108 
yellow on face below ocelli and on frontal 
shelf, dark brown shading present around 
bases of ocelli; antennae tan, base of each 
flagellum shaded with dark brown; eyes tan. 
The thorax dark brown, with light yellow 
markings on pleura and sternum. Mem- 
brane of each fore wing hyaline, with dark 
brown shading around  crossveins, this 
shading most extensive in discal area of 
wing; membrane of hind wing almost or 
quite completely stained dark brown; veins 
and crossveins of fore wing strong, stigmal 
crossveins anastomosed; each fore leg dark 
yellow-brown, with tarsus slightly lighter, 
middle and hind legs yellow, faintly shaded 
with red-brown on coxae, near apexes of 
femora, and at apexes of tarsal segments. 
Abdominal dorsum dark brown at base and 
on apical tergites, intervening tergites lighter 
brown; basal and apical sternites brown, 
intermediate sternites almost white, with 
vague tan mark near lateral margin of each 
sternite; genitalia, fig. 245, yellowish tan; 
caudal filaments uniformly gray-brown near 
bases, fading to almost white at apexes, 
articulations in apical part of each filament 
faintly stained with brown. 
FEMALE.—Length of body 11-14 mm., of 
fore wing 13-15 mm. Color pattern much as 
in male, except generally slightly lighter. 
Each fore leg faintly darker than middle 
and hind legs; fore wing as in male, hind 
wing with membrane not quite entirely 
shaded with brown, small hyaline areas pres- 
ent in center of most cells. Middle abdom- 
inal segments only slightly lighter than an- 
terior and posterior ones; apical margin of 
terminal abdominal sternite produced pos- 
teriorly, evenly rounded from side to side, 
or margin very slightly irregular on meson; 
caudal filaments uniformly tan throughout, 
or occasionally becoming a little lighter in 
shade toward apexes. 
NympH.—Length of body 11-16 mm., of 
caudal filaments 6-8 mm. Color pattern of 
thorax much as in adult, with coloration of 
adult wings distinctly visible; legs showing 
only a faint indication of darker shaded 
areas present in adult legs. Dorsum of ab- 
domen light, with a pair of submedian dots 
at anterior margin of each tergite; postero- 
lateral, spinelike projections of tergites usu- 
ally not darkened at tips; each sternite with 
a pair of broad, longitudinal, brown bands 
near lateral margins and four submedian, 
brown dots; gills borne by abdominal seg- 
Ittinoris NaturAt History Survey BULLETIN 
Vol. 26, Art. 1 
ments | and 2 double, others single, fig. 223, 
caudal filaments each with a broad, dark 
brown crossband at mid-length. 
Known from Arkansas and Illinois. 
Illinois Records.—ALto Pass: April 30, 
1942, Mohr & Burks, 16, 4 N; Jan. 25, 
1947, Burks, Stannard, & Riegel, 1 N. 
Dixon Sprincs: March 13, 1946, Ross & 
Burks, 7 N; April 4-6, 1946, Burks & San- 
derson, 2¢,19,5 N. Fountain BLufr: 
May 15, 1932, Ross & Mohr, 1 N. Grant 
Ciry State Park: April 2-21, 1942, Ross & 
Burks, 56, 42, 23 N; May 16-29, 1946, 
Burks & Sanderson, 36, 12,9 N. Gorg- 
VILLE, Fern Cliff: March 24, 1939, Ross & 
Burks, 1 N; April 4-23, 1942, Ross & Burks, 
26, 19, 5 N. Herop: June 1-3, 1939) 
Burks & Riegel, 29,2 N. 
ISONYCHIINAE new subfamily 
The subfamily Isonychiinae is here erected 
for the reception of a single North Ameri- 
can genus, Jsonychia, which has long been 
the cause of radical disagreement among 
mayfly workers. This genus has been con- 
sidered to have both heptageniid and baetid 
relationships. Ide (19306:227) and Spieth 
(1933:329) included it in the family Hepta- 
geniidae; Ulmer (1933:210) placed it in 
his superfamily Heptagenioidea; and Traver 
(1935a:477) placed it in the Baetidae. 
Isonychia is, in my opinion, clearly baetid in 
its family relationships and shows some simi- 
larity to Siphlonurus. However, both 
nymphal and adult characteristics are too — 
greatly at variance with those of Siphlonurus 
to permit the two genera to be placed in the 
same subfamily. : 
The adults of Isonychia have gill rem- 
nants persisting at the base of each vestigial 
maxilla and at the base of each fore coxa. — 
The fore tarsus in the males is approxi- 
mately as long as the fore tibia. In the fore 
wing, fig. 217, the cubital intercalaries are — 
a series of short, sinuate, and forked veins — 
which extend from Cu, to the anal margin — 
of the wing, much as in Hexagenia and — 
Potamanthus. In the hind wing, vein M is 
forked very near the outer margin of the 
wing. The male genitalia, figs. 248-253, are — 
of a type quite different from those of all 
other baetid mayflies. The nymphs bear 
tufted, filamentous maxillary and fore coxal 
gills, and unique abdominal gills, each of 
which is composed of an upper, platelike 
