May, 1953 
& Sanderson, 56, 29. Peorta: July 13, 
1940, F. F. Hasbrouck, 1 6. Quincy: June 
2, 1939, Burks & Riegel, 1 3 ; June 25, 1940, 
Mohr & Riegel, 1 ¢. St. CHar-es: at light, 
July 8, 1948, Ross & Burks, 26. SAVANNa: 
July 22, 1892, Hart & Forbes, 1¢; July 
feos. L. H: Frison, 12. Wesr’ Cui- 
caco: July 9, 1948, Ross & Burks, 19. 
Wuirte HeatH: Sangamon River, Aug. 2, 
1939, Ross & Riegel, 2¢. WILMINGTON: 
at light, Aug. 6, 1947, Burks & Sanderson, 
73,429. 
6. Isonychia bicolor (Walker) 
Palingenia bicolor Walker (1853: 552). 
Chirotenetes albomanicata Needham 
(1905: 31). 
Mave.—Length of body and of fore wing 
10-12 mm. Head brown; scape and pedicel 
of each antenna light brown, flagellum tan 
at base, becoming yellow toward apex. 
Thorax dark red-brown, almost black; fore 
leg same color, with tarsus white, segments 
shaded with brown at apexes; wings hyaline, 
veins and crossveins colorless except at costal 
margin, where they are tan. Abdomen very 
dark red-brown, the apical segment lighter 
brown, a narrow, transverse, black band at 
posterior margin of each tergite; genitalia, 
fig. 253, tan; caudal filaments light vellow 
or white, a few basal articulations brown. 
FEMALE.—Length of body and of fore 
wing 12-16 mm. Head yellow or tan, 
shaded with brown. Thorax yellow-brown, 
darker on venter; wings with veins and 
crossveins light brown. Abdomen bright red- 
brown, with transverse, black-shaded stripe 
at posterior margin of each tergite; black, 
longitudinal line and spot at each spiracle; 
caudal filaments white. 
Known from the northeastern and mid- 
western states and the eastern Canadian 
provinces. 
Illinois Records. — Eppyvitte: Lusk 
Creek, May 15-23, 1946, Mohr & Burks, 
36. KANKAKEE: May 31, 1938, Burks & 
Mohr, 14; June 5, 1932, Frison & Mohr, 
1¢; June 15, 1938, Ross & Burks, 24; 
June 17, 1939, B. D. Burks, 1 ¢, 19; July 
O29, 1. H.-F rison,: 5 ds; July 18, 1925, 
T. H. Frison, 16; Aug. 1, 1933, Ross & 
Mohr, 1 ¢ ; Aug. 2-4, 1938, Burks & Boesel, 
94,72; Aug. 16, 1938, Ross & Burks, 14, 
32. Momence: June 15, 1938, Ross & 
Burks, 1¢, 22. Oaxwoop: June 6, 1925, 
Burks: THE Mayr ties or ILLINOIS 113 
T. H. Frison, 1; June 9, 1926, Frison & 
Auden, 1¢; July 24, 1939, B. D. Burks, 
36, 22. Poprar BiurrF: June 20, 1943, 
T. H. Frison, 146. RockKrorp: June 13, 1931, 
Frison & Mohr, 16. Rock Istanp: June 
7, 1937, Burks & Riegel, 13; June, 1933, 
C. O. Mohr, 28. Witmincton: at light, 
Aug. 6, 1947, Burks & Sanderson, 24, 39. 
BAETINAE 
The most simplified of North American 
mayflies belong to the subfamily Baetinae. 
The wing venation is always reduced, both 
through complete loss of some veins and 
through partial atrophy of the veins that 
persist. The basal part of the outer branches 
of vein Rs and the base of vein M, of the 
fore wing are always atrophied, as in figs. 
31, 220-222. The hind wing, figs. 220-222, 
270-284, is greatly reduced in size and 
venation, or wanting entirely. The homol- 
ogies of the longitudinal veins that persist 
in the hind wing have not been conclusively 
determined. When three longitudinal veins 
persist, however, they perhaps represent the 
remnants of Sc, R, and M. The middle and 
hind tarsi, in both sexes, have only three 
clearly differentiated segments, fig. 15. The 
male genitalia, figs. 260, 267-269, 289-297, 
are greatly reduced, the penis lobes being 
virtually amorphous, membranous, internal 
structures. There is, between the bases of 
the forceps, a flaplike penis cover which, in 
some species of the genus Baetis, is obscure. 
Each arm of the forceps has four segments; 
the separation between the second and third 
segments often is so obscure that each arm 
appears to have only three segments. The 
adults uniformly have the median caudal 
filament vestigial in both sexes. 
In the members of this subfamily, antig- 
eny is more pronounced than in other 
mayflies. The compound eyes in the male 
adults are greatly enlarged and divided, each 
eye consisting of two distinctly separated 
portions, figs. 255-257. The eyes in the 
females are relatively small and _ simple. 
This hypertrophy of the eyes in the males 
has led to the development of a marked dif- 
ference between the two sexes in the shape 
of the head. This difference in head shape 
can be seen in the nymphs as well as in the 
adults. The nymphs in even the early in- 
stars show this difference in head shape. 
Because of this, the male nymphs in all stages 
