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7 
May, 1953 
antennae white, tinged with brown. Thorax 
dark red-brown, lighter on pleura; each 
fore leg brown, with apex of femur darker; 
wings hyaline, veins faintly tinged with tan, 
crossveins hyaline. Abdomen dark brown, 
each tergite and sternite with a large, light 
yellow-brown spot at either anterolateral 
angle; each tergite also with a smaller, 
median, light spot on anterior margin; 
minute, longitudinal, black lines in spirac- 
ular region; genitalia, fig. 248, yellow- 
brown; caudal filaments light yellow or 
white. 
FEMALE.—Length of body 10-13 mm., of 
fore wing 12-15 mm. Head yellow-brown. 
Thorax light red-brown; wings hyaline, 
veins and crossveins brown. Abdomen dark 
red-brown, with large, conspicuous, light 
tan markings: light spot at either anterolat- 
eral angle of each tergite, tenth tergite 
usually entirely light, and entire lateral 
third of each sternite light; black spot near 
each spiracle; posterior margin of apical 
abdominal sternite entire. 
Holotype, male.—Rock Island, Illinois, 
Walsh, 1863. Specimen dry, on pin; geni- 
talia on a microscope slide. 
Allotype, female—Same data as for 
holotype. Specimen dry, on pin. 
The holotype and allotype are in the col- 
lection of the Museum of Comparative 
Zoology and the paratypes listed below are 
in the collection of the Illinois Natural His- 
tory Survey. 
Known from Indiana, Illinois, and Kan- 
sas. 
Paratypes.—ILLINoIs.—DIxon: June 27, 
1935, DeLong & Ross, 1¢. GULFPoRT: 
Crystal Lake, June 10, 1939, J. S. Ayars, 
ma. OQUAWKA: June 13, 1932, H. L. 
Dozier, 22. PrRopHETSTOWN: Rock River, 
July 24-25, 1947, Burks & Sanderson, 2°. 
RockrorD: at light, June 29, 1938, B. D. 
Burks, 19. Rock Istanp: B. D. Walsh, 
mo, 29. 
2. Isonychia diversa TVraver 
Isonychia diversa Traver (1934: 244). 
This species is included here because it 
represents one of the four types of male 
genitalia to be found in the genus. Head 
and body dark red-brown; wings hyaline, 
with hyaline veins and crossveins; genitalia, 
fig. 249, and caudal filaments white. 
Known from Tennessee. 
Burks: THe MAyrties or ILLINOIS 
111 
3. Isonychia arida (Say) 
Baetis arida Say (1839:42). 
The original description of this species 
mentions only one character that is specific 
rather than generic. This is “anterior tibiae 
whitish, obscure at base and tip.” Unfor- 
tunately, the species identified as arida by 
Walsh (1862:370) has the anterior tibiae 
completely brown. Hagen (1863:191) no- 
ticed this discrepancy, but concluded that 
the normal range of variation in the species 
would include forms with brown and with 
white tibiae. All workers since Walsh’s 
time have followed his determination of 
arida, although to my knowledge a form of 
it with white tibiae has never been found. 
McDunnough (1931c:159) stated that there 
was considerable doubt in his mind that 
Walsh’s determination of arida was correct, 
but that “there seems nothing to be gained 
by altering his determination and changing 
the generally accepted idea of the species.” 
I certainly would have followed the same 
course were it not for the fact that we have 
an Illinois specimen with white fore tibiae 
that matches Say’s description of arida in 
all particulars. This specimen is quite dif- 
ferent from the species determined by Walsh 
as arida and must either be determined as 
arida Say or be described as a new species 
of extremely doubtful validity. After full 
consideration of the problem, I have decided 
that the former course is preferable. Ac- 
cordingly, the species identified as arida by 
Walsh is renamed sayi on page 110 and the 
name arida is here applied to this Illinois 
specimen, which fully agrees with Say’s de- 
scription. 
Ma.e.—Length of body 9 mm., of fore 
wing 11 mm. Head light tan, antennae tan, 
becoming white at apexes of flagella. 
Thorax red-brown; wings hyaline, veins and 
crossveins hyaline, stigmal crossveins rela- 
tively few, not anastomosed. In fore leg, 
femur brown, lighter at base; tibia white, 
darkened with brown at base and tip; tarsus 
white, segments slightly shaded with brown 
at apexes. Abdomen red-brown, a trans- 
verse stripe of black shading at posterior 
margin of each tergite; genitalia, fig. 250, 
white; caudal filaments white. 
This species is known from Illinois and 
Indiana. 
Illinois Record.—Momence: at light, 
August 16, 1938, Ross & Burks, 1¢. 
