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May-flies or Shad-flies — Part 1

(Order Ephemerida.)

The so-called May-flies, or shad-flies, are the insects which constitute this order. They are known by one name or the other by almost every one who has seen them swarming about the electric lights during the summer or who lives in the vicinity of some large watercourse in which these insects breed in incalculable numbers. Structurally speaking, the adult insects are very interesting creatures. They are distinguished from other insects by their short antennae, their extremely large front wings as compared with the very small hind wings, the absolutely atrophied mouth-parts and the long, slender filaments, two or three in number, at the end of the abdomen. The transformations are complete and the early stages are always passed in the water.

The larvae are active creatures with long, strong legs, and breathe by means of tracheal gills. They both swim and crawl and feed very largely upon vegetable matter, diatoms and confervae being found in their stomachs. They may be found under stones in running streams or swimming among water plants in quiet waters, or they may live at the bottom more or less covered with slime or mud. Some forms burrow into the sand-banks of rivers. The pupa or nymph is also active and feeds. It has small wing pads, and, when ready to transform, it floats upon the water, and the skin of the back opens and the winged insect flies out. The emergence is extremely rapid and the insect flies away almost immediately after the skin cracks. A very curious phenomenon occurs with these insects which is not found with any other insect, and that is that there is a molt after the fly issues from the nymph. It flies away to the shores in the condition called the sub-imago stage and again the skin splits and the true-imago, or adult, issues. This shedding of the sub-imago skin is said sometimes to take place while the insect is flying in the air.

The life of the adult is short; in fact, these insects were named after the Ephemerides of the Greek mythology, creatures which lived but a day, and from this root comes our common word, ephemeral. But they live longer than a day. They do not eat, since, as we have stated, the mouth-parts are atrophied and the alimentary canal is not fitted for the digestion of food; but it has been shown that where the air is not too dry some of them can live for several days. It is stated that Curtis kept one alive for three weeks; but in general they die within three or four days, and frequently in a few hours, or even less time. The males are readily distinguished from the females from the fact that with the males the hind legs only are atrophied or feeble, while with most females the fore legs as well as the hind legs are too feeble to support the body. When at rest the front legs are generally extended straight in front of the head, and frequently very close together, the anal setae usually slanting upwards. As a rule they remain quiet during the day, limiting their flight to the cooler hours of sunlight or extending it later in the evening, just after sunset. Where there is a strong artificial light they will fly until late in the night. The numbers in which these creatures swarm are often extraordinary. I drove through a dense cloud of them once on the banks of the St. Lawrence River in the month of June when both the air and the ground and our clothes looked as though the heaviest kind of a snow-storm were raging. Along the shores of the Great Lakes their bodies are frequently massed in great windrows miles in length and of a very considerable thickness.

Most of them couple during flight, the male undermost. Egg-laying is usually performed in fresh water, though one exotic species lays in brackish water. The females of some species discharge the contents of their ovaries at once in the form of a pair of egg-clusters. These upon reaching the water rapidly disintegrate, and the eggs sink to the bed of the river or stream. Others drop their eggs gradually, a few at a time, the female either alighting on the surface of the water at intervals to wash off the eggs that have issued, or she creeps down into the water, enclosed within a film of air, to lay her eggs on the under side of stones, after which she floats up to the surface and either flies away or is drowned. In one case a female has been seen to deposit living larvae.

These insects can readily be bred in fresh water aquaria, but Eaton says that it is important not to grow Ranunculus in the aquaria, because the sap exuding from its broken stems appears to be poisonous to the larvae. After the eggs hatch the larvae live upon mud or the small vegetation which grows on stones and the stems of large water-plants. Some of them are apparently predaceous. They cast their skins several times during the first few days after their birth.