May, 1953 
Some mayflies, such as Ameletus ludens 
and lineatus, normally are parthenogenetic, 
males being either unknown or extremely 
rare. Some, such as species of Callibaetis 
and Cloeon dipterum, are said to be ovovivip- 
arous. Edmunds (1945:170) and Berner 
(1941: 32) have observed a process approx- 
imating ovoviviparity in species of Calli- 
baetis. ‘They found that the eggs are re- 
tained within the abdomen of the fertilized 
female for several days, during which time 
the embryos develop. Then, when the fe- 
male alights on the surface of a suitable body 
of water, she expels the eggs, and the 
nymphs hatch within a few minutes. 
Food Habits 
With very few exceptions, the nymphs 
are herbivores or scavengers, living on vege- 
table detritus and microscopic aquatic organ- 
isms, principally diatoms. A few, such as 
those of Jsonychia, are partly predaceous, 
eating apparently almost anything that 
comes within their grasp, including other 
mayflies. Others, such as the supposed 
nymph of Anepeorus, fig. 394, and the 
nymph of Metreturus pecatonica, fig. 312, 
have long, sharp mandibles which indicate 
that they are entirely predaceous in habit. 
In the subimago and adult stages, mayflies 
do not feed. 
Emergence Peaks 
The adults of many species consistently 
appear year after year in the same localities 
on about the same dates for those localities. 
The species of Callibaetis, which develop in 
ponds and woodland pools, emerge contin- 
uously throughout the open growing season 
from April to October, but they usually 
have a peak of emergence in late May or 
early June. Some species of Baetis, de- 
veloping in small, well-aerated streams and 
along the margins of small rivers, have 
two peaks of emergence in a season, in late 
April or May and in August or September. 
Other species of Baetis have three peaks 
of emergence, in May, July, and September. 
In Hexagenia bilineata, which develops 
in large, slow rivers such as the Mississippi 
and Ohio, the adults emerge in late June, 
in July, or, rarely, in August. The swarms 
which appear in mid-July are the largest. 
H. limbata, which may be found in the 
largest rivers but which prefers somewhat 
smaller ones such as the Kankakee or IIli- 
Burks: THe MaAyrties oF ILLINOIS 
nois, emerges in greatest numbers in late 
June or July. AH. rigida, restricted to 
fairly rapid, well-aerated rivers such as 
the Rock and Kankakee, appears in greatest 
numbers in June. Pentagenia vittigera, 
which seems to prefer large, slow rivers 
but is also known to develop in smaller 
numbers in a great variety of streams, 
emerges in greatest numbers in July or 
early August but never in such tremendous 
numbers as does Hexagenia. Ephemera 
simulans, which inhabits lakes with consid- 
erable wave action as well as fairly sluggish 
streams, emerges in greatest numbers in 
early June. It should be noted, however, 
that occasional specimens of all these larger 
ephemerines are to be taken from April to 
October. 
Dispersal 
Mating usually occurs between males and 
females from the same brood of a species, 
and oviposition most often is carried out in 
the same body of water in which the in- 
dividuals have developed. From this fact, 
it would seem that the chances are rather 
small that new genetic factors will be in- 
troduced into a given mayfly population by 
cross-breeding with other populations. 
However, on exceptionally warm, humid 
summer evenings, females still carrying their 
egg masses will sometimes be found late at 
night flying to lights that are located several 
miles from any body of water in which they 
could have developed as nymphs. At times, 
the flight range of fertilized females may 
thus be fairly long. Evidently by means of 
long flights of such females, mayflies are 
able to establish themselves in new breed- 
ing grounds fairly far removed from their 
places of origin. As an example, a large 
swarm of adults of Hexagenia bilineata 
was observed to emerge from Lake Glen- 
dale, an impounded body of water in Pope 
County, on June 18, 1942. HA. bilineata 
ordinarily is a large-river species, common 
in the Ohio River, 12 air-line miles from 
Lake Glendale. The Ohio River is the 
nearest known source of this species. Adults 
from the Ohio evidently had flown the 12 
miles to Lake Glendale in the summer of 
1940 and established a colony of nymphs 
there. The valve in the dam which formed 
Lake Glendale was closed in the autumn 
of 1939. ‘This artificial lake is not, how- 
ever, a very suitable place for the develop- 
