8 Inuinors NaruraAt History Survey BULLETIN 
zation is accomplished. Nymphs can_ be 
hatched from these fertilized eggs. This 
has been done successfully with several 
species of mayflies, among them Hexagenia 
limbata and bilineata and Isonychia bicolor. 
During the subimago state, the degeneration 
of the alimentary tract becomes almost com- 
plete. This tract then becomes, in the adult 
male, fig. 3, only an air-filled and transparent 
sac which serves to make the body buoyant. 
In the female, the degenerated digestive tract 
is crowded and depressed by the eggs but, 
as eggs are expelled, it becomes inflated 
again, possibly aiding in the expulsion. It is 
the bursting of these inflated digestive tracts 
that produces the familiar popping noise 
living mayfly adults make when they are 
stepped on. 
Most mayflies spend the subimago state 
resting in the shade among plants near 
water. In many species, the subimaginal 
pellicle is shed in the early evening hours and 
in others during the night. It is left behind 
as an extremely delicate and fragile skin 
adhering to the support the subimago has 
occupied. After this molt, adults of most 
species continue to rest quietly among the 
concealing vegetation until the next sun- 
down; then the males swarm, mating oc- 
curs, and the females deposit all their eggs 
before midnight. In such forms as Caenis 
and Ephoron, which shed the subimaginal 
pellicle immediately after emerging from the 
nymphal stage, mating and egg laying occur 
within a very few hours of emergence, at 
most during the same night. In these forms, 
the total winged life is thus but a few 
hours. In most mayflies, however, the life 
in the winged stages endures for 2 or 3 days 
at summer temperatures. There are records 
in the literature of its being prolonged to 5 
or 6 days, but such length of life is unusual, 
except in the females of Callibaetis and 
Cloeon. In these forms, the female adults 
have been observed to live from 1 to 3 
weeks. 
Mayflies deposit their eggs in the water 
in a number of different ways. In many 
baetines, such as Baetis intercalaris, the 
female crawls beneath the surface to oviposit 
on stones or other objects on the bottom. 
This phenomenon is often referred to in the 
literature, and I have observed it several 
times in Illinois. In many genera, such as 
Ameletus, Siphlonurus, and Leptophlebia, 
and in some species of Stenonema, the female 
Vol. 26, Art. 1 
flies near the surface of the water and dips 
the end of the abdomen into it at intervals, 
by this action permitting a few eggs to be 
washed off at a time, much as in the ovi- 
position of some dragonflies. In other gen- 
era, such as Ephemerella, the female ex- 
trudes all the eggs during flight, and tempo- 
rarily holds them in a mass beneath the re- 
curved tip of the abdomen. As she flies 
along near the surface of the water, she 
darts quickly down to break the surface 
film momentarily, and drops the entire 
packet of eggs into the water. The eggs 
sink instantly and adhere to rocks or other 
objects on the bottom. 
The female of other mayflies, such as 
Heptagenia, flies a short distance above the 
surface of running water, and then alights 
on the surface for a few seconds, permitting 
the current to carry her a short distance 
downstream. During this time, she ex- 
trudes a few eggs, which are washed into 
the stream. ‘Then she flies up from the sur- 
face for a few minutes, returning again to 
it to deposit more eggs. This alternate fly- 
ing and dropping of eggs continues until the 
female is spent, or, as I have seen so often 
happen, until some bird or fish eats her. 
In most of the large Ephemeridae, such 
as Hexagenia, the female simply alights flat 
on the surface of the water, with wings out- 
spread, and extrudes all the eggs at once in 
two elongate packets. These eggs sink 
almost instantly, and the female remains 
on the surface until she drowns or, more 
likely, is eaten by a fish. 
The eggs of most mayflies hatch in from 
1 to 2 weeks, depending on temperature, 
and nymphal development begins at once. 
In a few genera, however, such as Ameletus 
and Siphlonurus, the eggs, deposited in the 
spring, do not hatch until the following 
February or March. The long period be- 
fore hatching is due to the fact that, during 
the summer and fall, the breeding sites for 
these genera become completely dry. The 
eggs of these genera evidently can tolerate 
such desiccation. Clemens (1922) pointed 
out this phenomenon for 4 meletus ludens in 
New York, and his findings agree well with 
my own field observations made in Illinois. 
Mayfly eggs are of a great variety of 
forms (Smith 1935:67), but, characteris- 
tically, most possess long, coiled, adhesive 
filaments which serve to attach them to 
stones or other objects in the water. 
