1902 Encyclopedia > Dragonfly
DRAGON-FLY (German, Wasserjungfer; Swedish, Trollsündal;Danish, Guldsmed; Dutch, Scherpstekendevlieg; French, Demoiselle), the popular English name applied to the members of remarkable group in insects which formed the genus Libellula of Linnae and the ancient authors. In some parts of the United States they appear to be known as devils Darning needles," and in many parts of England are termed "Horse-stinger." It is almost needless to say that (excepting to other insects upon which they prey) they are perfectly innocuous, though some f the larger species can inflict a momentarily painful bite with their powerful jaws. Their systematic position is at present contested and somewhat uncertain. By most of the older systematitsts they were placed as forming part of the heterogeneous order Neuroptera. Fabricius, however, elevated them to the rank of a distinct order, which he terms Odonata; and what-ever may be the difference of opinion amongst authors at the present day, that term is almost universally employed for the group. Erichson transferred all the groups of so called Neuroptera with incomplete metamorphoses, hence including the dragon-flies, as a division of Orthoptera, which he termed Pseulo-Neuroptera, terming those groups in which the earlier states are sun-aquatic Orthoptera amphibiotica. It is not necessary to enter into an examination here of the merits or demerits of those various systems, and it will suffice to say that all are agree into an examination here of the merits of demerits of those various, systems, and it will suffice to say that all are agree in maintaining the insects as forming a group marked by characters at once extraordinary and isolated in their nature.
Fig. 1. (left) -- The anterior portion of the body of Aeschna cyanea freed from the puparium.
Fig. 2. (right) -- The tail being extricated.
The group of Odonata (using the term as a matter of convenience) is divided into three families, and each of these again into two sub-families. The families are the agrionide, Aeschinilde, and Liberllulide, - the first including the sub-families Calopterygina and Agrionina, the second Gomphina and Aeschnina, and the third Cordulina and Libellulina.
The structure of a dragon-fly being so very remarkable, it is necessary to enter somewhat extensively into details. The head is comparatively small, and excavated posterior, connected very slightly with the prothorax, on which it turn almost as on a pivot. The eyes are, as a rule, enormous, often contiguous, and occupying nearly the whole of the upper surface of the head, bit sometimes (Agrionidoe and Gomphina) widely distant; occupied by innumerable facets, which are often larger on the upper portion In front of them is a portion termed the vextex, sometimes (Libellulidoe) forms a swollen vesicle, before which are placed the three very small ocelli, and on either side of which are inserted the antennae, which are smaller in proportion than in the almost any other insects, consisting only of two short swollen basal joints and a 5 or 6-jointed bristle-like thread. The front of the head is vertical, and consists of a large, often dilated upper portion, which is commonly termed the nasus, followed by a transverse portion termed the rhinarium, and this again by the large labrum, which conceals the jaws and inner mouth parts. The lower lip, or labium, is attached to every small chin piece (or mentum), and is generally very large, often (Agrionidoe) divided almost to its base into two portions, or more frequently entire or nearly so; on each side of its are two usually enormous hypertrophied pieces, which form the "palpi" and which are often furnished at the tips with an articulated spine (or terminal joint), the whole structure serving to retain the prey. Considerable diversity of opinion exists with respect to the composition of the mouth parts, and by some authors the "palpi" have been termed the side pieces of the lower lip. In a dead dragon-fly the parts are closed on each other, and for a just appreciation of their structure and power, it is necessary to take a living example in the fingers by the thorax, slightly lateral pressure on which caused the insect of display the formidable arrangement. The prothorax is extremely small, consisting of only of narrow ring, the upper portion of which is often elevated into lobes. The rest o the thorax is very large, and consolidated into a single piece, with oblique sutures on the side beneath the wings; the portion in from of the wing extremely robust, and offers a median carina or suture above, and broad transverse sinus posteriorly. The interalar portion if somewhat excavated, and on each side of its above are nodosities forming the attachments of the powerful muscles that work the wings; on each side is a large and distinct spiracle. The abdomen varies excessively in form, the two extremes being the filiform structure observable in most agrionidoe, and the very broad and depressed formation seen in our familiar Libellubla depressa. It consists of ten district segments, whereof the basal two and those at the apex are short, the other elongate, the first being excessively shorts. In a slit on the under side of the second, in the male, accompanied by external, protuberances, are concealed the genital organs: on the under side of the eight in the female is scale0like formation, indicating the entrances to the oviduct. The tenth is always provided in both, and often furnishing the best specific (and even generic) characters; by some authors these appendages are considered as representing a modified eleventh segment. The basal segments often have additional transverse sutures, and in the common triquetrous abdomen there is a fine longitudinal dorsal carina, and prominent lateral angels; invariably the central surface has a longitudinal membranous space connecting the here divided chitinous portion o the eternal skeleton. The legs vary in length and stoutness, but may, as a rule, be termed long and slender, and in a measure that appear disproportionate to the necessities of the insect; for a dragon-fly can scarcely be said to walk after the short promenade it takes on emerging from its puparium. The anterior pair probably assists in capturing and holding ants insect prey, but the greatest service all the legs render is possibly in enabling the creature to rest lightly, so that it can quit a position of repose in chases of passing prey in the quickest possible manner, in which the majority of the species are aided also by the horizontally extended wings. The coax is shot and stout, followed by a still shorter trochanter; the femora and tibiae long and slender, almost invariably furnished on their under surface with two series of strong spines, as also are the tarsi, which consists of three slender joints, he last having tow long and slender claws, usually (but not invariably) with a small tooth internally below the tops; the palm are absent or nearly so, and naturally are not necessary in a non-ambulatory insect. The wings are always elongate, and furnished with strong longitudinal neuration and dense transverse nervules strengthening the already strong (although typically transparent) membrane. In the Agrionidoe both pairs are nearly equal, and are carried vertically and longitudinally in repose, and the neuration and membrane are less strong; hence the species of this family are not so powerful on the wings as are those of the other groups in which he wings are horizontally extended in a position ready for instant service. The neuration is peculiar, and in many respects without precise analogy in other groups of insects, but it is not necessary here to enter into more than some special points. On the costal margin (excepting in some Calopterygina) there is a small dark space limited by nervules, termed the pterostigma, and between this and base o the wing is point termed the "nodus," at which the sub-costal nervure is suddenly arrested. The arrangement of the nervures at the base of the wing is very singular, and slight differences in it form useful aids to classification. In the Aescnidoe and Libellulidoe this pace, (know as the "triangle"), which is either open or traversed by nervules; but in many Agrionidoe this space, instead of being triangular, is on long a elongate quadrate, or with its upper edge partly staring and partly oblique. This fixitude of type in neuration is not one of the least important of the many peculiarities exhibited in these insects.
The internal structure is comparatively simple. The salivary glands appear to be absent, and the whole digestive apparatus consists of the elongate canal extending from mouth to anus, comprising the oesophagus, stomach, and intestine, with certain dilatations and constrictions; the characteristics. Malpighian vessels are stated to number about forty, placed round the posterior extremity of the stomach. Dragon-flies eat their prey completely, and do not content themselves by merely sucking its juices, the harder portions are rejected as elongate, nearly, dry, pellets of excrement.
But the most extraordinary feature in the economy, -- one which has attracted the attention of naturalists from remote times, -- is the position of the genital organs, and the corresponding anomalous manner in which the pairing of the sexes and impregnation is effected. In the male the intermittent organ is (as stated above) situated in a slit on the under surface of the second abdominal segment; it is usually very crooked or sinuous in form, and is accompanied by sheaths, and by external hooks or secondary appendages, and also by seminal vessels. But the ducts of the vessels connected with the testes unite and open on the under surface of the ninth segment; hence, before copulation can take place, it is necessary that the vessels in the second segment be charged from this opening, and in the majority of cases this is done by the male previously to seeking the female. In the latter sex the entrance to the oviduct and genital organs is on the under surface if the eight abdominal segment. The act of pairing may be briefly stated as follows. He male, when flying, seizes he prothorax of the female with the strong appendages at the extremity o the abdomen, and the abdomen of this later sex is then curved upward so as to bring the under side of the eight segment into contact with the organs of the second segment of the male. This act must have been observed by all, though but few non-entomologists are acquainted with the reason for this most extraordinary position. In the more powerful Libellulidoe, &c., the act is of short duration, and it is probable that polygamy and polyandry exits, for it possibly requires more than one almost momentary act to the fertilize all the eggs in the ovaries of a female. But in many Agrionidoe, and in some others, the male keeps his hold of the prothorax of the female for a lengthened period, retaining himself in flight in an almost perpendicular manner, and it may be that the deposition of eggs and pairing goes on alternately. There is, however, much yet to be learned on these points. He gravid female usually lays her eggs in masses (but perhaps sometimes singly), and the operation may be witnessed by any one in localities frequented by these insects. She hovers for a considerable time over nearly the same spot, rapidly dipping the apex of her abdomen into the eater, or at rate touching it, and often in places where there are non water-weeds, so that in all probability the eggs, fall at once to the bottom. But in some of the Agrionidoe the female has been often notices y trustworthy observers to creep down the stems of aquatic plants several inches below the surface, emerging after the act of oviposition has been affected; and in the case of Lestes sponsa, Von Siebold saw the male descend with the female. The same exact observer noticed also in this species that the female makes slight incisions in the stems of leaves of water plants with he double serrated apparatus (vulva) forming a prolongation of the ninth segment beneath, deposition an egg in each excision. He has seen two pars thus occupied beneath the surface on one and the same stem.
Fig. 3. -- The whole body extricated.
The duration of the sub-aquatic life of a dragon-fly is not doubt variable, according to the species. In the smaller forms it is probably less than a year, but precise evidence is wanting as to the occurrence of two broods in one year. On the other hand, it is certain that often a longer period is requisite to enable the creature to attain its full growth, and three years have been stated to be necessary for this in the large and powerful Anax formosus. Like all insects with incomplete metamorphoses, there is no quiescent pupal condition, no sharp line of demarcation between the larval and so-called nymph" or penultimate stage. The creature goes on eating and increasing in size from the moment it emerges from the egg to the time when it leaves the water to be transformed into the aerial perfect insect. The number of moults is uncertain, but they are without doubt numerous. At probably about the antepenultimate of these operations, the rudimentary wings begin to appear as thoracic buddings, and in the full-grown nymph these wings overlap about on-half of the dorsal surface of the abdomen. In structure there is a certain amount of resemblance to the perfect insect, but the body is always much shorter and shorter, in some cases most disproportionately so, and the eyes are always separated; even in those genera (e.g. Aeschna) in which the eyes of the imago are absolutely contagious, the most that can be seen in the larva is a prolongation towards and more fitted for crawling about water plants and on the bottom. In the mouth parts the mandibles and maxillae are similar in form to those of the adult, but there is an extraordinary and unique modification of the lower lip. This is attached to an elongate and slender mentum articulate to the posterior portion of the lower of the surface of the head, slightly widened at its extremity, to which is again articulate the labium proper, which is very large, flattened, and gradually dilated to its extremity; but its form differs according to group as in the perfect insect. Thus in the Agrionidae it is deeply cleft, and with comparatively slender sidepieces (or palpi), and strongly developed articulated spines; in the Aeschnidoe it is at the most notched, with narrow side-pieces and very strong spines; in the Libellulidoe it is entire, often triangular at its apex, and with enormously developed palpi without spines, but having the opposing inner edges furnished with interlocking serrations. The whole of this apparatus is commonly termed the mask. In a state of repose it is applied closely gains the face, the elongated mentum directed backward and lying between he anterior pair of legs; but when an approaching victim is seen the whole apparatus is suddenly projected, and the prey caught by the raptorial palpi; in some large species it is capable of being projected fully half an inch in front of the head. The prey, once caught and held by this apparatus, is devoured in the usual manner. There are two pairs of thoracic spiracles, but respiration is mostly affected by a peculiar apparatus at the tail end, and there are two different methods. In the Agrionidoe there are three elongate flattened plates, or false gills, full of tracheal ramifications, which extract the air from the water, and convey it to the internal tracheae (in Calopteryx these plates are excessively long, nearly equaling the abdomen), the plates also serving as means of locomotion. But in the other groups these external false gills are absent, and in their place are five valves, which by their sudden opening and closing force in the water to the rectum, the wall s of which are finished with bronchial lamellae. The alternate opening and closing of these valves enables the creature to make quick jerks or rushes (incorrectly termed "leaps") through the water, (Footnote 1) and, in conjunction with its mouth parts, to make sudden attacks upon prey from a considerable distance. The lateral angles of the terminal abdominal segments are sometimes produced into long curved spines. In colour these larvae are generally muddy, and they frequently have a coating of muddy particles, and hence are less likely to be observed by their victims. It among insects the perfect to be observed by their victims. If among insects the perfect dragon-fly may be termed the tyrant of the aim so many its larva be styled that the water. Aquatic insects the perfect dragon-fly may be termed the tyrant of the air, so may its larva be styled that of the water. Aquatic insects and larvae form the principal food, bit there can be no doubt that worms, the fry of fish, and even younger larvae of their own species, form part of the bill of fare. The "nymph" when arrived at its full growth sallies forth from the water, and often crawls a considerable distance frequently many feet up the trunks of trees) before it fixes itself for the longitudinally down the back, through which fissure the perfect insect gradually drags itself. The figures ion last page indicated this process as observer in Aeschna cyanea.
Fig. 4. -- The perfect insect (the wings having acquired their full dimensions) resting to dry itself, preparatory to the wings being horizonatally extended.
For a considerable time after its emergence a dragon-fly is without any of its characteristic colours, and is flaccid and weak, the wings (even in those groups in which they are afterwards horizontally extended) being held vertically in a line with the abdomen. By degrees, the parts harden, and the insect essays its first flight, but even then the wings have little power and are semi-opaque in appearance, as if dipped in mucilage. In most species of Calopterygina, and in some others, the prevailing colour of the body is a brilliant bronzy green, blue, or black, but the colours in the other groups vary much, and often differ in the sexes. Thus in Libellula depressa the abdomen of the fully adult male is covered with a bluish bloom, whereas that of the female is yellow; but several days elapse before this pulverulent appearance is attained, and a comparatively young male is yellow like the female. The wings are typically hyaline and colourless, but in many species (especially Calopterygina and Libellulina) they may be wholly or in part opaque and often black, due apparently to gradual oxydization of a pigment between the two membranes of which the wings are composed; the brilliant iridescence, or metallic lustre, so frequently found is no doubt due to interference-the effect of minute irregularities of the surface-and not produced by a pigment.
A beautiful little genus (Chalcopteryx) of Calopterygina from the Amazon is a gem in the world of insects, the posterior wings being of the most brilliant fiery metallic colour, whereas the anterior remain hyaline.
These insects are pre-eminently lovers of the hottest sunshine (a rew are somewhat crepuscular), and the most powerful and daring on the wing in fine weather become inert and comparatively lifeless when at root in dull weather, allowing themselves to be captured by the fingers without making any effort to escape. Many of the larger species (Aeschna, &c.) have a habit of affecting a particular twig or other resting place like a fly-catcher among birds darting off after prey and making long excursions, but returning to the chosen spot. Mr Wallace, in his Malay Archipelago, states that the inhabitants of Lombock use the large species for food, and catch them by means of limed twigs.
They are distributed over the whole world excepting the polar regions, but are especially insects of the tropics. At the present day about 1700 species are known, dispersed unequally among the several sub-families as follows; Agrionina, 490 species; Calopterygina, 170; Gomphina, 210; Aechnina 150 ; Corduliina, 100; Libellulina, 580. In Europe proper only 100 species have been observed, and about 46 of these occur in the British islands. New Zealand is excessively poor, and can only number 8. Species, whereas they are very numerous in Australia. Some species are often seen at sea, far from land, in calm weather, in troops which are no doubt migratory; our common Libellula quadrimaculata, which inhabits the cold and temperate regions of the northern hemisphere, has beer, frequently seen in immense migratory swarms. One species (Pantala flavescens) has about the widest range of any insect, occurring in the Old World from Kamtchatka to Australia, and in the New from the Southern States to, Chili, also all over Africa and the Pacific islands, but is not found in Europe. The largest species occur in the Aeschnina and Agrionina; a member of the former sub family from Borneo expands to nearly 6_ inches, and with a moderately strong body and powerful form; in the latter the Central American and Brazilian Megaloprepus caerulatus and species of Mecistogaster are very large, the former expanding to nearly 7 inches, and the latter to nearly as much, but the abdomen is not thicker than an ordinary grass-stem and of extreme length (fully 5 inches in. Mecistogaster). Among living entomologists the dragon flies have received, and are receiving, great attention, especially from the Baron de Selys-Longchamps of Li4ge and from Dr H. A. Hagen, formerly of Königsberg, now of Cambridge, Massachusetts.
It is impossible to prepare dragon-flies for the cabinet so as to retain all the brilliant colours the bodies have in life. They are excessively brittle when dry, and in the smaller species it is advisable to run a bristle into the under side of the thorax, pushing it down till it reach the extremity of the abdomen, when the other end can be cut off close to the thorax. But the larger species should be disembowelled through a slit along the under surface of the abdomen, and then filled (but not too tightly) with clean white cotton wool. The colour stand a much better chance of not greatly altering if the insects be not killed until some hours after they are captured, so as to allow the contents of the, intestinal canal to be naturally passed away, for it is the decomposition of the food that assists materially to alter or obliterate the colour and markings.
Among fossil insects dragon-flies hold a conspicuous position. Not only do they belong to what appears to have been a very ancient type, but in addition, the large wings and strong dense reticulation are extremely favourable for preservation in a fossil condition, and in many cases all the intricate details can be as readily followed as in a recent example. In this country they have been found more especially in the Purbeck beds of Swanage, and the vales of Wardour and Aylesbury, in the Stonesfield Slate series, and in the Lias and Rhaetic series of the west of England. But the richest strata appear to be those of the Upper Miocene at CEningen, in the Rhine valley; the Middle Miocene at Radaboj, in Croatia; the Eocene of Aix, in Provence; and more especially the celebrated Secondary rocks furnishing the lithographic stone of Solenhofen, in Bavaria. This latter deposit would appear to have been of marine origin, and it is significant that, although the remain of gigantic dragon flies discovered in it are very numerous and perfect, no traces of their sub-aquatic conditions have been found, although these as a rule are numerous in most of the other strata, hence the insects may be regarded as having been droned in the sea and washed on shore. Many of these Solehhofen species differ considerably in form those now existing. So that Dr Hagen, who has especially studies them, says that for nearly all if necessary to make new genera. A notice of fossil forms should not be concluded without the remark that indications of at least two species have been found in amber. A number disproportionately small if compared with other insects entombed therein; but it must be remembered that a dragon-fly is. As a rule, an insect of great power, and in all probability those then fisting were able to extricate themselves if accidentally entangled in the resin.
See De Selys-Longchamps, Monographie des Libellulidées d Europe, Brussels, 1840; Synopses des Agrionines, Caloptérygines, Gomphines, et Cordulines, with Supplements, Brussels, from 1853 to 1877; De Selys-Longchamps and Hagen, Revue des Odonates dEurope, Brussels, 1850; Monographie de Caloptérygines et des Gomphines, Brussels, 1854 and 1858; Charpentier, Libellulinae europeae, Leipsic, 1840. (R. McL.)
(1) A similar contrivance was suggested and (if the winter mistakes not) actually tried as a means of propelling steam-ships.
The above article was written by Robert McLachlan, F.R.S., editor of the Entomologist's Monthly Magazine; author of A Monographic Revision and Synopsis of the Trichoptera of the European Fauna.