The nymphs of the Ephemeridae being creatures adapted for existence in water, the details of their transformation into creatures having an entirely aerial existence cannot but be of much interest. In the nymphs the tracheal system is well developed, but differs from that of air-breathing Insects in the total absence of any spiracles. Palmén has investigated this subject,[[360]] and finds that the main longitudinal tracheal trunks of the body of the nymph are not connected with the skin of the body by tracheae, but are attached thereto by ten pairs of slender strings extending between the chitinous integument and the tracheal trunks. When the skin is shed these strings—or rather a chitinous axis in each one—are drawn out of the body, and bring with them the chitinous linings of the tracheae. Thus notwithstanding the absence of spiracles, the body wall is at each moult pierced by openings that extend to the tracheae. After the ordinary moults these orifices close immediately, but at the change to the winged state they remain open and form the spiracles. At the same time the tracheal gills are completely shed, and the creature is thus transformed from a water-breather to an Insect breathing air as usual. In addition to this change there are others of great importance, such as the development of the great eyes and the complete atrophy of the mouth-parts. The precise manner of these changes is not known; they occur, however, within the nymph skin. The sudden emergence of the winged Insect from the nymph is one of the most remarkable facts in the life-history of the may-fly; it has been observed by Sir John Lubbock,[[361]] who describes it as almost instantaneous. The nymph floats on the water, the skin of the back opens, and the winged Insect flies out, upwards and away; "from the moment when the skin first cracks not ten seconds are over before the Insect has flown away." The creature that thus escapes has not, however, quite completed its transformation. It is still enveloped in a skin that compresses and embarrasses it; this it therefore rapidly gets rid of, and thus becomes the imago, or final instar of the life-cycle. The instar in which the creature exists winged and active, though covered with a skin, is called the sub-imago. The parts of the body in the sub-imago are as a whole smaller than they are in the imago, and the colour is more dingy; the appendages—wings, legs, and caudal setae—are generally considerably shorter than they are in the imago, but attain their full length during the process of extraction. The creatures being, according to Riley, very impatient and eager to take to the wing, the completion of the shedding of the skin of the sub-imago is sometimes performed while the Insect is flying in the air.
Fig. 283.—Lingua of Heptagenia longicauda, × 16. m, Central; l, lateral pieces. (After Vayssière.)
The food of young Ephemeridae is apparently of a varied and mixed nature. Eaton says[[362]] that though sometimes the stronger larvae devour the weaker, yet the diet is even in these cases partly vegetable. The alimentary canal frequently contains much mud; very small organisms, such as diatoms and confervae, are thought to form a large part of the bill of fare of Ephemerid nymphs. Although the mouth is atrophied in the imago, yet it is highly developed in the nymphs. This is especially notable in the case of the lingua or hypopharynx (Fig. 283); indeed Vayssière[[363]] seems to incline to the opinion that this part of the mouth may be looked on in these Insects as a pair of appendages of a head-segment (see p. [96] ante), like the labium or maxillae.
The life-history has not been fully ascertained in the case of any species of may-fly; it is known, however, that the development of the nymph sometimes occupies a considerable period, and it is thought that in the case of some species this extends to as much as three years. It is rare to find the post-embryonic development of an Insect occupying so long a period, so that we are justified in saying that brief as may be the life of the may-fly itself, the period of preparation for it is longer than usual. Réaumur says, speaking of the winged fly, that its life is so short that some species never see the sun. Their emergence from the nymph-skin taking place at sunset, the duties of the generation have been, so far as these individuals are concerned, completed before the morning, and they die before sunrise. He thinks, indeed, that individuals living thus long are to be looked on as Methuselahs among their fellows, most of whom, he says, live only an hour or half an hour.[[364]] It is by no means clear to which species these remarks of Réaumur refer; they are doubtless correct in certain cases, but in others the life of the adult is not so very short, and in some species may, in all probability, extend over three or four days; indeed, if the weather undergo an unfavourable change so as to keep them motionless, the life of the flies may be prolonged for a fortnight.
The life of the imago of the may-fly is as remarkable as it is brief; in order to comprehend it we must refer to certain peculiarities of the anatomy with which the vital phenomena are connected. The more important of these are the large eyes of the males, the structure of the alimentary canal, and that of the reproductive organs. We have already remarked that the parts of the mouth in the imago are atrophied, yet the canal itself not only exists but is even of greater capacity than usual; it appears to have much the same general arrangement of parts as it had in the nymph. Its coats are, however, of great tenuity, and according to Palmén[[365]] the divisions of the canal are separated by changes in the direction of certain portions anterior to, and of others posterior to, its central and greater part—the stomach—in such a manner that the portions with diverted positions act as valves. The stomach, in fact, forms in the interior of the body a delicate capacious sac; when movement tends to increase the capacity of the body cavity then air enters into the stomachic sac by the mouth orifice, but when muscular contractions result in pressure on the sac they close the orifices of its extremities by the valve-like structures we have mentioned above; the result is, that as complex movements of the body are made the stomach becomes more and more distended by air. It was known even to the old naturalists that the dancing may-fly is a sort of balloon, but they were not acquainted with the exact mode of inflation. Palmén says that in addition to the valve-like arrangements we have described, the entry to the canal is controlled by a circular muscle, with which are connected radiating muscles attached to the walls of the head. Palmén's views are adopted, and to a certain extent confirmed, by Fritze,[[366]] who has examined the alimentary canal of the may-fly, and considers that though the normal parts of the canal exist, the function is changed in the imago, in which the canal serves as a sort of balloon, and aids the function of the reproductive organs. The change in the canal takes place in an anticipatory manner during the nymph and sub-imago stages.
The sexual organs of Ephemeridae are remarkable for their simplicity; they are destitute of the accessory glands and diverticula that, in some form or other, are present in most other Insects. Still more remarkable is the fact that the ducts by which they communicate with the exterior continue as a pair to the extremity of the body, and do not, as in other Insects, unite into a common duct. Thus in the female there is neither bursa copulatrix, receptaculum seminis, nor uterine portion of oviduct, and there is no trace of an ovipositor; the terminations of the ducts are placed at the hind margin of the seventh ventral plate, just in front of which they are connected by a fold of the integument. The ovary consists of a very large number of small egg-tubes seated on one side of a sac, which forms their calyx, and one of whose extremities is continued backwards as one of the pair of oviducts. The male has neither vesiculae seminales, accessory glands, nor ductus ejaculatorius. The testes are elongate sacs, whose extremities are prolonged backwards forming the vasa deferentia; these open separately at the extremity of the body, each on a separate intromittent projection of more or less complex character, the two organs being, however, connected by means of the ninth ventral plate, of which they are, according to Palmén, appendages. We should remark that this authority considers Heptagenia to form, to some extent, an exception as regards the structures of the female; while Polymitarcys is in the male sex strongly aberrant, as the two vasa deferentia, instead of being approximately straight, are bent inwards at right angles near their extremities so as to meet, and form in the middle a common cavity, which then again becomes double to pass into the pair of intromittent organs.
According to the views of Exner and others, the compound eyes of Insects are chiefly organs for the perception of movement; if this view be correct, movements such as those made during the dances of may-flies may, by the number of the separate eyes, by their curved surfaces and innumerable facets, be multiplied and correlated in a manner of which our own sense of sight allows us to form no conception. We can see on a summer's evening how beautifully and gracefully a crowd of may-flies dance, and we may well believe that to the marvellous ocular organs of the flies themselves (Fig. 274) these movements form a veritable ballet. We have pointed out that by this dancing the peculiarly formed alimentary canal becomes distended, and may now add that Palmén and Fritze believe that the unique structure of the reproductive organs is also correlated with the other anatomical peculiarities, the contents of the sexual glands being driven along the simple and direct ducts by the expansion of the balloon-like stomach. During these dances the momentary conjugation of the sexes occurs, and immediately thereafter the female, according to Eaton, resorts to the waters appropriate for the deposition of her eggs. As regards this, Eaton says:[[367]] "Some short-lived species discharge the contents of their ovaries completely en masse, and the pair of fusiform or subcylindrical egg-clusters laid upon the water rapidly disintegrate, so as to let the eggs sink broadcast upon the river-bed. The less perishable species extrude their eggs gradually, part at a time, and deposit them in one or other of the following manners: either the mother alights upon the water at intervals to wash off the eggs that have issued from the mouths of the oviducts during her flight, or else she creeps down into the water to lay her eggs upon the under-side of stones, disposing them in rounded patches, in a single layer evenly spread, and in mutual contiguity." The eggs are very numerous, and it is thought may sometimes remain in the water as much as six or seven months before they hatch.
The number of individuals produced by some kinds of may-flies is remarkable. Swarms consisting of millions of individuals are occasionally witnessed. D'Albertis observed Palingenia papuana in countless myriads on the Fly River in New Guinea: "For miles the surface of the river, from side to side, was white with them as they hung over it on gauzy wings; at certain moments, obeying some mysterious signal, they would rise in the air, and then sink down anew like a fall of snow." He further states that the two sexes were in very disproportionate numbers, and estimates that there was but a single female to every five or six thousand males.
Ephemeridae in the perfect state are a favourite food of fishes, and it is said that on some waters it is useless for the fly-fisher to try any other lure when these flies are swarming. Most of the "duns" and "spinners" of the angler are Ephemeridae; so are several of the "drakes," our large E. danica and E. vulgata being known as the green drake and the gray drake. Ronalds says[[368]] that the term "dun" refers to the pseud-imago condition, "spinner" to the perfect Insect. E. danica and E. vulgata are perhaps not distinguished by fishers; Eaton says that the former is abundant in rapid, cool streams, while E. vulgata prefers warmer and more tranquil rivers.