May, 1953 
Fig. 390.—Rhithrogena sp., mature nymph, 
ventral aspect. 
rounded projection on either side of this 
indentation, fig. 391. The penis lobes vary 
considerably in shape, but are always fused 
on the meson at the base; apical teeth and 
lateral spines are often present on the penis 
lobes. The posterior margin of the apical 
abdominal sternite of the female is broadly 
rounded, without a median emargination. 
In the nymphs, fig. 390, the frontal margin 
of the head is very slightly emarginate on 
the meson, exposing a small part of the la- 
brum when viewed in dorsal ‘aspect; each 
maxillary palp has the distal segment broad 
and clavate, with the apex obliquely truncate 
and bearing multiple rows of pectinate spines 
and teeth on the inner surface and a dense 
row of hairs on the outer margin; the crown 
of each maxillary galea-lacinia bears a row 
of minute, stout teeth or spines; the apical 
segment of each labial palp is greatly en- 
larged and has a dense row of fine hairs 
Burks: THe MAyrties or ILLINOIS 197 
along the outer, apical margin, below which 
is a bank of pectinate spines. In the legs, the 
femora are only moderately flattened, and 
have a row of closely set bristles along the 
posterior margin of each; each tarsal claw is 
short, broad at the base, and bears one or 
two ventral spines near the base and two 
or three ventral denticles near the tip. The 
gills are borne by segments 1-7, each gill 
being composed of a ventral, platelike ele- 
ment and a dorsal tuft of filaments, fig. 324. 
The platelike element of each gill has the 
outer margin irregularly fissured. The gills 
are held spread out beneath the venter of 
the abdomen in such a way as to form an 
adhesive disc, the first and last pairs of gills 
meeting on the meson to complete this disc 
at the anterior and posterior ends of the 
abdomen. The median caudal filament is 
somewhat longer than the cerci; all fila- 
ments are bare except for minute setae on 
the mesal margin of each cercus. 
The species of Rhithrogena are relatively 
numerous and common in the western states 
and in the Northeast. Only one species has 
been collected in Illinois. 
Rhithrogena pellucida Daggy 
Rhithrogena pellucida Daggy (1945:383). 
Mave.—Length of body and of fore wing 
6-7 mm. Compound eyes in life dark gray- 
green; scape and pedicel of each antenna 
light yellow, flagellum shaded with gray; 
vertex and frontal shelf of head dark brown, 
except for a light yellow, transverse band ex- 
tending from eye to eye at level of bases of 
antennae. Pronotum dark red-brown; meso- 
notum dark brown, almost black, with faint, 
greenish tinge, apex of scutellum red-brown; 
pleura brown, with yellow markings. Legs 
yellow or tan-yellow, each femur with a 
vague, brown, median band, and each tarsal 
segment shaded with gray-brown at apex; 
first fore tarsal segment one-third as long as 
second; wings hyaline, veins C, Sc, and R of 
fore wing shaded with gray-brown at bases, 
all other veins and all crossveins hyaline; 
stigmatic crossveins only slightly anasto- 
mosed. Abdominal tergites 2-6 each with a 
pair of large, brown, lateral spots; tergites 
7-10 brown, with faint orange cast; ster- 
nites dirty white or yellow; genitalia, fig. 
391, greatly reduced, the penis lobes lacking 
all spines, teeth, or tubercles; caudal fila- 
ments white, articulations not darkened. 
