May, 1953 
differ significantly from that of Caenis: veins 
R, and R, diverge relatively close to the 
wing base, veins M, and M, and the median 
intercalary vein all arise at the wing base, 
and there are relatively few crossveins. The 
legs are long and slender, with the femora 
very little stouter than the tibiae; legs con- 
spicuously more slender than in Caenis. 
The abdomen in the males is about as long 
as the thorax, while, in the females, the 
thorax is slightly shorter than the abdomen. 
The segments of the abdomen bear long 
and filamentous, lateral projections. ‘The 
male genital forceps base bears a pair of 
accessory lateral projections; otherwise the 
male genitalia are similar to those of Caenis. 
The posterior margin of the terminal ab- 
dominal sternite in the females is evenly 
rounded from margin to margin. The caudal 
filaments of the males are much longer than 
those of the females. 
The nymphs, fig. 113, are flattened, heavy- 
bodied forms with conspicuously long, slen- 
der legs and tuberculate heads. Each an- 
tennal pedicel is three times as long as the 
scape, and the flagellum is as long as the 
head and pronotum combined. ‘The tarsal 
claws are long, slender, and entirely without 
ventral denticles, fig. 93. The lateral mar- 
gins of the abdominal tergites are produced 
as broad, flat, and thin projections. The 
first abdominal segment bears a pair of 
single, filamentous gills; the gills of ab- 
dominal segment 2 are operculate and semi- 
quadrate; and the gills borne by segments 
3-6 are single, the margins of each gill being 
provided with a fringe of long filaments. 
Reliable characteristics for the separation 
to species of females and nymphs of this 
genus have not yet been found. 
KEY TO SPECIES 
Aputtr Ma tes 
Abdominal tergites white, entirely without 
blackumiarkines cid. cecsh le acca 1. prudens 
Abdominal tergites white; a black, transverse 
line at posterior margin on each of the apical 
tergites, and traces of a median, longi- 
tudinal, black line on each of the middle and 
EA GAIBTCEOTECS = Aah cia cet e ears 2. lacustris 
1. Brachycercus prudens (McDunnough) 
Eurycaenis prudens McDunnough 
(1931e: 264). 
Mate.—Length of body and of fore wing 
3 mm. Head below antennae yellow; vertex 
Burks: [THE MAyr ies or ILLINOIS 49 
shaded with brown, and with a median, 
longitudinal, black line, lateral areas near 
ocelli dark gray-brown. Pronotum light 
yellow, almost white, with dark brown 
shading on median area of posterior margin; 
fore coxa vaguely shaded at base and apex 
with brown, fore femur gray-brown, fore 
tibia gray at base; mesonotum yellow-brown, 
with faint, dark brown shading on dorsal 
sutures; each pleuron and sternum of meso- 
and of metathorax light yellow-brown; met- 
anotum yellow-brown; middle and hind legs 
white, with coxae sometimes slightly shaded 
with gray-brown. Abdomen entirely light 
yellow, almost white, either entirely without 
darker markings or with faint, pinkish-brown 
shading at posterior margins of caudal ter- 
gites; genital forceps long and flat, faintly 
gray in color; caudal filaments white. 
FEMALE.—Length of body and of fore 
wing 3.5 mm. Coloration identical with 
that of male except that mesonotum is a 
slightly darker brown, and middle and hind 
coxae are extensively shaded with brown. 
Known from Illinois, Kansas, and Sas- 
katchewan. Apparently develops in large 
rivers. 
Illinois Records.—SAvANNA: at light, 
July 20, 1892, Hart, Forbes, & McElfresh, 
76. SHETLERVILLE: at light, Aug. 10, 1898, 
Co Ans art A Ses: i: 9 
2. Brachycercus lacustris (Needham) 
Caents lacustris Needham (1918: 249). 
Eurycaents pallidus Ide (19305: 218), 
not Tschernova. Name preoccupied. 
Brachicercus idei Lestage (1931a:119). 
New name for pallidus Ide. 
Caenis lacustris was described from 
nymphs only, and the types have been lost. 
I follow Lyman (1944:3) in placing idei as 
a synonym of Jacustris. 
Mate.—Length of body 4-5 mm., of fore 
wing 4.5-6.0 mm. Head, pronotum, and 
abdomen white, meso- and metathorax light 
brown; each of the apical abdominal ter- 
gites marked with a transverse, black line 
at posterior margin, and traces of a median, 
longitudinal, dorsal, black line present on 
middle and posterior tergites; fore leg 
shaded with brown, other legs white; 
claspers of genitalia tan, and penis lobes 
white; caudal filaments white. 
Known from Michigan, Minnesota, New 
York, Ohio, and Ontario; should eventually 
be found to occur in Illinois. 
