6 Ittinois NATURAL History Survey BULLETIN 
Vol. 26, Art. 1 
Fig. 5.—Gibbons Creek at Herod, from which Ameletus lineatus emerges in early spring. 
In late spring, Baetis herodes and Leptophlebia cupida emerge here. Stenonema tripunctatum 
emerges here throughout spring and summer months. 
10. Large lakes, represented by Lake 
Michigan: Ephemera simulans. 
11. Small, relatively shallow glacial lakes, 
such as those found in Lake County: 
Ephemerella lutulenta and temporalis. 
12. Small, temporary brooks, which flow 
into larger streams in late winter and early 
spring. This type of stream may furnish 
a habitat favorable for an occasional small 
nymph of Stenonema or Caenis but no 
mayfly has been found to mature in such a 
place. Stoneflies and craneflies, however, 
often occur abundantly in these streams. 
Another category might have been made 
for the various bodies of impounded water 
which now exist in considerable numbers 
in central and southern Illinois, but no may- 
fly can be said to be characteristic of such 
bodies of water. Mayflies that occur in 
such waters indicate the ecological charac- 
teristics of the individual impoundments. 
Some of these impoundments are stagnant 
and produce large flights of Caenis simulans 
only; others apparently have the character- 
istics of a large, slow river, and produce 
flights of Hexagenia bilineata or limbata. 
Life History 
Mayfly nymphs require a relatively long 
time to develop from egg to full-grown 
nymph. The shortest known length of 
nymphal life is in species of the genus 
Callibaetis which, in summer, mature from 
egg to adult in 5 to 6 weeks. Some of the 
smaller baetines require 4 to 5 months, a 
length of time that results in the production 
each year of two waves of adults for a 
species, one in early spring, the other in late 
summer or early autumn. Some, as was 
shown by Murphy (1922) for Baetis vagans, 
have a complex, overlapping series of broods. 
The summer brood matures in 6 months, the 
winter brood in 9. The large ephemerids, 
such as Hexagenia, figs. 2 and 3, require 2 
years to mature; the annual appearance of 
a given species in a locality is due to over- 
lapping broods of the species. Many other 
mayflies, such as Stenonema and Heptagenia, 
emerging as they do year after year in the 
same locality at about the same dates, may 
be inferred to require 1 year to mature from 
egg to adult. 
During nymphal life, the developing may- 
fly passes through a very large number of 
instars. Baetis vagans, which has a rela- 
tively short nymphal life of 6 to 9 months, 
passes through 27 instars (Murphy 1922). 
Other mayflies, such as Callibaetis, have 
been estimated to go through about 20 
nymphal instars (Needham ef al. 1935:15). 
