The May-flies and Stone-flies 



67 



eggs reach the bottom separately. From each egg hatches soon a tiny 

 flattened, soft-bodied, six-legged creature called a nymph, without wings 

 or wing-pads, and looking very much like a Campodea (the simplest 

 living insect, see p. 61). This nymph crawls about, feeds, grows, moults, 

 grows, moults again and again (in a species observed by Lubbock there 

 were twenty-one moultings), and finally at the end of a year, or of two or 

 three years, depending on the species, is ready to issue as a winged adult. 

 During the nymphal life wings have been slowly developing, visible as 

 short pads projecting from the dorsal margins of the meso- and meta-thorax, 

 and appearing visibly larger after each moulting (Fig. 102). Respiration is 

 accomplished by flat, leaf-like gills (Fig. 102) (these do not appear in some 

 species until after one or two moultings), arranged segmentally along the 

 sides of the abdomen. The mouth-parts are well developed for biting 

 and chewing, with sharp-pointed jaws (mandibles). During its aquatic 

 life at the bottom of stream or pond the May- 

 flv has to undergo all the vicissitudes of an 

 exposed and protracted life; it is eagerly sought 

 after by larger, fierce, predaceous insects, 

 stronger of jaw and swifter than itself; it is 

 the prized food of many kinds of fishes, and it 

 has to struggle with its own kind for food and 

 place. 



At the end of the immature life the nymphs 

 rise to the surface, and after floating there a 

 short time suddenly split open the cuticle along 

 the back and after hardly a second's pause 

 expand the delicate wings and fly away. Some 

 nymphs brought into the laboratory from a 

 watering-trough at Stanford University emerged 

 one after another from the aquarium with 

 amazing quickness. Almost all other insects 

 require some little time after the final moulting 

 for the gradual unfolding of the wings, and dry- 

 ing and strengthening of the body-wall, before 

 flight or other locomotion. Most of the May- 

 fly species go through another moulting after 

 acquiring wings, a phenomenon not known to occur in the case of any other 

 insect. The stage between the first issuance from the water with expanded 

 wings and the final moulting is called the subimago stage, and may last, 

 in various species, from but a few minutes to twenty-four hours. Such 

 is, in general, the life-history of the May-flies. As a matter of fact, the 

 life-history of no single May-fly species has yet been followed completely 



Fig 



. — Young (nympW of 

 May-fly, showing (g) tracheal 

 gills, {.^fter Jenkins and 

 Kellogg; three times nat- 

 ural size.) 



