May, 1953 
Stenonema possibly has 30 nymphal instars. 
Hexagenia, with its 2-year life cycle, has 
an unknown but quite large number of 
nymphal instars. Mayfly nymphs have been 
observed to grow relatively little during each 
stadium. 
The developing adult wings, eggs, and 
genitalia can clearly be seen inside the later 
nymphal instars. Feeding terminates with 
the next to the last nymphal instar. When 
the last nymphal instar is reached, develop- 
ment of the adult structures is almost com- 
plete. During the last instar, the nymph is 
quiescent, and the alimentary canal degen- 
erates rapidly. “The vestigial mouthparts 
and short antennae of the adult can clearly 
be seen developed beneath the nymphal 
cuticle. 
In such forms as Hexagenia, the mature 
nymph when ready to molt comes to the 
surface of the water, the nymphal skin splits 
rapidly, and the subimago emerges quickly. 
The subimago rests for a short time on the 
shed nymphal skin, which floats like a raft. 
Then it is ready to take flight for a place 
of safety. The whole process requires only 
about 2 minutes. This molt from the nymph 
to the subimago represents a very dangerous 
time in the life of the mayfly. Many mature 
nymphs, as they are swimming to the surface 
of the water to molt, and many more subi- 
magoes while resting on their nymphal skins, 
are devoured by fish. Some, even as they 
take flight, fall prey to fish that jump out of 
the water to catch them. Some of the may- 
flies that elude their fish enemies at this 
time are likely, as they flutter up from the 
surface of the water, to be eaten by birds. 
In other mayflies, such as [sonychia, Am- 
eletus, and Siphlonurus, the nymph crawls 
out of the water onto stones, sticks, or other 
convenient objects, the nymphal skin splits, 
and the subimago emerges fairly slowly, the 
process requiring 3 to 5 minutes. ‘The 
empty nymphal exoskeleton is left clinging 
to the support where the subimago emerged. 
If the shed last nymphal skin is examined, 
it will be found to contain, almost intact, 
the nymphal structures for which the adult 
has no use. The nymphal mouthparts are 
complete and still contain some of the mus- 
culature in only a partly disintegrated state. 
The nymphal gills also are intact, even in 
such forms as Isonychia, which retain gill 
rudiments in the adult. 
The subimaginal stage in most mayflies 
Burks: THE MAyr igs or ILLINoIs 7 
normally lasts 1 or 114 days. This is subject 
to some prolongation at low temperatures. 
Siphlonurus marshalli requires 214 days 
when the daytime temperature is from 45 
to 50 degrees F. When daytime tempera- 
tures rise to 70 degrees F., the imago appears 
in 114 days. During the subimaginal stage, 
almost 25 per cent of the body weight is 
lost, probably due principally to losses of 
water through evaporation and respiration. 
It can be shown that subimagoes must lose 
water before the adults can emerge, as subi- 
magoes kept in a too-moist atmosphere are 
never able to emerge as adults. On the 
other hand, water loss must not be too 
rapid, or the subimagoes will die without 
producing the adults. My experience in 
rearing mayflies has been that relative 
humidity is the most critical single factor 
in the maturing of subimagoes to adults. 
In some mayflies, such as Ephoron, Tor- 
topus, and Caenis, the subimago stage is 
greatly abbreviated. In them, the sub- 
imaginal skin or pellicle is shed almost im- 
mediately after the emergence from the 
nymph. In Ephoron and Tortopus, which 
do not have functional legs in the adults, 
the males shed the subimaginal skins in 
flight, but the females remain as subimagoes. 
In Caenis simulans, | have observed the 
subimaginal pellicle to be shed in flight, but 
Needham (Needham et al. 1935:99) states 
that in Caenis sp. the subimagoes alight to 
shed the subimaginal skin. The observations 
probably were made on different species of 
the genus. At any rate, in the females of 
all species of Caenis which have been ob- 
served, the subimaginal pellicle is only 
partly shed, but in the males it is shed com- 
pletely. 
In the great Papuan mayfly of the East 
Indies, Plethogenesia papuana (Eaton), 
both males and females remain subimagoes, 
never attaining the adult stage. However, 
of the many preserved specimens of this 
species I have seen, the subimaginal pellicle 
is differentiated from the enclosed adult 
structures. “The same is true of the sub- 
imago females of our American species of 
Ephoron and Tortopus which, as has already 
been mentioned, do not attain the ultimate 
adult stage. 
By the time the subimago stage is reached, 
the eggs and sperm are already mature and 
can be stripped from subimagoes and mixed 
in normal saline solution, by which fertili- 
