Fertilization of Plants.Such is the importance of insects in the economy of nature, and as conferring indirect benefit on man, in this particular, that this subject might have been alluded to under the preceding heading. That the action of insects in fertilizing plants was often necessary had long been known. But it is owing to the patient and laborious researches of living naturalists (amongst whom the names of Darwin, Hermann Müller, and Lubbock stand prominently forward) that the vast importance of the subject has come to be understood. They have proved incontestably that in a multitude of plants the condition of the reproductive organs is such that self-fertilization is impossible ; but what is of greater import-ance is the proof afforded that, although many plants are per-fectly capable of self-fertilization, the weight and number of the seeds or fruit are often vastly increased when cros-s-fertilization is effected, and that this is mainly done by the action of insects, the wind and other causes playing only a minor rôle. It may be truly said that such is the correlation between plants and insects that the majority of the former would more or less gradually disappear from the earths surface were the latter to be destroyed. In New Zealand the red clover has been introduced and flourishes, but all hopes of spreading it there have to be abandoned; the plant never perfects its seeds, owing to the absence of humble bees, which appear
Parasitism.Among the varied relations of insects to other classes of the animal kingdom and their mutual relations, no subject is more interesting than is that of parasitism. It occurs in almost all the orders, but in very different degrees. Whole groups are naturally epizoic, other entozoic, while a few (such as fleas and bed bugs) can scarcely be arranged in either of these divisions, inasmuch as, although in one sense epizoic, it appears probable that they may occasionally be able to go through the whole of their life cycle without contact with the animals to which they otherwise appear especially attached. As true epizoa the whole group of true lice, Anoplura (which are probably degraded Hemiptera), and bird lice (Mallophaga, a group of uncertain affinities) are especially familiar. These cannot exist without their hosts, and their whole life is passed on them, each mammal or bird having its especial parasite (or more than one), which affects it only, or is at any rate confined to it and allied species. Such also are certain degraded forms of Diptera, including the bat para-sites (Nycteribia), the bird flies (Ornithomyia), and others. Such also is a curious creature (Platypsylla) parasitic upon the beaver, the affinities of which are so little marked that it has been formed into a distinct order (Achreioptera) by Westwood, placed in the Hemiptera by Ritsema, and declared to be a true beetle by Leconte. Such also is a curious little moth (Epipyrops, Westwood), an external parasite upon certain homopterous insects; another moth (Tinea vastella) lives in its larval state on the horns of living animals; and many others might be cited.
As entozoic insects, the large dipterous family (Estridae is especially characteristic, all its members living at the expense of Mammalia, in very varied manners, the stomach, throat, frontal air passages, the subcutaneous system, and even the genital organs being attacked by various species, but only as larvae, the perfect insects being winged and strong flyers. Furthermore, a genus of Diptera (Batrachomyia) belonging to quite another family (Mucidae) is said to attack frogs. It is scarcely just, however, to class as true parasites certain insects whose larvae have been discharged (still living) from the nostrils, intestines, or urethra of man. Many such cases have been perfectly authenticated, but the insects have been such as certainly do not of necessity require such conditions, and these latter are not natural habitats. Accident introduced them, and they were fitted to exist, at any rate for short periods, in the interior of the human body. But the largest class of insect parasitism is that which exists between insects themselves, as exhibited in an enormous number of certain families (Ichneumonidae, Evaniidae, Proctotrypidae, Chalcididae, &c.) of Hymenoptera, &c. These are essentially parasitic in their preparatory stages, and the parasitism is of the class that may be termed entozoic. The eggs are laid either in or on the bodies of the larvae (chiefly) of other insects, and even in the eggs, the young larvae of the parasites feeding mostly on the adipose tissue of their hosts, often enabling the latter to undergo most of their transformations (but very rarely that to the perfect insect). To such a class belong also many dipterous insects, chiefly belonging to the Tachinidae. Hyper-parasitism exists in many minute species of Chalcididae, which do not directly affect tlie hosts themselves, but which feed in the bodies of other parasites.
Luminosity.This is another subject that should have more than passing notice bestowed upon it. Modern scientific travellers have not succeeded in confirming Madame Merians well-known statements with regard to the luminosity of the so-called lantern-flies (Fulgora), hence these have to be eliminated from the category of luminous insects. It is among the Coleoptera that the phenomenon especially occurs, and in them is almost confined to certain skip-jack beetles (the genus Pyrophorus), and probably the entire family of glow-worms (Lampyridae). The luminosity is confined to certain, dis-tinct patches, differing in position and number according to the species and also according to sex,usually most observ-able in the female, although this does not appear to be always the case. The property is distinctly under the control of the insect, and is often exercised in an inter-mittent manner at stated intervals, when the insects are not under the influence of extraordinary excitement. It is probable that luminosity exists in some Diptera, and also in the larvae of certain exotic Lepidoptera, a recent traveller having assured us that in South America a larva of this order has luminous patches along either side, so that when in motion it has been compared to a lighted-up railway train. Some occasional instances of luminosity appear to be accidental, probably owing to the insects having been feeding upon, or otherwise in contact with, decaying phosphorescent matter. With regard to the nature of the luminous substance no very precise results have been arrived at by investigators. That it is phos-phorus in some form or other appears certain, and the latest experimenter (Jousset de Bellesme) asserts as his belief that it is no other than phosphoretted hydrogen gas stored up in the cellular tissue, and in direct communica-tion with the nervous and respiratory systems.
Galls.These well-known insect-productions are alluded to chiefly in order to call attention to the mystery that surrounds their growth. Galls are occasioned by the presence of the larvae of certain species in nearly all orders of true insects, though it is amongst the Cynipidae in Hymenoptera and the Cecidomyiidae in Diptera that they are most characteristic. And they may be in almost any position on a plant, according to the species of gall-maker. The most striking, however, are clearly modified leaf or flower-buds.
The mystery surrounding galls is their cause. The in-direct cause is the puncture of the insect, and the presence of its eggs or larvae, but no explanation has been offered of the reason why this presence sets up the growth termed a gall. Two insects of differing species will deposit their eggs in the same position : in the one case no abnormal growth follows; in the other some peculiar irritation sets up a tumour, often enormous in size. Two insects, also of different species, but both gall-makers, do the same : in both cases a tumour ensues, but its form is totally different in the two. A most noticeable recent discovery is that by Dr Adler (since confirmed by others) to the effect that in certain European Cynipidae dimorphism to a remarkable extent occurs, and that certain genera are only conditions of others, the two forms of insects, and the totally different galls occasioned by them, being alternate in appearance.
External Structure.Taking any large insect, we recog-nize in it three more or less distinctly separated divisions, the head, thorax, and abdomen. Taking the majority of insects, and especially of their larvae, we recognize thirteen segments or somites, counting the head as one, the thorax consisting of three, and the abdomen as nine. From a classificatory point of view, it is probably convenient to retain this idea, though in the abdomen of a dragon-fly (for instance) there are 10 quite distinct segments. But, seen in the light of embryology and morphology, a different aspect is put on, The abdomen in the embryo of some insects clearly consists of 11 segments. Moreover if each appendage of the head be considered as a modified limb, we get in some insects as many as 7 segments in this portion of the body alone. Thus although 13 segments is a usual and convenient number as regards the structure of an insect, this number must be vastly increased if we consider the animal in regard to other divisions of the Arthropod series. In the abdomen the actual number is sometimes very much reduced, owing to several of the segments becoming obsolescent, coalescent, or retracted.
The exoskeleton, or outer covering, is more or less horn-like in its nature. But its elements are by no means similar to those of either horns or bones. It is composed to a varying extent of phosphate of lime, with the addition of a peculiar substance termed chitine, especially character-istic of, though not strictly confined to, the Insecta. According to recent analysis, the constituents of chitine are said to be as follows:
The head, or anterior of the three main divisions of the body, of a perfect insect is of very varying form and structure, both as regards outline, the condition of its attachment to the trunk, and the details of its special appendages. No account of these variations can be given here; they will be briefly alluded to in the classificatory portion of this article. The various organs and appendages may be stated as follows. On each side of the anterior portion are inserted two long and usually multi-articulate processes termed "antenneae," which are tubes containing nerve-prolongations and tracheae, and undoubtedly associated in a high degree with the special senses; but, notwithstanding all the controversy on the subject that has existed and still exists, we do not yet know clearly what is their special function. They have been considered organs of touch, of hearing, of smell, or simply as balancers assisting and directing flight. No one who has watched the proceedings of many insects (and especially of ants), when meeting others of their kind, can doubt that they act in some way (but perhaps not in all insects) as means of intercommunication, and thus take a high rank as important structures. They are, as a rule, much less developed in those insects having very large eyes; and in the larvae of those that undergo a complete metamorphosis they are usually rudimentary only, notwith-standing their often enormous development in the perfect insects produced from the same larvae.
The compound eyes are two in number (though each is occasionally divided into two portions), usually of large, sometimes of enormous size, and each consisting of very numerous facets, which but indicat~ the faces of so many independent angular tubes separated by layers of pigment. In the larval state the eyes are ordinarily simple, and each eye is usually a congregation of separate eye-spots. Besides the compound eyes, there are two or three (or no) small simple eyes, "ocelli"or "stemmata," each with a simple nerve, and never present in the larvae or (probably) pupae.
The organs of the lower surface of the head are of a most complicated nature, and are excessively modified according as the insect takes nutriment by biting or by sucking Below the eyes is the "front"; this is succeeded by a piece termed the "clypeus" (or "epistome" or "nasus"), which is followed by the "labrum" or upper lip. On either side are the "mandibles" (usually dentate within) articu-lated to the cheeks, and below these a second pair of jaws, compound in structure, and consisting of a hinged base, afterwards frequently dividing into two portions, the "maxillae" and maxillary lobes, and provided externally with articulated appendages known as the maxillary palpi. Below the mouth is the "labium" with its labial palpi, articulated to the "mentum" or chin-piece; lying within this lower mouth-covering is the "lingua" or tongue. The same general arrangement is present throughout all insects, and also in the larval and pupal stages; but the differing conditions of the food cause extreme modification, not only between differing groups or orders of the perfect insects, but also in the metamorphic stages of the one and the same species. In some insects there are additional small structures, such as the "paraglossae."
The "thorax" is the next main division. It is composed of three distinct portions, the prothorax, mesothorax, and metathorax, all subject to excessive modifications ; but the last is, on an average, the smallest; at any rate it seldom exceeds the intermediate, and is usually very much smaller. According to surface, each portion receives two different names; thus the upper side consists of the pronotum, mesonotum, and metanotum, the lower of the prosternum, mesosternum, and metasternum. It will be noticed also that each subdivision is again subdivided by more or less distinct grooves, especially above and on the sides, indicating its compound nature, and each of these has its special term, so that some authors go so far as to say that each thoracic division is formed of nine separate pieces (a text book on entomology, which this article cannot be, should be consulted as to these). In those insects in which the wing-power is great, the attachments of the muscles are strongly indicated externally.
The appendages of the thorax are the legs and wings. The legs are articulated members, of which one pair is attached to the sides of each subdivision. All true insects have but six actual legs, but in the larvae of some orders there are simple fleshy prolegs on the abdominal segments, considered as representing the homologues of those abdominal legs so conspicuous in the Myriopoda. Of the true legs the anterior (or prothoracic) pair are directed forward, the two other pairs backward. Each leg consists of a basal joint or coxa (frequently not movable) inserted in sockets termed the acetabula; this is followed by a small joint termed the trochanter placed between the coxa and femur or thigh, which is ordinarily the largest joint, and is enormously developed in saltatorial insects. To this succeeds the tibia, followed again by the tarsus,-which is ordinarily compound, but may consist of any num-ber of joints from one to five. The tarsus is terminated by a pair (seldom one only) of claws, between which are more or less membranous arolia or plantulae (much marked in the feet of Diptera, which climb polished surfaces, &c., by means of them), and also a pulvillus or cushion.
Wings are appendages of the mesothorax and metathorax (never of the prothorax), and, viewed simply as organs of locomotion, may be considered as expansions of the integument, though some morphologists object to this simple definition, and one at least (F. Plateau) regards them as tracheal extensions. Although in all orders there are cases in which they are never developed, the exceptions being so few as abundantly to prove the rule, yet the posterior (or "hind" or "under") pair may be absent, and the anterior (or "fore" or "upper") ample. So strongly are they attributes of a perfect insect, that in some cases in which neither pair is developed the creatures strongly incline to retain their larval form. Normally the first external indications may be said to appear in the pupal stage (but we will show that in insects with imperfect metamorphoses the line of demarcation between larva and pupa is not marked), and they only attain their full development some little time after the exclusion of the perfect insect. A wing consists of an upper and lower membrane (readily separable in a recently excluded insect, or afterwards by maceration), strengthened by more or less numerous strong ribs, more or less connected transversely, termed nervures or veins (neither term being very appropriate), which are chitinous tubes (containing special tracheae), through which the blood circulates. The varying condition of the wings will be alluded to in the systematic portion of this article, as also to some extent the scheme of neuration, one of the most important factors in systematic entomology, but rendered unsatisfactory in consequence of the utterly different nomenclature employed by writers on special orders, though doubtless the general scheme is capable of being homologized.
The last of the three great divisions of the body is the abdomen, which consists of a number of segments (nor-mally nine), having an upper (dorsum) lower (venter) chitinous surface, which two surfaces (in the most char-acteristic condition) are connected by a membranous lateral line, with lateral stigmata or spiracles. But almost every conceivable modification is presented both in its attachment to the thorax, its general outline, and the number of segments present. Of the appendages of the abdomen it is necessary to say but little. In a perfect insect there are no abdominal legs, and rarely any indications of breathing plates (so usual in some groups of aquatic larvae). The appendages are therefore almost entirely connected with the sexual apparatus, which vary enormously, and occasionally there are terminal articulated thread-like tails, strongly simulating antennae both in form and structure.
Nervous System.This may be said to consist of a more or less double cord lying along the ventral portion of the body, connected at intervals by thickened masses termed ganglia. But the large mass in the head is termed the brain, in contradistinction to the others. The brain usually consists of a bilobed mass giving off nerve masses to the eyes, and threads to the other cephalic appendages or organs ; recent researches prove that, at any rate in some cases, the brain has convolutions analogous to those of the higher animals. Immediately below the brain is a large ganglion, usually termed the infra-oesophagal, con-nected with the mouth organs and digestive functions. Then follow, in the thorax and abdomen, a series of ganglia, each of which gives off numerous lateral threads. But the number of these ganglia varies very greatly, not only in insects of different orders, or in species of the same order, but also in the larvae, pupae, and perfect insects of the same species; and it is impossible to enter here into the most rudimentary analysis of these variations. It has been said that normally there should be a ganglion for each segment (or for each movable segment) of the body, and to some extent this would appear to hold good, for, in those insects in which some of the segments coalesce, a similar arrange-ment is seen in the system of ganglia, but this would not appear to be a universal law, and in some the abdominal ganglia are virtually obsolete. Similar variations exist in the extent to which the double central column becomes united or remains divided. In addition to this column, a simple sympathetic nerve is also distinguished, without ganglia, but giving off threads to the respiratory and other systems. This lies above the main ganglionic chain. In minute structure the nervous cord of the rnsecta is analo. gous to that of higher animals. The simplicity of the nervous system has caused it to be believed that insects do not suffer pain in the sense of that experienced by higher animals, and their behaviour when subjected to treatment that should cause intense pain, in the ordinary sense of the word, appears to warrant such an opinion; but the existence of such a condition cannot be held to justify wanton cruelty. Those who desire minute informa-tion on the nervous system should especially consult Newports article "Insecta" in Todds Cyclopaedia of Avatomy and Physiology, and a series of articles by E. Brandt, now appearing in the publication of the Russian Entomological Society.
Respiratory System.Respiration by tracheae is one of the main characteristics of an insect. Tracheae are tubes ramifying in the interior of the body, the walls of which are composed of two membranes with a spiral thread between, and extending into the wings and other append-ages; but in the perfect insect the main tracheae are subject to modification, and are more or less expanded into vesicles to suit the requirements of creatures with great powers of flight, or of strong movement in other ways. The manner in which air is communicated to these tracheae, in order that the necessary oxygen may be obtained from it, is twofold in its nature. In insects that live in free air the latter is received through lateral openings termed spiracles or stigmata, which vary in number in different insects, but there is usually one on each side of most of the segments. A spiracle usually consists of a longitudinal slit in a membrane, protected by delicate mechanism, and also by special muscles, which can close it hermetically if necessary. Many aquatic insects also breathe through spiracles, and in these cases a quantity of air is collected (or entangled) in delicate pubescence on the surface of the body, the insect coming to the surface to obtain a fresh supply at intervals. But in the majority of aquatic insects, and especially of their larvae or pupae, air is obtained by means of external threads or plates, expansions of the integument, the function of which is to absorb air from water and convey it to the tracheae by means of delicate ramifications of the tracheal system in their substance. The number and position of these external appendages (or branchiae) is as varied as are the conditions under which the insects live ; in some only a single elastic tube is present, which can be protruded to the surface of the water, and its length adapted to the varying depth of that element; in some (as in many dragon-flies) the plates are in the rectum, and the air is obtained by the forcible taking in and expalsion of water by means of powerful anal valves (which serve also for locomotion). It is obvious that those larvae that exist parasitically in the substance of the body of other larvae, &c., must still obtain air, and it is presumed that this is sometimes effected at the expense of the respiratory system of their hosts. It has long been known that rudimentary branchiae exist in aerial insects, and, though this was at one time supposed to be an attribute of one or two forms only, it is now known to occur frequently. According to the researches of Gegenbaur and Palmén, those branchiae exist side by side with the ordinary spiracles; hence they conclude that there is no direct connexion between the branchial system of the larva and the spiracles of the imago. It is still perhaps an open question whether these branchiae in the imago serve any functional purpose.
Alimentary and Digestive Systems.The food of insects is either solid or liquid, and the parts of the mouth are modified, according to requirements, into two main condi-tions, termed mandibulate and haustellate ; but the latter term is somewhat vague, inasmuch as the modifications are by no means homologous in all haustellate insects, although the structure is subservient to the same function. Again, m both divisions the food may be either vegetable or animal in its nature, and according as this may be the parts of the digestive system are modified. The most simple digestive systern consists merely of a tube extending from mouth to anus, with no very distinct division into parts. But in insects the arrangement is considerably more complex, yet varying enormously. The most complete system consists of aesophagus, with the salivary glands (modified into silk-producing glands in Lepidoptera, &c.), crop or proventriculus, gizzard, stomach, small and large intestines, and an arrangement of small canals termed the Malpighian tubes. Some authors distinguish also other divisions of the intestines answering to those of higher animals. By some the term proventriculus is applied to the crop, by others to the gizzard. The gizzard is usually absent in haustellate insects; but, as most of these are truly mandibulate in their larval stage, much modification is undergone during metamorphosis. The digestive secretion of all parts of the system appears to be essentially alkaline, and assimilation goes on from all (excepting perhaps the lower intestine) pari passu with digestion, the latter being commenced in the crop. The Malpighian tubes are a set of long slender vessels (varying much in number) situated in the lower portion of the system at the junction of the small and large intestines. Their function has been warmly contested amongst physiologists, many considering them biliary organs, while probably an equal number maintain they are solely urinary, and a few bold that both these functions may be attributed to them. The recent researches of Plateau and others are in favour of their being solely urinary. Von Siebold has asserted that the biliary system consists of certain cells in the walls of the stomach. It is possible the whole intestinal canal is at times called upon to play a rôle quite independent of digestion and assimilation ; it may be made subservient to metamorphosis through being distended with air, thus assisting the rupture of the integuments for the escape of the imago; but this can probably only obtain in insects with incomplete metamorphoses.
Circulatory System.Almost as much uncertainty exists, or has existed, as to the true nature of this system as in other points of internal structure and physiology. Originally it was believed that no circulatory system existed, an idea that was speedily dissipated. If we examine a larva of which the integuments are tolerably transparent, we perceive, even without dissection, a large vessel running along the dorsal portion of the creature just beneath the integument, and we perceive also that it dis-tinctly pulsates. This is the "dorsal vessel" or "heart, and it terminates anteriorly in a cephalic aorta. Examined more minutely by dissection, it is seen to consist of a number of chambers and constrictions, each chamber having a lateral valvular opening on either side, through which the blood is received into the vessel by regular currents and conveyed to the cephalic aorta, whence it escapes into the body in currents which have no vascular walls, and is again received into the dorsal vessel from lateral currents,such, at least, is the most generally received opinion. Certain it is that the blood (which is ordinarily a colourless liquid) circulates through all parts of the body, even to the antennae, legs, and wings, and the circulation can be well observed in the wings of some insects in which these organs are unusually transparent, in that case distinctly following the course of the nervures. But many physiologists have believed that the blood is conveyed over the body by means of the tracheae, some distinguishing certain tracheae to which this function alone, and not that of respiration, is proper. The majority of these, however, state that the blood simply flows between the two integuments of which the walls of a trachea are composed, and to this system the term "peritracheal" has been given. According to the results obtained from the experiments of the most recent observers, we prefer to doubt the existence of this peritra-cheal circulation. The relative frequency of pulsations varies much according to the insect and its state of activity or excitement. They disappear almost entirely in insects in a state of hibernation, and are much reduced in the pupae of those that undergo complete metamorphoses.
Muscular System.The muscles are attached to the inner side of the chitinous integument, and lie just beneath it. They are composed of numerous parallel fibres without any tendinous sheaths, but the fibres are apparently sometimes united at their extremity into a kind of tendon, which has been considered as only an extension of the chitinous integument. According to their position and function, they act variously, as do those of higher animals, and have received similar names. Their number is often enormous, and when we consider the great powers of flight, or of locomotion by other means, possessed by many insects, it is not difficult to understand that their strength must be proportionately great. Lyonets celebrated treatise on the anatomy of Cossus remains a masterpiece of research on this subject, and in England Lubbocks recent memoir on the subcutaneous muscles of Pygaera bucephala is equally remarkable, and should be studied by those desiring minute information on the complex muscular system.
Generative System.In all insects the sexes are separate. True hermaphrodites do not exist, though individual mon-strosities, in which the form, coloration, and even internal organization of both sexes are combined, are not rare. The external organs are placed at or near the extremity of the abdomen, and are usually accompanied by secondary or accessory appendages often of most complex structure, serving to ensure complete contact during the sexual act, and probably also to some extent excitatory. In the dragon-flies, however, the intromittent organ of the male is in the under side of the second abdominal segment, which explains the extraordinary position of the sexes when coupled. In the male the testes are very varied in form, ordinarily separated, but sometimes united into one mass, each of the two halves of which has its special duct. But the separate form is by far the most usual. As in higher animals, there are the usual parts, the ductus ejaculatorius, the vesiculae seminales, and the vasa deferentia, the conditions of which vary infinitely in different insects. Whether the intromittent organ is always traversed by an inner canal or not is a little doubtful. Ordinarily such is no doubt the case, but in others it would appear probable that the ductus ejaculatorius does not end absolutely in the organ, and that a groove on the surface of the latter receives the sperm. Some such arrangement must certainly exist in dragon-flies, in which the testes and the opening of the duct have no direct connexion with the intromittent organ. In the female the ovaries occupy much of the abdomen that is not taken up by the intestinal canal. Each consists of a very varying number of tubes, branching off externally, in which the eggs are contained ; these eggs are conveyed by oviducts, and before extrusion receive the fertilizing fluid stored in the spermatheca, which latter may be simple or compound; they pass out by the vagina. In close connexion with these parts in the female is the poison gland and sting found in some insects. In the gravid female of Termes the ovaries become enormously distended, so that the entire insect may be said to consist of little else than eggs. The rudiments of the sexual organs maybe detected in the larva when in a very young state, and the sex of the future perfect insect determined,a sufficient answer to those who assume that sex can be controlled by the nutriment furnished to the larva. It was formerly con-sidered that, pairing once effected, the male died almost immediately, and the female followed after having deposited her eggs. Recent observations go to prove that this is to a large extent erroneous, that pairing may be effected several times by both sexes (the female laying her eggs intermittently), in effect that polygamy and polyandry exist.
There are certain anomalous conditions of the generative system that may be conveniently noticed here, under different headings.
Neuters or Workers.In bees, wasps, and ants, and also in Termes (or white ants), the majority of the members of a colony is made up of individuals which as a rule have no reproductive powers. In the first three, these are aborted females, and it has been proved (at any rate for ants) that occasionally these workers lay eggs, which, how-ever, always produce males, the production of a queen depending apparently upon special feeding in the larval stage. In Termes the conditions are different. There are both workers and soldiers, both incapable of reproduction, but not exclusively consisting of aborted females, since both sexes are represented. Also in Termes there are what have been termed complementary males and females, distinct from the pair that were once supposed to be the exclu-sive founders of a new colony ; of these the females lay comparatively few eggs, their ovaries not acquiring the extraordinary development of those of the true queen.
Viviparous Insects.Oviparous generation is the rule in insects ; but there are certain departures from the rule. In the Aphides it is well known that both the oviparous and viviparous exist in the same species. In Lepidoptera there is a well-authenticated instance of an Australian insect closely allied to the clothes-moth bringing forth larvae already hatched. A similar condition is asserted to exist in a species of cockroach. In Coleoptera, Schiödte has noticed that two species of Staphylinidae, living in the nests of white ants in Brazil are viviparous, as is likewise Oreina in Chrysomelidae; so also are the Strepsi-ptera. In Diptera flesh-flies of the genus Sarcophaga are known to be viviparous. But the most extraordinary instance is in certain minute flies, to be noticed below, of which the larvae produce living larvae.
Alternation of Generations, Parthenogenesis or Agamo-genesis.In the bee, ant, many gall-flies, some Lepidoptera and (as is now known) also some Coleoptera, and insects of other orders, females are capable of producing fertile eggs without any contact with the male, and the produce of these eggs is frequently male. This property varies considerably in details. In the case of the bee or the ant, it would appear that one impregnation suffices for the life of the queen (which may last for seven or eight years), but the power of producing females does not probably extend beyond the immediate influence of the impregnation. In some hymenopterous gall-flies a true alternation probably sometimes occurs, combined with dimorphism; but absolute parthenogensis, in which females are produced generation after generation, is the common condition in many lepidopterous insects. This process is effected by internal budding. In the Aphides the conditions are still more remarkable, owing to the existence of both winged and apterous forms of both sexes, and of both oviparous and viviparous generation ; but it is not proved that the same individual insect is capable of producing both forms. In the case of the minute fly (Miastor metroloas) mentioned above, the production of larvae from larvae is continued throughout the winter and spring, until in June the brood goes through its ordinary metamorphosis, and results in mature males and females, and so the cycle recommences. Of all the marvels in the history of insects, this is the most astonishing; no wonder that the assertions of R. Wagner (the discoverer) were met with incredulity from the best physiologists until abundantly confirmed by others, and in other species.
Metamorphoses.Hundreds of volumes have been written on this fascinating subject, one or more of which are in almost every library; hence there is no necessity for giving more than a rudimentary outline here. All true insects may be said to undergo a metamorphosis. Such a condition is absent in the small groups known as Thysanura and Collembola; and, although these are here retained amongst Insecta as a matter of con-venience, the writer is disposed to agree with Lubbock that they are outside the pale of true insects. Metamorphosis may be broadly grouped into two main divisions(1) that in which the larva changes into a pupa which, as a rule, is inactive, and which never eats, and (2) that in which there is no true pupa state, the animal continuing active, and eating from the egg to the perfect insect. In the former the larva changes its skin, or moults, several (often many) times before it changes to the pupa, the last moult (or ecdysis) happening when it is in what may be called the pseudo-pupal condition (which may last from a few hours to several months) immediately before the pupa state is assumed ; this division is usually characterized as "metabolic." In the latter ecdysis goes on continuously at intervals from the egg to the winged-insect, and the form of the larval condition much resembles that of the perfect, the wings budding out gradually as the creature approaches maturity; the term "hemimetabolic" is applied to this division. In the first division the conditions are rather more varied than in the second. In the pupa of some Diptera the larval skin hardens, and within this the true pupa is formed (such a pupa is termed "coarctate") ; in other Diptera the pupa is not contained within the larval skin (which much resembles a true cocoon), but is free and even sometimes active, the various appendages not being connected with the body, as is usually the case in that of a lepidopterous insect. The pupae of Hymenoptera and Coleoptera are also much in the same condition, but they are not strictly active. Many writers have attempted to draw a broad distinction between such a pupa as that of a moth and that of an ant-lion or caddis-fly, cited here as extremes, because in the latter the members are free, and the pupa is really active shortly before the change into the perfect state, and thus the metamorphosis is supposed to be in some respects intermediate between that of true Metabola and true Hemimetabola. But such distinctions are more apparent than real. In many of the small moths the limbs and other appendages are scarcely consolidated with the body, but simply concealed in sheaths of which the ends at any rate are free. Also in the second (or hemimetabolic) division distinction has been drawn between the larva of a May-fly and that of a bug (as instances), because the changes from an absolutely apterous condition to one in which the wings are rudimentary and from this to the perfect state are more marked in the latter. This is probably due to the number of moults being less; the form with rudimentary wings is in no way a true pupa.
The metamorphosis of the internal organs, and even of the mouth parts, is much more marked in the Metabola than in the Hemimetabola. Respiration is maintained by means of spiracles or branchiae, as in larvae.
"Hypermetamorphism" is a term applied to certain con-ditions in which the larva atone period of its life assumes a very different form and habit from those of another period. Such a condition exists in several Coleoptera, such as Meloe and Cantharis, in which the larva is at first very active, with long legs, slender form, and anal setae, and attaches itself to the bodies of bees, afterwards becoming almost apodal, short, and stout, and living in the bees-nests. Other Cantharidae live in the egg-tubes of Orthoptera. In Sitaris a still more remarkable intermediate condition has been observed : the larva, after having attained its second con-dition assumes that of the coarctate pupa of a fly, from which it changes again to a state more analogous to the second condition before finally transforming to a pupa. This kind of metamorphosis has been closely observed by Newport, Fabre, Lichtenstein, Riley, and others. Brauer has recorded a somewhat similar condition in the larva of Mantispa (Neuroptera), which is at first free and very active, and afterwards becomes nearly apodal and obese, and lives parasitically in the nests of spiders. Advanced evolutionists hold the idea that larvae are only acquired conditions.
It is necessary to reduce what may be termed the systematic portion of this article to the smallest possible limits. The various orders are noticed under separate articles, and similar articles are devoted to the con-sideration of many of the more prominent, interesting, and familiar insects. All we can do here is to allude briefly to classification as a whole, with indications of the higher groups under each order. We also have nothing to do here with Crustacea, Arachnida, and Myriopoda, now considered as distinct classes, although American writers have recently again included the last two in Insecta as orders, placing the more subordinate groups (or orders in the general acceptation of the term) as suborders. Still more impossible is it for us to enter into an examination of the history of classification ; those of our readers who are specially interested in this subject cannot do better than consult vol. iv. of Kirby and Spences Introduction to Entomology, where a most full and painstaking "history of entomology" is to be found up to the date (1826) at which it was published; or they may consult with equal advantage Westwoods Introduction to Modern Classification, and Burmeisters Manual of Entomology (Shuckards English translation). The different classifications proposed by authors mainly resolve themselves under three headings,-the "metamorphotic" (of which Swammerdam may be considered the founder), the "alary" (or wing-system, due to Linnaeus), and the "cibarian" (or mouth-systern, originating with, or at any rate elaborated by, Fabricius). The metamorphotic system divides insects into those that undergo complete and incomplete metamorphoses; the alary is based upon the presence of two or four wings, or their absence altogether; the cibarian depended upon the conditions of the mouth organs, and more especially as to their being fitted for biting or sucking (mandibulate or haustellate). But experience proved that each of these systems had its defects ; there were always some groups, of more or less importance and extent, that would never fit satisfactorily into any of the proposed systems. To remedy this varying means were adopted, such as a combination of the several systems into what has been termed the "eclectic" system, the erection of numerous orders for certain aberrant groups, and that most ingenious idea of MacLeay, the author of what is termed the "circular" system. We are disposed to consider that of all systems the one that combines the greatest amount of convenience with the nearest approach to being natural is the metamorphotic, and this we shall accordingly follow here. It is not intended to acknowledge the subsidiary orders, excepting the Collembola and Thysanura, which are probably scarcely true insects, but which it is necessary to place here, were it only to avoid the risk of their being overlooked alto-gether, inasmuch as the writers on the other classes of Arthropoda are not likely to recognize them as coming within their scope.
The stumbling-block of all systems has been the Linnaean order Neuroptera, inasmuch as its members combine the characters of most of the other orders, and ingenious American writers have attempted to overcome this difficulty by considering it a collection of "synthetic types." In adopting metamorphosis as the basis of classification, we prefer to take another course, and to follow Erichson, who (in 1839) boldly transferred all those Neuroptera with incomplete metamorphoses to the Orthoptera as a suborder, although, in dealing with the Neuroptera in the light of a specialist, division into several orders appears the more natural course.
The sequence of orders we propose to follow is as under:
The obvious innovation in this arrangement is the position assigned to the Diptera, rendered necessary by the intimate relationship of Lepidoptera and Trichoptera, but in a metamorphotic sense no particular outrage on more generally adopted systems is occasioned, and we see no alternative other than that of widely separating the two subdivisions of Neuroptera.
HYMENOPTERA.In accordance with the system adopted by many modern writers, this order heads the scale as containing amongst its members those insects that appear to be endowed with the highest intellectual faculties. But at the same time it must be remembered that if the economy of the Termitidae in the Pseudo-Neuroptera had been as fully investigated as has that of bees, wasps, and ants, it is probable that the importance of this idea might be considerably weakened. The main characteristics are as follows:
Wings four (frequently absent altogether in ants, &c.), membranous, naked, transparent, with open reticulation and very few transverse nervules. Mouth mandibulate. Metamorphosis complete, but the pupa has its members free. Larva mostly apodal, but in the saw-flies much re-sembling that of Lepidoptera.
A convenient subdivision into three great groups is generally adopted, viz., Aculeata, Entomophaga (or Pupivora), and Phytophaga.
The Aculeata may be again divided into four:Mellifera (or Bees), Fossores (Wasps, &c.), Heterogyna (Ants and allies), and Tubulifera (Ruby-tailed Flies), but the last is perhaps more generally considered as forming a special division. In these the females (and workers, when present) are provided with a sting at the apex of the abdomen, connected with a poison gland. The abdomen is petiolate. The antennae are mostly thirteen-jointed in the males and twelve jointed in the females. In the bees the mouth arts are greatly modified, so as to form a suctorial apparatus, by the elongation of the maxillae, labium, and lingua, the small palpi being borne at its end. The neuration of the wings is tolerably complete. The legs are much modified, according to requirements, such as pollen-gathering, burrowing, &c. The larvae are apodal, hatched in cells constructed by the parent insects, the food usually provided by them being either honey or other insects. The habits of the group are frequently social (in this case neuters or workers are present) ; many are parasitic on insects of their own group (in the broad sense), and in many instances the parasites strikingly resemble those species in the nests of which they live (as in the familiar instance of Bombus and Apathus). Each division includes several families (to which we cannot allude here), and the group as a whole includes some of the most familiar insects, such as bees, wasps, and ants.
The Entomophaga are invariably true parasites in the larval stage (excepting the Cynipidae), the perfect insects depositing their eggs in or on the larvae or eggs of other insects, and their larvae, living upon the adipose tissue or contents of the eggs. There are no true sting and poison gland, but the female usually has the end of the abdomen provided with a long slender ovipositor, with which she can (in some cases) pierce the skin if roughly handled (but no inflammatory symptoms follow). The antenme are usually long, slender, and multiarticulate. The abdomen is strongly petiolate. The neuration of the wings is variable (often almost absent). The main divisions are Ichneumonidae, Proctotrypidae, Chalcididae, and Cynipidae, chiefly founded on the neuration, which in Chalcididae (and in a lesser degree in Proetotrypidae) is almost absent. Many members of this group are of extreme interest in consequence of their economy, and especially some extremely minute species (in Proctotrypidae) that infest the eggs of other insects, some of which can swim by means of the wings in search of the eggs of aquatic insects. The Cynipidae, although agreeing in main points of structure with the other divisions, are totally different in habits, and the term Entomophaga as applied to them is erroneous (a few, however, are parasitic upon aphides). They lay their eggs in the tissues or buds of plants, and there results therefrom a swelling termed a gall, inside which the larva feeds, either solitarily, or many in one gall in separate cells.
True Phytophagous Hymenoptera (or Terebrantia) comprise the two divisions known as Tenthredinidae and Siricidae. In these the abdomen, instead of being petiolate, is sessile ; the female is pro-vided with a double saw in the Tenthredinidae, and with a borer in Siricidae. The antennae have seldom. more than ten joints. The neuration of the wings is complete. The larvae differ from those of all other Hymenoptera in possessing Well-developed thoracic legs, and in addition (excepting in the Siricidae) a varying number of abdominal prolegs, and are so like those of Lepidoptera as often to require a practised eye to distinguish them therefrom. All are phytophagous, but their habits are very varied ; in fact, all the con-ditions known in Lepidoptera are probably here present also. Some species cause galls. Some (such as the Turnip Saw-Fly) occasion great damage. The Siricidae are wood or stem borers ; the familiar Sirex gigas often appears in the midst of large towns, through the larvae or pupae having been brought in with pine timber.
COLEOPTERA.This is probably the largest, and cer-tainly the best studied, of all the orders.
Four-winged insects, but the upper pair of wings are modified, hard and horny in texture, and are termed "elytra," lying longitudinally over the meso- and meta-thorax and abdomen, and when closed divided by a line or suture (occasionally the elytra are united, and in this case the second pair of wings is usually absent, and the insects are incapable of flight; more rarely the wings are absent altogether, both the elytra and hind-wings). Mouth mandibulate. Antennae seldom more than 11-jointed (often much less). Metamorphosis complete; the pupa having its members free. Larva extremely variable as to form; usually with thoracic legs, sometimes apodal.
Latreille divided the enormous amount of materials comprised in this order into four great groups according to the number of joints in the tarsi. Thus the Pentamera have five joints in all the tarsi; the Heteromera have five in the anterior and intermediate tarsi, and four in the pos-terior; the Tetramera have four in all the tarsi, the Trimera three in all. More minute investigation and better know-ledge have proved, however, that this system is essentially artificial, and in part founded on misapprehension; and it is the custom amongst many modern entomologists to ignore these great divisions, and to consider the order as composed of about 75 families, without collecting them into larger groups. It would be impossible to notice here each of these families in detail, and the old system, still regarded with favour by some of our most intelligent coleop-terists as the most useful, will be followed.
The general structure is so marked that but little con-troversy has been occasioned. A beetle is recognized as such universally, notwithstanding the great diversity of details that exists. The only disputed elements are the Strepsiptera (Bee-parasites) and Platypsylla (an epizoic parasite on the beaver). The diversity in the larval con-dition is much greater, and as extremes may be cited the larvae of the Staphylinidae (in which there is little difference in form and structure from those of the imago, excepting the absence of wings) and the apodal maggots of the Curculionidae. In the pupal condition this divergence mainly disappears.
The Pentamera, are usually considered to comprise the following superior subdivisions. Adephaga alone possess an inner palpiform lobe to each maxilla ; the larvae are predaceous, and feed on other insects and on flesh generally: they include the Cicindelidae (Tiger-Beetles) and Carabidae (these two groups often termed Geodephaga), which are terrestrial, and the Dytiscidae and Gyrinidae, familiar aquatic groups, with the addition of Amphizoa, an anomalous Ameri-can genus, recently referred to Dytiscidae. Palpicornia have short clavate antennae, and comprise a number of small and mostly aquatic genera, Brachelytra (including Staphylinidae, known as Rove-Beetles or Devils Coach Horses) are known by the very short elytra (usually much shorter than the abdomen), and form an exceedingly numerous group of often minute insects of which the larvae are predaceous, and differ but little in form from the imago ; with these the curious and aberrant Pselaphidae are usually united. Clavicornia have clavate antennae, and are sometimes termed Necrophaga, from the habit many of the species have of feeding upon dead and decaying animal matter; the forms are very numerous, and include the well-known Burying Beetles (Necrophorus) and the destructive Dermestidae (in which is the Bacon-Beetle); Parnidae are aquatic; Paussidae are wonderfully curious creatures with singularly formed antennae, living in ants nests : in this division are now placed the almost microscopic Trichopterygidae, most of which are not larger than small grains of sand, and remarkable for their narrow ciliated hind wings, and also other groups sometimes placed in the Trimera. Lamellicornia have the apex of the antennae pectinate or provided with lamelliform plates: they include the Stag-Beetles and the very numerous Dung-Beetles, amongst which is the Scarabaeus of the Egyptians, together with the numerous Chafers. Sternoxi have the prosternum produced and pointed, and mostly filiform antennae, with ordinarily an elongate oval form : in this division are the beautiful Buprestidae and the familiar Elateridae, the latter known as Click-Beetles, and able to spring by means of the process of the prosternum acting on a peculiar structure of the mesosternum: the larvae are all vegetable feeders, and have the legs only slightly de-veloped; those of Elateridae are known as Wire-Worms, and are often very destructive; some of the exotic Elateridae are brilliantly luminous. Malacodermi are a group of mostly soft-bodied insects very variable in form and habits ; the prosternum is not produced, and the antennae are usually pectinate or serrate : included in it are the Telephoridae (Soldier-Beetles), Lampyridae (Glow-Worms), Cleridae (often parasitic in the nests of bees), the wood-eating Ptinidae (in which is Anobium, or the Death-Watch), and Bostrychidae.
The Heteromera are a large group of forms connected together by the tarsal structure, but otherwise of the most diversified nature ; in fact it may be said that, so far as outward appearance is concerned, there are forms in it that might be readily mistaken as belonging to almost all the other principal groups, so protean are they both in structure and inhabits. Two principal points of structure have been used for separating them into two great divisions. In one of these they have been divided into Globicoxae and Conicoxae, accord-ing to the form of the anterior coxae; in the other the two divisions are termed Atrachelia, in which the head does not form a neck, and Trachelida, in which the head is narrowed into a neck. To some extent both these points of structure are correlated ; we adopt the latter ; but it must be remarked that the protean nature of the group as a whole is equally exemplified in its primary subdivisions. They appear to be invariably terrestrial, and for the most part phytophagous. The Atrachelia are mainly composed of a large number of genera of which Tenebrio (the Meal-Worm) may be taken as a type, and Blaps (the Churchyard Beetle) is also amongst its members. In the Trachelida the forms are more varied, and include some of the most remarkable instances of anomalies of form and struc-ture, and even of metamorphosis, that exist amongst Coleoptera. Here are placed the Cantharidae (Blister-Beetles, &c.), so remarkable for the hypermetamorphosis that exists in the larvae, and parasitic in the nests of bees and locusts ; the extraordinary genus Sitaris (equally hypermetamorphotic), a parasite in bees nests; Meloe (the Oil--Beetles, the history of which reads almost like a romance, the very young larvae being active little creatures living on the bodies of bees, but afterwards becoming obese and almost footless, and feed-ing on the bee larvae in the nests) ; and Rhipidius, parasitic on cockroaches. As a crowning point of eccentricity the extraordinary Strepsiptera (or Stylopidae) seem likely to find their resting place here, after having been considered a distinct order, as Diptera, as Hymenoptera, and as Neuroptera ; but even yet it is probable their position may be warmly disputed. These anomalous creatures are parasitic in the bodies of bees, and the female, which is vermiform, and without antennae, legs, or wings, never leaves its host, and is viviparous. The male has very large eyes, and extraordinary short antennae; the anterior wings (or elytra) are represented by small narrow processes, not unlike. the halteres of Diptera ; the posterior wings are folded in repose, but when expanded are extraordinarily broad, whitish in colour, and almost without nervures. The meta-thorax occupies the greater part of the body; the abdomen is termi-nated by a short stout process. The very young larvae are minute, active, and not unlike the young larvae of Cantharis and Meloe in form, and escape from the body of the mother by a slit in the neck; they are conveyed into the nests of their hosts, and penetrate the larvae of the latter, where they undergo hypermetamorphosis. Although originally supposed to be exclusively parasites on Hymenoptera, one species has recently been detected in the body of an homopterous insect.
Tetramera.Although the beetles of this great division have apparently only four-jointed tarsi, it was long ago demonstrated by Westwood that there is actually a very minute joint between the lobes of the third joint, so that they are actually pentamerous, but the concealed joint is probably functionless; thus the terms Pseudo-Tetramera and Crypto-Tetramera have been proposed in lieu of Tetramera. The chief groups are the Rhyncophora, Xylophaga, Longicornia, Phytophaga, and Clavipalpi. All are vegetable feeders. The Rhyncophora (or Weevils) have the head produced into a ros-trum, varying enormously in length, and in its lowest condition scarcely appreciable. The larvae are footless grubs, feeding almost universally in the interior of the stems or seeds of plants, and occasionally causing galls. Some exotic members of this group are amongst the most beautiful of insects. A peculiarity exists in the antennae, which are attached to the rostrum, and usually elbowed, the basal joint being ordinarily very long (and termed the "scape") and the rest shorter, the terminal joints usually forming a club (the portion between the scape and the club is termed the "funiculus"). Rhyn-cophora have been very variously subdivided. Sehönherr separated them into Orthoceri and Gonatoceri, according to the absence or pre-sence of an elbow to the antennae; Westwood has three families, Bruchidae, Attelabidae, and Curculionidae, founded on the antennae and palpi ; Lacordaires groups are Adelognathes and Phanerognathes, founded on the covered or uncovered mentum. Adopting Westwoods system, we have three families. Bruchidae have only a short flattened snout, unelbowed antennae, and filiform palpi ; they are probably universally seed or nut feeders in the larval stage. Bruchus gra-narius causes great destruction to grain; species of the genus Caryoborus affect palm-nuts, some even living in the so-called vegetable- ivory nuts; Anthribidae, which form a subfamily of Bruchidae accord-ing to Westwood, live in dead wood. Attelabidae have the antennae unelbowed, the palpi conical, and the rostrum long and curved, the genera Brenthus and Attelabus form the types of two subfamilies differing chiefly in the form of the club of the antennae ; the first of these is almost entirely extra-European, and its members appear to feed on dead wood ; the second includes the brilliant species of Rhynchites (the larvae of which roll up leaves and feed on them, or live upon fruits) and the minute species of Apion, of which the larvae variously attack seeds, the interior of the stems of plants, &c. Curculionidae have elbowed antennae and conical palpi, and are further subdivided into two main groups according to the length of the rostrum, each again forming numerous smaller groups ; the family contains many of the most familiar weevils, and some of the most destructive; the habits are extremely diversified : Sitophilus feeds on grain, Calandra in the stems of palms, sugar-cane, &c., Balaninus on nuts, Hylobius on the wood of pine trees, and a mul-titude of other instances of peculiar habits might be cited. It must be remarked that other main groupings of Rhyncophora considerably modify both the sequence and family position of many genera to an extent that cannot even be glanced at here. Xylophaga form a small group sometimes united with the Rhyncophora, consisting of small wood-boring beetles, in which the rostrum may be termed obsolete, and the insertion of the antennae is close to the eyes. Some of the members of this group (of which Tomicus and Scolytus are familiar examples) are supposed to cause great damage through their larvae feeding beneath the bark of trees; but it is possible they only appear when an unhealthy condition has been set up from other causes. The Longicornia may be mainly distinguished by their elongate elegant form, long antennae, which are generally filiform, but often pectinate, serrate, or ornamented with tufts of hair, the head not rostrate and armed with powerful jaws, the femora often clavate, and the tarsi having the basal three joints cushioned beneath. The larvae mostly feed on dead or dying timber, boring into its interior, and but seldom on living healthy wood, the females being provided with an ovipositor ; these larvae are fat, with very strong mandibles, and extremely short legs ; in some species several years elapse before they attain their full growth. The most modern classification recognizes three families, Prionidae, Cerambycidae, and Lamiidae. The Prionidae have the sides of the prothorax margined, and usually toothed, and comprise the largest known beetles in length, even if not in bulk. Cerambycidae have the head porrect, and the prothorax not margined; some authors separate from these a family Lepturidae as of equal value. Lamiidae have the head vertical. The Phytophaga form a large group of beetles feeding essentially, in all their stages, on the leaves of plants (hence the name). They are usually of short and thick form, with filiform, moniliform, or serrate (never clavate) antennae, the head ordinarily immersed in the prothorax and without rostrum, and the elytra covering the sides of the abdomen ; the mandibles comparatively weak ; the femora often enlarged, The larvae are usually external feeders, with well-developed legs, and often very curious in form. Much diversity in the classification of Phytophaga exists, and as to the number of families and the value of their characters. By Latreille they were divided into Eupoda (Parameca, Westwood) and Cyclica, of which the most prominent character is in the external form, the Eupoda having the head and thorax narrower than the abdomen, whereas in the Cyclica the base of the elytra is not broader than the hinder part of the thorax, hence the form is more rounded. Another grouping is according to the insertion of the antennae at the sides, or on the middle, of the front. The number of families varies from four to nine according to different authors. A glance at the prominent forms, according to later ideas, is here given. Crioceridae (by some divided into Crioceridae, Donaciadae, and Sagridae) belong to the Eupoda, and include the well-known Asparagus Beetle (Crioceris asparagi), the genus Donacia (and allies), which is aquatic in its earlier stages, and the brilliant exotic genus Sagra (remarkable for its enormously thickened bind femora), the larva of which forms galls on the stems of plants and lives therein. Cryptocephalidae (including Clythridae) are remark-able for the habits of the larvae, which form hard cases of excrement (?) in which they live. Chrysomelidae are a group of often beautiful insects, mostly remarkable for their nearly hemispherical form; they include, inter alia, Timarcha (the Bloody-nosed Beetle), the well-known genus Chrysomela, and the Colorado Potato-Beetle (Doryphora or Longitarsus); most of them distil an acrid fluid; the larvae are short and obese, feeding exposed and the pupae often have the remarkable peculiarity of being suspended by the tail. Halticidae are noted for their thickened hind femora and their jumping powers ; though small in size, some of the members (e.g., the Turnip-Flea) are most destructive. Cassididae (or Tortoise- Beetles) usually have the sides of the elytra expanded ; the larvae have the very singular habit of concealing themselves under a covering formed of their own excre-ment, which is sustained by means of a forked appendage at the extremity of the body. Other families (such as Hispidae and Galeru-cidae) must be passed over. The last division of the Tetramera is the Clavipalpi, often placed with the Trimera, and forming a connecting link there with. They area small group, with the last three joints of the antennae forming a compressed club, and the last joint of the maxillary palpi also broadly clavate. The family Erotylidae mainly constitutes the group ; the larvae probably all live upon fungi. In proof of the wide divergence of opinion as to the re-lationship of special groups, it may be mentioned that one author places certain genera of this group amongst the Clavicornia of the Pentamerous division.
The last of the great divisions of Coleoptera forms the Trimera. As in the Tetramera, it was discovered that the term is not strictly applicable, and that a minute joint is concealed between the lobes of the second joint of the tarsi, hence they have been called Pseudo--Trimera and Crypto-Trimera. Some authors have made this group a place of refuge for many almost isolated forms, the natural position of which it is difficult to suggest. At present, however, only a small number of more homogeneous materials are usually located here, and these are divided into Aphidiphaga and Fungicola. The Aphidiphaga comprise the familiar Lady-Birds (Coccinella) and allies; these have the last joint of the maxillary palpi hatchet-shaped, have short-clubbed antennae, and have the body remarkably hemi-spherical. They appear to feed chiefly on other small insects in both larval and perfect stages, and aphides are especial articles of diet with them ; but the writer has seen a larva of Coccinella with its head deeply immersed in the juicy body of a recentlv formed pupa of its own species. Fungicola have the last joint of the maxillary palpi filiform, with longer antennea, and, as a rule, less hemi-spherical body. They contain a number of mostly small and little familiar forms, and, as their name indicates, are often found in fungi, on which they no doubt principally subsist
DIPTERA.Only the anterior (mesothoracic) wings pre-sent, membranous, usually naked, with varying longitudinal nervures and but few transverse nervules. Posterior wings replaced by knobbed filaments termed "halteres." Mouth consisting of a rostrum formed chiefly by the extended labium, forming a canal in which the other usual organs, modified into lancet-shaped pieces, are contained, the whole forming a sucker; only the maxillary palpi developed. Thorax consisting almost entirely of the greatly enlarged mesothorax, the two other divisions very small, and scarcely separable frorn the mesothorax. Tarsi 5-jointed , the end joint with a pair of more or less disciform. pulvilli. Meta-morphosis complete.
Although it is the generally received opinion that the halteres are the representatives of the posterior wings, there have been those who regard them rather as connected with respiration or hearing, and by some they have even been considered as belonging to the base of the abdomen rather than to the thorax; this latter idea results from the diffi-culty of defining the true limits of the metathorax. At the base of each anterior wing is a small membranous portion termed an "alulet," not absolutely connected with the wing itself, but which must be considered an adjunct, and ;
certainly not representing a posterior wing.
Diptera form. one of the most extensive orders. The Genuina are commonly divided into two great groups, according to the structure of the antennae, and termed Nemocera and Brachycera respectively ; but it has long been seen that these divisions are not natural, especially with regard to metamorphoses, and Brauer proposed division according to whether the larval skin at the last moult opens by a slit down the back or in a curvilinear manner, a proposal be has since extended by demonstrating that those two divisionsOrthorhapha and Cyclorhapha-are rendered the more natural by the pupal condition being correlated with differences in the larva and perfect insect, and eventually it is probable his views will be generally adopted. Another basis for division is according to whether the pupa is free (sometimes active, but not taking nutriment) or enclosed within the hardened skin of the larva ("obtected" or "coarctate"). The division into Nemocera and Brachycera is here followed.
In the Nemocera the antennae are long and slender, and composed of a considerable number of small joints, which are often verticillate or plumose. They include the families Culicidae, Chironomidae, Psychodidae, Cecidomyiidae, Mycetophilidae, Tipulidae, and Bibio-nidae, but some authors make a more minute subdivision. All these, and a multitude of others, fall into Brauers suborder Orthorhapha, and the pupa is obtected. Here come some of the most familiar and blood thirsty members of the order, such as Gnats (or Mos-quitos), Midges, Gall-Midges, "Daddy Long Legs," &c., and some of them cause incalculable mischief to the agriculturist. In those species in which the earlier stages are aquatic, the pupa is active.
The Brachycera have the antennae short and thick, not more than three-jointed, but the terminal joint has a bristle (arista) which is sometimes articulate. These again have been divided into Hexachaetae, Tetrachaetae, and Dichaetae, according to the number of setae concealed in the mouth. They are "cyclorhaphous," and the pupa is coarctate. It is not possible to enumerate here all the families, nor to allude to the extreme diversity of habit and structure that exists. House-Flies, Blow-Flies, Flesh-Flies, Bot-Flies, and Gad-Flies, the beautiful Syrphidae (many of which devour aphides in the larval state), the parasitic Tachinae, the plant-eating Phytomyzae, &c., all belong to this division.
The Pupipara are a small group distinguished especially by the fact that the larvae and pupae are developed in the a body of the mother, and the head is sunk in the thorax ; they have even been considered a distinct order termed Homaloptera. All are parasitic. They include Ornithomyia (Bird-Flies), Melopkagus (the Sheep--Tick), the extraordinary wingless genus Nycteribia, parasitic upon bats, and the perhaps still more extraordinary genus Braula, a minute creature known as the Bee-Louse. Brauer terms them Cyclorhapha pupipara.
With the Diptera (as a distinct section) it is now the all but universal practice to include the Aphaniptera or Fleas, at one time considered as forming a distinct order. They differ from true Diptera in their laterally compressed form, well-defined thoracic divisions, absence of wings (which are represented only by scales), aborted antennae, developed labial palpi, &c. The mouth of the imago is (as is too well known) formed for suction, and its parts can be homologized with those of the true Diptera. The larva is slender and worm-like, and is mandibulate, in all probability feeding on the scaly débris or scurf from the skins of the animals attacked; it isnot found on the animals themselves, but in their beds or other resting-places. The pupa is inactive. For all practical purposes the Aphaniptera include only two generaPulex and Sarcopsylla, the Flea and the Chigoe or Jigger. Many mammals and some birds have each its own peculiar species of flea, or more than one, and the size of the tormentor is often in an inverse proportion to that of the tormented, the flea of the mouse being of gigantic pro-portions. The chigoe is notorious in tropical America for its habit of penetrating the skin of man, especially on the feet, the abdomen of the insect swelling and causing troublesome ulcers. Formerly it was believed that the eggs were deposited in the ulcers and that the larvae fed therein, but more recent observations tend to prove that the habits are much those of ordinary fleas.
LEPIDOPTERAFour membranous wings (frequently rudimentary, or sometimes wanting, in the female) clothed with flattened striated scales ; neuration open ; transverse nervules few; at the base of the anterior margin of the posterior pair is frequently a bristle used for connecting the two pairs in flight. Mouth haustellate, the maxillae -being much elongated and very slender, forming two closely- applied pieces, which together make the suctorial apparatus by means of which the nectar of flowers is pumped up for food ; in some groups this apparatus is rudimentary. Labial palpi strongly developed, the maxillary ordinarily rudimentary (but more developed than the other pair in a few small groups). Prothorax very narrow, with a pair of lateral organs termed patagia. Legs slender; tibiae spurred. Metamorphosis complete, all the appendages of the pupa enclosed in common covering with the body, but leaving the parts visible (occasionally, in some of the lower groups, the extremities of the appendages are free). Larva (termed a caterpillar) with six thoracic legs, and with a varying number (never more than 4 pairs) of fleshy abdominal and two anal prolegs (rarely the larva is apodal). With few exceptions, they are phytophagous.
These insects are familiarly known as Butterflies and Moths, and the order contains the most beautiful insects that exist, and forms the most popular and attractive of all for collectors. The scales of the wings (and other parts of the insect) are really modified hairs. Each is a flattened sac, striated on one side, containing variously coloured pigments, the arrangement of scales of different colours causing the beautiful markings and patterns so universal ; but metallic colours are due also to interference caused by minute inequalities of the surface, combined with the contained pigment.
The classification of Lepidoptera is still in a state of much uncertainty. By collectors they are fancifully divided into Macro- and Micro-Lepidoptera. A more familiar division is into Butterflies and Moths,the former being termed Rhopalocera (or Diumi), the latter Heterocera (or Nocturni).
The Rhopalocera are especially distinguished by their clubbed antennae. The following great groups are tolerably well marked (but each has been much subdivided):viz., Papilionidae, Nymphalidae, Erycinidae, Lycaenidae, and Hesperiidae. Some split the division into two, according as the pupa is suspended by the tail only, or has a thread round the body as well (these groups are termed Suspensi and Succincti respectively); the Nymphalidae are especially characteristic of the first of these. In the Nymphalidae the anterior legs are not fully developed in either sex ; in the Erycinidae and Lycaenidae this occurs only in the males. The Hesperiidae have the club of the antennae terminated by a hook, and the position of the wings in repose differs ordinarily from that in the other groups; the pupa is enclosed in a rudimentary cocoon, and may even be subterranean.
The Heterocera are subdivided roughly into Sphingidae, Bomby-cidae, Noctuidae, Geometridae, Pyralidae, Tortricidae, Tineidae, and Pterophoridae; but much more minute subdivision is adopted by specialists. In these the form of the antennae is very variable (as the name implies), and the bristle on the posterior wings is usually (not always) present. The Sphingidae (which comprise some of the largest and most robust moths) usually have the antennae fusiform towards the tips. Bombycidae generally have the antennae of the male strongly pectinate, and those of the female simple or nearly so ; the larvae with four pairs of abdominal prolegs (and the anal pair); but this is a group of extremely heterogeneous materials, if taken in its widest sense ; it includes the Silk-Worm Moths as familiar examples, and many extraordinary forms, amongst which may be mentioned the curious Psychidae, in which the larvae manu-facture portable cases wherein they live, and in which the females are apterous. Perhaps allied to this group, or intermediate between it and the Sphingidae, is the curious and abnormal collection of pretty insects termed Castniadae, at one time considered to be butterflies, and even yet included with them by some authors. The haustellum is rudimentary in the true Bombycidae. The Noctuidae are stout-bodied moths, mostly (but by no means always) of nocturnal habits ; the antennae greatly varying, but not thickened ; the bristle on the posterior wings present ; the hau-stellum present (in one genus, Ophideres, it is strong enough to pierce the skin of oranges). They are mostly divided into two groups accord-ing, to the number of the prolegs in the larvae (four, or only three, abdominal pairs), and the neuration of the posterior wings. The larvae are usually nearly smooth (those groups with hairy larvae are by some transferred to the Bombycidae), and the pupae subter-ranean. Geometridae are especially distinguished by the presence of usually only one pair of abdominal prolegs, occasioning a peculiar form of locomotion, termed "looping"; antennae varying; bristle of posterior wings present ; the wings usually expanded when at rest. With apparent relations to these is the small group Uraniidae, consisting of beautiful papilioniform insects, still by some placed with the butterflies. Pyralidae form a special group of varying, and for the most part rather small, insects, with simple (or nearly shaple) antennae; long slender legs; the bristle of the posterior wings present; long palpi ; larvae with three to five pairs of abdominal prolegs, and mostly smooth and glossy in appearance. Tortricidae are small insect ts of nocturniform mien when at rest (the wings being horizontal and not expanded) ; antennae simple bristle of posterior wings absent ; haustellum short ; palpi short larvae with four pairs ol abdominal prolegs. Many of the species of this group do immense damage to trees and garden plants. The Tineidae is an immense group of mostly small (often very minute) insects, with extremely varying structure and habits. They may always be distinguished from the Tortricidae by the long palpi (the maxillary pair being sometimes strongly developed and exceeding the labial) ; the fringes of the wings are usually very long. Some of these minute forms are excessively beautiful. The group as a whole is made up of very incongruous materials. Pterophoridae are a small group at once distinguished by the wings being split up into linear divisions, hence they have been termed "plumes." By some they are not considered distinct from the Pyralidae, with which there is considerable structural affinity.
NEUROPTERA.Four membranous and for the most part densely reticulate wings, more or less clothed with hairs, but without true scales ; very frequently the hairs are on the neuration only. Mouth mandibulate. Metamor-phosis complete, but the pupa has its members free.
In the outline of classification (at p. 147) proposed to be adopted in this article, it is stated that the Neuroptera as there indicated are considered as forming a single order, more as a matter of convenience than from any conviction of the homogeneity of the two divisions.
The Trichoptera (or Caddis-Flies) form a very natural and sharply defined group distinguished by their rudimentary mouth-parts, with the exception of the two pairs of palpi, which are strongly developed, the maxillary pair being the longer, and with often the greater number of joints ; the antennae setaceous; wings with com-paratively simple neuration and but few transverse nervules, ordi-narily covered with hair (which sometimes simulates scales) ; larvae (known as Caddis-Worms) with well-developed thoracic legs, and anal crotchets, but without prolegs, living in tubes covered with extraneous materials ; pupa lying free in the case, or occasionally in a special cocoon, only active just before its metamorpliosis ; habits (with one or two exceptions) aquatic.
It is considered by the writer that there is direct relationship of the Trichoptera with the Lepidoptera, and this idea acts as the key to the scheme of classification adopted. They are divided into seven families, viz., Phryganeidae, Limnophilidae, Sericostomatidae, Leptoceridae, Hydropsychidae, Rhyacophilidae, and Hydroptilidae, chiefly according to the structure of the maxillary palpi. In the Rhyacophilidae and Hydropsychidae the larvae inhabit fixed cases, in the others the cases are free, and caried about by the inmates ; in the Rhyacophilidae the pupa is enveloped in a special cocoon. The neuration shows strongly-marked homology with that of Lepidoptera.
The Planipennia (or true Neuroptera according to modern ideas) have strongly-developed mandibulate months ; for the most part moniliforin or filiform (often clavate) antennae; the wings ordinarily densely reticulate, with very numerous transverse nervules, the membrane hairless or nearly so. The larva is more divergent from the Lepidopterous type. The pupa is ordinarily in a cocoon; it is active just before its transformation. A convenient subdivision is into Panorpidae, Sialidae, and Megaloptera.
The Panorpidae (Scorpion-Flies, &c.) are remarkable for the mandibles, &c., being situated at the end of a long beak, formed by the much-elongated clypeus above and the lower lip beneath. The wings have open reticulation, and the larva is more, vermiform than in the succeeding groups, so that the relationship to the Trichoptera is close. They are carnivorous both in imago and larva, and the latter is subterranean. Panorpa is remarkable for the cheliform termination of the abdomen, Bittacus for its tipuliform aspect, Boreus for its nearly apterous condition.
The Sialidae form a heterogeneous group of small subdivisions with setaceous antennae (which are sometimes pectinate); strongly developed prothorax; the third or fourth joints of the tarsi cordate. They are again divided into two sections (or families), of which Rhaphidia and Sialis may be taken as the types. The former (Snake-Flies) are especially remarkable for the enormously elon-gated prothorax (the anterior legs at its posterior extremity); the larvae sub-cortical. The latter comprise mostly large insects with strong (but not greatly elongated) prothorax and ample wings, the larvae, of which are aquatic, and provided with lateral branchial plates ; the genus Corydalis is remarkable for the enormously elongated mandibles of the male (but not in all species).
Megaloptera contain many groups of insects, with mostly monili-form (or clavate) antennae; densely reticulate broad wings; varying prothorax ; tarsal joints not dilated. The number of subfamilies is large. The most prominent forms are the Mantispidae, with their long prothorax (the anterior legs at its anterior end), the larvae, of which live in the nests of spiders (and also tree wasps), and in Mantispa undergo a kind of hypermetamorphosis ; the Nemopteridae, with their linear posterior wings; the Ant-Lions, with their clavate antennae and trap-forming larvae ; the Chrysopidae and Hemerobiidae, known to feed on aphides in their larval stage,-the former often emitting a disgusting odour ; and the very curi-ous little Coniopterygidae, covered with a whitish waxy secretion, and differing from all others in the extremely simple neuration, In Osmylus and Sisyra the larvae are aquatic; and those of the latter have been found in the interior of the freshwater sponge.
ORTHOPTERA.Typically with four densely reticulate unequal wings (or apterous), whereof the anterior are more or less coriaceous, the posterior folded under them, and membranous; in the most typical groups they are deflexed, and closely applied to the body longitudinally in repose. Mouth mandibulate. Metamorphosis incomplete.
Having adopted metamorphosis as a basis for classifica-tion, it became necessary to view this order after the manner universal amongst German systematists, and to include in it many groups that are ordinarily accepted as neuropterous, the only plan possible without the erection of independent orders for their accommodation. The result is that more absolute homogeneity from a general point of view is attained, and we have to deal with an order made up of otherwise most incongruous elements, but somewhat relieved by the sharpness with which the great groups are defined. Regarding the two great divisions, Pseudo-Neuroptera and Orthoptera genuina, as a whole, the main distinction really consists in the fact that in the former the four wings are equally membranous, whereas in the latter the anterior pair are more or less coriaceous; another difference is in the head, which in the former is horizontal, whereas in the latter it may be described as vertical ; but this distinction only applies to the typical groups. Thus there is really little more difference than exists between the two great divisions of Hemiptera, now almost universally placed in one order.
Naturally allowing the Pseudo-Neuroptera the first place as following on from the true Neuroptera (though some would say the Dragon-flies are really the typical Neuroptera of Linnaeus), these must be first considered. Adopting the descending scale, the main groups may be glanced at as follows:
The Odonata (Dragon-flies ; constituted an order by Fabricius) may be considered the most highly organized, with regard to their powerfully mandibulate mouth, strong, densely reticulate wings, &c. The special peculiarities of this group (including the extra-ordinary structure of the mouth in the preparatory stages, and the anomalous position of the genital organs in the male) have been fully discussed in the article DRAGON-FLY (q.v.).
The Ephemeridae (see EPHEMERIDAE) follow. After these come the Perlidae, aquatic insects in their preparatory conditions, remarkable for the comparatively weak development of the month parts (shared with the Ephenteridae) in the perfect state, the four wings longitudinally horizontal and overlapping, the stout quadrate or oblong prothorax, the frequent presence of two articu-lated tails, the long setaceous antennae, &c. This group is also re-markable as being the first in which the persistence of external branchiae in the imago was detected (a peculiarity since found to be of frequent occurrence in them, and extending to other orders). They are known familiarly as Stone-Flies, and form a large portion of the stock-in-trade of an angler.
The Embidae constitute a very small group, which in general form much resemble Perlidae, but have, on the other hand, affinities with the white ants. The larvae live habitually under stones in little galleries, and a recent discovery appears to prove that they feed on roots.
Termitidae (or White Ants, placed by some authors in the true Orthoptera) are social insects living in immense communities, and forming nests on the round or on trees. In some respects there is analogy between these and social Hymenoptera, and the diversity of condition in a single species is even greater. Winged forms of both male and female exist (the wings being shed at a certain time), and there are also commonly apterous forms known as workers and soldiers, whose office it is to build the dwellings and protect the inmates, the soldiers having the head provided with a powerful horn or elongated mandibles. Each condition has its own special immature form, so that it is probable no more specially polymorphic insects exist. The wings are carried horizontally and overlapping in repose ; the prothorax is well developed, and the tarsi are four-jointed. Recent observations tend to show that speci-ally prepared food (comminuted wood) is provided for the larvae.
Psocidae are a small group of small insects remarkable for their swollen face, setiform antennae, narrow prothorax, and large mesothorax, four-jointed maxillary palpi, and rudimentary labial palpi, two or three-jointed tarsi. The wings are deflexed (often absent), with ordinarily very open neuration. These insects live on fungi and débris and also occasionally on dry animal substances, as in the case of the Book-Louse, ordinarily so abundant in neglected collections of insects, and erroneously supposed to occasion a ticking like that of Anobium.
There remain two groups the position of which has occasioned much controversy, but which are now often placed here, not, perhaps, because the affinities are very marked, but more to accommodate them with a resting-place in a division of Insecta the characters of which are so very elastic.
The first of these are the Thysanoptera, considered a distinct order by Haliday, the founder of the name, and by Burmeister placed in his order Gymnognatha as a distinct group termed Physopoda ; by some authors they are placed in the Hemiptera. It is true that the mouth forms a short rostrum with only bristle-like mandibles, but the presence of distinct palpi would appear definitely to invalidate tile position in Hemiptera. The wings are four in number, lying horizontally on the back and crossing at the tips ; they are very slender, membranous, without nervures or nearly so, and strongly ciliated, or they may be wanting in some species, even in the perfect state. These insects are familiarly known as Thrips, and sometimes occasion much damage to various kinds of plants by sucking the juices, which the almost haustellate nature of the mouth enables them to do. They are mostly very minute insects, and have been divided into many sections and genera on structural characters.
The second of the above-mentioned groups is the Mallophaga (or Bird-Lice), which it is convenient to separate from the Anoplura (or true Lice) on account of the structure of the mouth, which is man-dibulate and also carries palpi. On account of the absence of metamorphoses, some place them (with the Anoplura) as outside the pale of the Insecta, but they may be regarded as degraded Pseudo-Neuro-ptera. They for the most part live on the feathers of birds (each bird having its particular parasites), but a few also on mammals.
What may be termed Orthoptera genuina consist of groups for the most part very sharply defined.
The Blattidae (or Cockroaches) form the order Dictyoptcra of Leach. These are insects of flattened form, with four horizontal wings (or apterous), of which the anterior pair are considerably coriaceous, but with distinct neuration ; the head small ; tarsi five--jointed. The eggs are not laid separately, but are contained in a common capsule which is carried about by the female at the extremity of her abdomen. There are many genera and species, of which latter the abundant Periplaneta orientalis is the most familiar.
Forficulidae (or Earwigs) form the order Euplexoptera of West-wood and the group Dermatoptera of Burmeister. Externally they much resemble Coleoptera of the family Staphylinidae in form (but with pincer-like appendages at the extremity of the abdomen), the anterior wings being abbreviated and coriaceous, separated by a suture, and concealing the ample but folded posterior wings (but some forms are apterous) ; the tarsi three-jointed. The eggs are deposited in cavities in the earth, and are guarded (at any rate in some) by the mother.
Mantidae are mostly large elongate insects with strongly de-veloped raptorial anterior legs (hence the insects are carnivorous). The prothorax very long; tarsi five-jointed ; wings often ample of frequently wanting, all reticulate, but the anterior pair slightly more coriaceous. The earlier states greatly resemble the perfect insect without wings. The eggs are contained in a kind of case formed of a secretion voided with them, in which they are arranged in rows, the whole mass being attached to twigs, &c.
Phasmidae (Spectres, or Walking-Sticks) have considerable exter-nal resemblance to the former, but the anterior legs are not rapto-rial, and the insects are phytophagous. The wings (when present) are usually much shorter than the abdomen. Most of the species mimic (as do those of the last family) leaves or twigs, often to such a degree that it is hard to imagine one is regarding an insect.
Gryllidae form the first of a division termed Saltatoria (as opposed to Cursoria or Gressaria), from the structure of the hind legs fitting them for jumping. The antennae are long and setaceous ; tarsi three-jointed ; anterior wings lying horizontally over the folded posterior. The males mostly produce sound by a special structure at the base of each anterior wing acting on the posterior. This family is made up of materials presenting considerable diversity, but may be grouped roughly in two, according as the anterior legs are formed for digging (Mole-Crickets) or for running (Crickets).
Locustidae have the antennae very long and slender; the tarsi four-jointed ; the anterior wings longitudinally deflexed. The female ordinarily has a broad curved ovipositor suitable for forming grooves in bark, or earth, in which the eggs are deposited. They are mostly phytophagous, but in some cases carnivorous also. The males are usually very noisy,with special sound-producing organs at the base of the anterior wing.
Acrydiidae differ from the last chiefly in the antennae, which are shorter and thick, and in the three-jointed tarsi. The female has no produced ovipositor. They are phytophagous, and the eggs are mostly laid in earthen tubes. This family includes (according to modern classification) the true Locusts, notwithstanding the appli-cation of the term to the last-mentioned. Sound is produced by friction of the hind thighs against the nervures of the anterior wings.
The Orthoptera have here been treated in a somewhat more de-tailed manner than other orders, on account of the bearing of the materials on the classification of insects in general, and the sharply differentiated nature of these materials in particular.
HEMIPTERA.This order consists of insects of very vary-ing structure. Primarily there are two great divisions, known as Heteroptera and Homoptera, by some considered distinct orders. The points in which they agree consist especially in an imperfect metamorphosis, and the structure of the mouth, which latter is of a very simple nature, consisting of an elongated articulate tube formed by an extension of the labium into a suctorial organ, concealed in which are bristle-like mandibles and maxillae, and probably rudiments of maxillary palpi. The tarsi have from one to three joints.
In tha Heteroptera (or true Bugs) the anterior wings are horizontal, and composed of two distinct parts, the basal portion (or corium) being coriaceous, and the apical portion (or membrane, often unde-veloped) being membranous with distinct longitudinal neuration, which latter is only faintly indicated in the coriaceous portion. In repose the membranous portion of one wing overlaps that on the other. The posterior wings are concealed under the anterior, folded, membranous, and with only few nervures. Apterous forms are not uncommon, and sometimes the posterior wings are wanting. This division is again divided into two, Gymnocerata and Cryptocerata, in the former of which the antennae are composed of few elongate slender joints, while in the latter the joints are still fewer, short and thick, and ordinarily concealed under the head. Modern writers have erected a multitude of small subdivisions which cannot be enumerated here. The Gymnocerata are broadly divided into the fol-lowing families, viz., Scutelleridae, Pentatomidae, Coreidae, Berytidae, Lygaeidae, Capsidae, Tingididae, Reduviidae, Emesidae, and Saldidae, founded on different points in the structure of the antennae, rostrum, scutellum of mesothorax, tarsi, &c. They are terrestrial, and suck the juices of plants or animals. The entire family Reduviidae are probably blood-suckers, and members of other families as above given are notorious for a similar habit, amongst which may be par-ticularly noticed the genus Acanthia (including the Bed Bug); but the greater part are plant bugs. Most of them are remarkable for emitting a peculiar and often disgusting odour. The Cryptocerata are entirely water bugs, often of extraordinary form, and sometimes gigantic in size. They include the families Hydrometridae, Gerridae, Galgulidae, Nepidae, and Notonectidae. They prey upon animals. One genus (Halobates) is remarkable for its pelagic habits being found on the surface of the ocean very far from land. Many others, such as Notonecta (Water Boatmen or Toe-Biters), Nepa, Ranatra, &c., are very familiar insects.
The Homoptera have the wings for the most part deflexed, and the anterior pair not separated into two parts. Often all the wings are membranous, with strong nervures ; in others the anterior pair is coriaceous. The division regarded as a whole is very polymorphic. The true Homoptera have three-jointed tarsi. They may be divided into Cicadidae (remarkable for the sound-producing organs at the base of the abdomen of the male), Fulgoridae (known as Lantern--Flies, but now known to produce no light ; having the head greatly prolonged in front), Lystridae, Cixiidae (comprising many little plant-hoppers), Issidae, Derbidae, Flatidae, Tettigometridae, Membracidae (often of most extraordinary forms), Cercopidae (included in which is the Cuckoo-Spit Insect), Ledridae, and Jassidae,all vegetable feeders. The more aberrant Homoptera include well-marked groups. The Psyllidae are small plant-sucking saltatorial insects with four membranous wings which lie longitudinally deflexed in repose, and with very narrow prothorax, and eight- to ten-jointed antennae ; they often occasion much damage ; the larve, are frequently covered with a cottony secretion. The Aphidae are the familiar Plant-Lice, the winged forms of which have those organs mem-branous, and often extended in repose. The antennae are five- to seven-jointed. The diversity in form and habits is enormous, and, as is well-known, there are winged and apterous forms in the same species, and parthenogenetic generation of the most extraordinary nature; and the same species may be both oviparous and viviparous. Most of them void a sweet secretion from abdominal tubes, known as honey-dew, for which they are "milked" by ants. The destruc-tion they occasion to plants is very great as a now too familiar instance of this, the Phylloxera vastatrix of the grape-vine may be cited. Coccidae (or Scale Insects) have the male two-winged, the female apterous, and living all her life as a fixed "scale" on plants, the organs being of the most rudimentary nature ; the eggs lying under the scale in great numbers ; the tarsi with only one joint ; parthenogenesis occurs also in this group ; the male in its earlier stages lives under a special scale. The Cochineal Insect is one of the best known in this group. The little family Aleurodidae consists of minute insects covered with a white waxy secretion. They have four almost nerveless wings in both sexes, two-jointed tarsi, the abdomen without secreting tubes, and do not live under scales.
In the Hemiptera it is now the fashion to include the Anoplura, or true Lice (some also place here the Mallophaga or Bird-Lice), a degraded form of this order, without metamorphosis. Here it is preferred to let them rest in this article, even although some writers do not consider them true insects. The mouth parts certainly have indications of a rostrum, and there are no palpi, and, but for the absence of metamorphosis, there would be little difficulty in fixing the position here as without doubt. All, as is well-known, are epizoic parasites on man and other Mammalia, each species being confine to a special host, while attempts have been made to prove that the Head-Louse (Pediculus capitis) varies according to the races of men to which it is attached. Perhaps the Crab-Louse (Phthirius pubis) is regarded with greater disgust than is bestowed upon any other living creature.
COLLEMBOLA and THYSANURA.In the introductory notes to this article (p. 141) it is stated that "although it is not difficult to define an insect, speaking broadly, there are certain small groups that do not satisfactorily fall into the class as limited by strongly-defined lines of demarcation." The writer there had especially in view those lice known as Mallophaga and Anoplura, and the two groups indicated in the heading of this section, groups in which metamor-phosis, the key of his ideas as to classification, and embody-ing an essential requisite in an insect according to common acceptance, is wanting. In the time of Linnaeus, when we were only outside the threshold of knowledge, it may have been sufficient (and perhaps prudent) to include these groups in an order Aptera. But accumulation of knowledge soon dispersed that incongruous order. Such of its elements as could with justice be considered insects have been distributed amongst the various orders. We have not hesitated here to regard the Mallophaya as degraded Pseudo-Neuroptera, nor the Anoplura as equally degraded Hemiptera, notwithstanding that some veterans in entomo-logical science may still dispute their position as true insects. There is a breaking-point to elasticity even in ideas of classification, and with regard to the Collembola and Thysanura we gladly avail ourselves of the assertion of Lubbock to the effect that they are scarcely within the pale of the true Insecta, notwithstanding the efforts made to locate them in that convenient refuge for the destitute, the -Pseudo-Neuroptera. It is certain that the writers in the present work on other classes of Arthropoda will not accept them, and it becomes necessary that they should not be forgotten. If insects at all, they have in the process of evolution lost the chief attributes of insects, or have never acquired them. Generally both groups are accepted as Thysanura, or as forming two familiesPoduridae (=Collembola) and Lepismatidae (=Thysanura).
In the Collembola the antennae are short, thick, and few-jointed the eyes are composed of groups of simple "eye-spots" (much as in the larvae of true insects) varying in number; the mouth organs mandibulate, subject to modifications of a haustellate nature ; the palpi quite rudimentary ; the abdomen consisting of six segments, an ordinarily provided beneath with a saltatory apparatus (which may, however, be rudimentary) ; no caudal setae. The body is often clothed with prismatic scales, not unlike those of Lepidoptera. Ordinarily they are minute animals, living in damp places, and sometimes found gregariously. An elongate form is the most general, but Smynthurus and Papirius are short and obese. Lubbock recognizes six families.
The true Thysanura are elongate creatures, not unlike the larvae of Ephemeridae in form. The antennae are long, slender, and multi--articulate ; the eyes large, compound, and contiguous (or absent); the mouth mandibulate ; the palpi well developed ; the abdomen consisting of ten segments, with long caudal appendages. Lubbock forms three families. ln some of the genera the scales form beautiful microscopic objects ; in others they are replaced by hairs. Lepisma saccharina-sometimes termed the "Silver Fish"is a fiamiliar example of Thysanura. The genus Campodea is especially interesting as being considered by some as the representative of the primitive form of insect, whence all others have been evolved.
Bibliography.Bibliothecae and Annual Records:H. A. Hagen, Bibliotheca Entomologica, 2 vols., Leipsic, 1862-3, notices all that has been written on the subject up to date; "Berichte über die wissenschaftlichen Leistungen im Gebiete der Entomologie." in Wiegmanns Archiv, 1835-1880; Zoological Record, London, 1865-1881. General Handbooks:W. Kirby and W. Spence, An Introducton to Entomology, 4 vols.,1815, 1817, and 1826, new edition in 1 Vol., 1856; the latter practically includes only vols. i. and ii. of the first edition; J. O. Westwood, An Introduction to the Modern Classification of Insects, 2 vols., 1839-1840 ; H. Burmeister, Mannal of Entomology, translated by Shuckard, London, 1836; A. S. Packard, Guide to the Study of Insects, 6th edition, New York, 1878; E. Blanchard, Histoire des Insectes, 2 vols., Paris, 1845 ; C. E. A. Gerstäcker, Handbuch der Zoologie, vol. ii., Leipsic, 1863 ; M. Girard Les Insectes: traité élémentaire dEntomologie, Paris, 1873-1880, in progress. Geographical Distribution:A.R. Wallace, The Geographical Distribution of Animals, 2 vols., London, 1876; Id., Island Life, London, 1880. Economic Entomology:J. Curtis, Farm, Insects, London, 1850; J. T. Ratzeburg, Die Forst-Insekten, 3 vols., Berlin, 1839-1844 ; J. H. Kaltenbach, Die Pflanzenfeinde aus der Klasse der Insekten, Stuttgart, 1874; B. Altum, Forstzoologie: Insecten, 2 vols., Berlin, 1874-1875; E. L. Taschenberg, Entomologie für Gärtner und Gartenfreunde, Leipsic, 1874 ; C. V. Riley, Reports on Noxious and Beneficial Insects of the State of Missouri, 6 vols., Jefferson City, 1870-1877; see also Kirby and Spence, ut supra. Insects and the Fertilization of Plants:C. Darwin, On the various Contrivances by which British and Foreign Orchids are Fertilized by Insects, 1862; Id., Animals and Plants under Domestication, 2 vols., 1868; Id., Different Forms of Flowers, 1880; J. Lubbock, British Wild Flowers in relation to Insects, 1875 ; H. Müller, Die Befruchtung der Blumen durch Insekten, Leipsic, 1873; Id., Alpenblumen, ihre Befruchtung durch Insekten und ihre Anpassungen an dieselben, Leipsic, 1881. Anatomy, Physiology, Chem-istry, &c.:Works by Kirby and Spence, Westwood, Burmeister, Packard and Blanchard, ut supra; G. Newport, article "Insecta" in Todds Cyclopaedia of Anatomy and Physiology, 1839; R. Owen, Lectures on the Comparative Anatomy and Physiology of Invertebrata, 2d edition, 1855 ; C. T. E. von Siebold, Lehrbuch der vergleichenden Anatomie der wirbellosen Thiere, Berlin, 1848 ; Id., A True Parthenogenesis in Moths and Bees, translated by W. S. Dallas, London, 1857; C. Gegenbaur, Grundriss der vergleichende Anotomie, 2 vols., Leipsic, 1877; T. H. Huxley, Manual of the Anatomy of Invertebrated Animals, 1877 ; E. Brandt, articles in the Horae Societatis Entomologicae Rossicae, St Petersburg; F. M. Bal-four, A Treatise on Comparative Embryology, 2 vols., 1880; A. Gamgee, Physiological Chemistry of the Animal Body, vol. i., 1880. Metamorphoses:J. Lubbock, Origin and Metamorphoses of Insects, 1874; M. Girard, Les Métammphoses des Insectes, Paris, 1874, and the general handbooks noticed above. Classification and Special Entomology:An analysis of the various schemes proposed by the principal systematists is to be found in the works noticed under the heading "General Hundbooks," up to the date of their publication; those by Kirby and Spence, Westwood, Burmeister, Packard, and Gerstäcker are most recommended. For a much condensed account, F. P. Fascoe;s Zoological Classi-fication, 2d edition, 1880, will be found useful. For information on special orders or groups of insects, reference should be made to the "Register" at the end of the second volume of Hagens Bibliotheca Entonwlogica, or to the German Berichte and the English Zoological Record.
The above article was written by Robert McLachlan, F.R.S.