1902 Encyclopedia > Ephemeridae

Ephemeridae




EPHEMERIDAE, a remarkable family of Pseudo-Neuropterous Insects, deriving the name from e^/xepos, in allusion to the very short lives of the winged insects. In some species it is possible that they have scarcely more than one day's existence, but others are far longer lived, though the extreme limit is probably rarely more than a week. The family has very sharply defined characters, which separate its members at once from all other neurop-terous (or pseudo-neuropterous) groups.

These insects are universally aquatic in their preparatory states. The eggs are dropped into the water by the female in large masses, resembling, in some species, bunches of grapes in miniature Probably several months elapse before the young larvae are excluded. The sub-aquatic condition lasts a considerable time : in Cloeon, a genus of small and delicate species, Lubbock proved it to extend over more than six months; but in larger and more robust genera (e.g., Palingenia) there appears reason to believe that the greater part of three years is occupied in prepara-tory conditions. The larva is elongate. The head is rather large, and is furnished at first with five simple eyes of nearly equal size; but as it increases in size the homologues of the facetted eyes of the imago become larger, whereas those equivalent to the ocelli remain small. The antennae are long and thread-like, composed at first of few joints, but the number of these latter apparently increase at each moult. The mouth parts are well-developed, consisting of an upper lip, powerful mandibles, ordinarily three-jointed maxillary palpi, a deeply quadrifid labium or lower lip, and three-jointed labial palpi. There are three distinct and large thoracic segments, whereof the prothorax is narrower than the others; the legs are much shorter and stouter than in the winged insect, with monomerous tarsi terminated by a single claw. The abdomen consists of ten segments, the tenth furnished with long and slender multi-articulate tails, which appear to be only two in number at first, but an inter-mediate one gradually develops itself (though this latter is often lost in the winged insect). Respiration is effected by means of external gills placed along both sides of the dorsum of the abdomen and hinder segments of the thorax. These vary in form: in some species they are entire plates, in others they are cut up into numerous divisions, in all cases traversed by numerous tracheal ramifications. According to the researches of Lubbock and of the Messrs Joly, the very young larvae have no breathing organs, and respiration is effected through the skin. Lubbock traced at least twenty moults in Cloeon; at about the tenth rudi-ments of the wing-cases began to appear. These gradually become larger, and when so the creature may be said to have entered its " nymph " stage; but there is no condition analogous to the pupa-stage of insects with complete meta-morphoses. There may be said to be three or four different modes of life in these larvaj; some are fossorial, and form tubes in the mud or clay in which they live; others are found on or beneath stones ; while others again swim and crawl freely among water plants. It is probable that some are carnivorous, either attacking other larvae or subsisting on more minute forms of animal life; but others perhaps feed more exclusively on vegetable matters of a low type, such as diatoms.





When the acpuatie insect has reached its full growth, it emerges from the water or seeks its surface ; the thorax splits down the back, and the winged form appears. But this is not yet perfect, although it has all the form of a perfect insect and is capable of flight; it is what is variously termed a "pseud-imago," "sub-imago," or "pro-imago." Contrary to the habits of all other insects, there yet remains a pellicle that has to be shed, covering every part of the body. This final moult is effected soon after the insect's appearance in the winged form; the creature seeks a temporary resting-place, the pellicle splits down the back, and the now perfect insect comes forth, often differing very greatly in colours and markings from the condition in which it was only a few moments before. If the observer take up a suitable position near water, his coat is often seen to be covered with the cast sub-imaginal skins of these insects, which had chosen him as a convenient object upon which to undergo their final change. In some few genera of very low type it appears probable that, at any rate in the female, this final change is never effected, and that the creature dies a sub-imago.
The winged insect differs considerably in form from its sub-aquatic condition. The head is smaller, often occupied almost entirely above in the male by the very large eyes, which in some species are curiously double in that sex, one portion being pillared, and forming what is termed a " turban ;" the mouth parts are aborted, for the creature is now incapable of taking nutriment either solid or fluid ; the antennae are mere short bristles, consisting of two rather large basal joints and a multi-articulate thread. The pro-thorax is much narrowed, whereas the other segments (especially the mesothorax) are greatly enlarged; the legs long and slender, the anterior pair often very much longer in the male than in the female ; the tarsi four- or five-jointed ; but in some genera (e.g., Oligoneuria and allies) the legs are aborted, and the creatures are driven help-lessly about by the wind. The wings are carried erect: the anterior pair large, with numerous longitudinal ner-vures, and usually abundant transverse reticulation; the posterior pair very much smaller, often lanceolate, and frequently wanting absolutely. The abdomen consists of ten segments ; at the end are either two or three long multi articulate tails ; in the male the ninth joint bears forcipated appendages ; in the female the oviducts terminate at the junction of the seventh and eighth ventral segments. The sexual act takes place in the air, and is of very short duration, but is apparently repeated several times, at any rate in some cases.

Ephemeridae are found all over the world, even up to high northern latitudes. Pictet, Eaton, and others have given us valuable works or monographs on the family, but the subject still remains little understood, partly owing to the great difficulty of preserving such delicate insects; and it appears probable they can only be satisfactorily investigated as moist preparations. The number of described species is less than 200, spread over many genera.

From the earliest times attention has been drawn to the enormous abundance of species of the family in certain localities. Scopoli, writing more than a century ago, speaks of them as so abundant in one place in Carniola that in June twenty cart-loads were carried away for manure ! Polymitarcys virgo, which, though not found in England, occurs in many parts of Europe (and is common at Paris), emerges from the water soon after sunset, and continues for several hours in such myriads as to resemble snow showers, putting out lights, and causing incon-venience to man, and annoyance to horses by entering their nostrils. In other parts of the world they have been recorded in multitudes that obscured passers-by on the other side of the street. And similar records might be multiplied almost to any exent. In Britain, although they are often very abundant, we have scarcely anything analogous.

Fish, as is well known, devour them greedily, and enjoy a veritable feast during the short period in which any particular species appears. By anglers our common species of Epliemera (vulgata and danict, but more especially the latter, which is more abundant) are known as the "May-fly," but the terms "Green Drake" and "Bastard Drake " are applied to conditions of the same species. Useful information on this point will be found in Ronalds's Fly-Fisher s Entomology, edited by Westwood.

A singular creature, with a carapace almost like that of a miniature tortoise, originally described by Latreille as a doubtful genus of Branchiopod Crustacea under the name of Prosopistoma, of which two species are known (one occur-ring in France, the other in Madagascar), is now almost proved by Messrs E. & N. Joly to be the aquatic condition of some insect of this family

Ephemeridae belong to a very ancient type of insects, and their fossil imprints are common, occurring even in the Carboniferous formation. (R. M'L.)






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