May, 1953 
posed, figs. 199, 212, 240, 247, 254, 
DOOP29 Ora ee | Se ae, eee 
10. Cerci uniformly clothed with short setae, 
BE epeenIO OM. McG 6 obs ik 
2A Lye ae Paneer Leptophlebiidae, p. 81 
Cerci bearing a dense row of setae only on 
the mesal margin of each, fig. 298. . 
ET Ae ee Baetidae, p. 97 
EPHEMERIDAE 
The family Ephemeridae, as defined here, 
corresponds to the families Ephemeridae, 
Polymitarcidae, and Potamanthidae in the 
classification of Ulmer (1933: 195), except 
Burks: THE MaAyrtiss oF ILLINOIS Zi 
as to be approximately parallel with Cu, 
in the basal area. The basal costal cross- 
veins are well developed, and the stigmatic 
crossveins usually are not anastomosed. In 
many ephemerids, each compound eye in the 
males is divided into a dark lower portion 
and a light-colored upper portion, but the 
facets usually are the same size in each 
portion of the eye. The eye of the male in 
some species is not much larger than the 
eye of the female. The fore tarsus in the 
males has five segments, the basal one very 
short, figs. 23, 24. The hind tarsus in both 
Fig. 48.—Hexagenia bilineata, doral aspect of head of nymph. 
Fig. 49.—Pentagenia vittigera, dorsal aspect of head of nymph. 
Fig. 50.—E phemera simulans, dorsal aspect of head of nymph. 
that Neoephemera, included by Ulmer in the 
Ephemeridae, is excluded from this family 
and placed in a new family, Neoephemeridae. 
The classification of the Ephemeridae used 
here is identical with that of Traver (1935a: 
240), except for her inclusion of Neoephem- 
era in this family. 
The Ephemeridae include the largest 
mayflies occurring in Illinois. These are 
the mayflies that on warm summer evenings 
commonly emerge in enormous numbers 
from our larger rivers and lakes, and cover 
bridges and water-front buildings. 
The wings of adult ephemerids have ex- 
tremely numerous crossveins, and, except in 
Tortopus and Campsurus, a band of fine, 
short veinlets along the outer margin of each 
wing, figs. 38-43, 46, 47. In the fore wing, 
vein Cu, is sinuate near the base, and the 
posterior branch of vein M, is curved so 
sexes has only four clearly differentiated 
segments. In Tortopus, the middle and hind 
legs are nonfunctional and almost completely 
degenerated; in Ephoron the legs are re- 
duced somewhat and are nonfunctional. 
The nymphs of all ephemerids are pro- 
vided with mandibular tusks and a row of 
completely exposed, biramous gills on either 
side of the abdomen, figs. 2, 55, 59. These 
nymphs live in the sand, gravel, or silt on the 
bottoms of our larger streams and lakes. 
Although they remain almost completely 
buried most of the time, they can sometimes 
be seen swimming freely in fairly deep water 
near shore. “They have been observed to 
swim with a characteristic darting and un- 
dulating motion at about a foot beneath the 
surface. The periods in which the nymphs 
swim freely in the water may be near the 
time of molting. Nymphs in an aquarium 
