When the time for the appearance of the perfect insect arrives the nymph crawls or swims to the surface : a fissure then appears in the dorsal cuticle and the winged insect issues and flies away in a few seconds. At this stage it is known as the sub-imago and it differs from the true imago by its duller appearance, its body and wings being covered by a very delicate pellicle. In this con dition the insect soon comes to rest, and after a period varying from a few minutes up to 24 hours or more, the pellicle is finally cast off and the imago or true may-fly emerges with transparent wings and mature reproductive organs. May-flies are always very short-lived as adults : some species emerge towards evening, pair and lay their eggs, and die before morning : others may live for a few days but they partake of no food during their fugitive life. These insects often issue from the water in great numbers about the same time: some species execute aerial dances con sisting of a fluttering swift ascent followed by a more passive and leisurely descent, the two processes being many times repeated. At times their numbers are so great as to darken the air and inconvenience passers-by, especially in Switzerland and Italy.
About 470 species of may-flies are known : they are very widely distributed and about 4o species inhabit the British Isles, but these insects have been very little studied in the tropics and many other parts of the world. In the fossil state true may-flies
appear first in Permian rocks and their nymphs are also known from that time. Some of the fossil may-flies are interesting in that the hind wings have undergone little or no reduction in size as occurs in living forms.
The order is to be regarded as beneficial to man : may-flies are eagerly devoured by fishes, while many of the "duns," "spinners," and some of the "drakes," made up by the fly-fisher, represent species of Ephemera, Palingenia, or other forms. The nymphs serve as an important source of food for fishes, and in New Zea land the introduced trout has lessened their numbers so much that some of the native may-flies are becoming extremely scarce.
The standard work on these insects is A. E. Eaton's monograph, Trans. Linnean Soc. (2) iii. (1883-85), which figures many species and their nymphs. See also L. C. Miall, Natural History of Aquatic Insects (1913); J. G. Needham in Ward and Whipples' Freshwater Biology (1918) ; and in Bull. 86 New York State Museum (19o5) ; F. Klapalek in Brauer's Susswasserfauna Deutschlands pt. 8 (Jena, 1909), and E. Rousseau, Les Larves et Nymphes Aquatiques (Brux elles, 1921) . (A. D. I.)