6 / 
The May-flies and Stone-flies 
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¬ 
fly 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. 102. —Young (nymph) of 
May-fly, showing (g) tracheal 
gills. (After Jenkins and 
Kellogg; three times nat¬ 
ural size.) 
