May, 1953 
Operculate gills elongate-oval.......... 
Sse cra ek as TEN ake 12. Leptohyphes 
3. Head bearing occipital and frontal tuber- 
Gless Howth csc os 10. Brachycercus 
Head without tubercles, fig. 114........ 
9 oid i Ae ee tee nae 11. Caenis 
9. TRICORYTHODES Ulmer 
Tricorythodes Ulmer (1920a:51). 
The species of Tricorythodes consist of 
small, fragile mayflies which resemble 
Caenis in habitus, but differ considerably in 
diagnostic characteristics. Tricorythodes is 
a New World genus related to Tricory- 
thus of the Palearctic and African regions. 
Tricorythodes and Tricorythus are distin- 
Fig. 96. — Tricorythodes atratus, mature 
nymph, dorsal aspect. 
Burks: THE Mayr ties or ILLINOIS 45 
guished in the adults by the characteristics 
of the legs. In the former, the legs are 
long and slender; in the latter, they are 
short and less slender. 
In the Illinois species of Tricorythodes, 
each adult has a pair of tubercles near the 
posterior margin of the vertex of the head. 
Each antennal pedicel in both sexes is two 
or three times as long as the scape; the 
flagellum is enlarged near the base and is 
four or five times as long as the pedicel. 
In the adult males, the fore leg is as long 
as the body, the fore femur is one-half as 
long as the fore tibia, and the fore tibia is 
one and one-half times as long as the fore 
tarsus; the second tarsal segment is one- 
half as long as the tibia and as long as 
tarsal segments 3-5 combined. In the fore 
wing, fig. 98, there are relatively numerous 
crossveins; veins R, and R, diverge in the 
center of the wing, and vein M, and the 
median intercalary arise some _ distance 
distad of the wing base. The posterior 
margin of the male genital forceps base 
has a wide, median excavation. Each arm 
of the forceps, figs. 99-101, has three seg- 
ments; segment | is columnar, segment 2 
has a bulbous, median enlargement at the 
base, and segment 3 is minute. The penis 
lobes are fused on the meson almost to the 
apexes, much as in some species of Ephem- 
erella. In the adult females, the caudal 
filaments are as long as those of the males, 
and these female caudal filaments usually 
retain the subimaginal setae only at the 
apexes. 
The nymphs, fig. 96, have smooth heads 
lacking tubercles. Each antenna is almost 
twice as long as the head and pronotum, 
when measured in dorsal aspect. The legs 
are relatively longer than in Caenis, fig. 114, 
but shorter than in Brachycercus, fig. 113; 
each claw is relatively long, hooked at its 
apex, and has a ventral row of denticles, 
fig. 95. Abdominal segment | lacks gills; 
segment 2 bears a pair of subtriangular, 
operculate gills, each of which has an addi- 
tional, ventral, membranous plate; each of 
segments 3-6 bears a pair of double, plate- 
like gills. All gills, fig. 90, have the margins 
entire. The three caudal filaments are 
relatively long and stout, and they have a 
whorl of setae at each articulation. 
I have observed the subimagoes of T'ri- 
corythodes atratus to shed the subimaginal 
