76 Intinors NarurAL History SurvEY BULLETIN 
The wings of some adults are washed 
with red or orange, but those of most are 
hyaline. In the fore wing, there are quite 
numerous, weak crossveins, fig. 30, numer- 
ous, short marginal intercalaries, and the 
outer wing margin is always slightly scal- 
loped. The median intercalary vein and M, 
extend almost to the wing base. There are 
no cubital intercalaries. Vein 1A extends 
to the outer wing margin, and a series of 
irregular, weak intercalary veins extends 
from 1A to the anal wing margin. The hind 
wing is almost circular in outline, fig. 176, 
has a broad costal projection near the wing 
base, and numerous weak crossveins and 
numerous marginal intercalaries; vein M 
is forked near the center of the wing. 
The abdomen is stocky in the basal half, 
but the segments are markedly more slender 
and elongate from the sixth segment pos- 
teriorly. The male genitalia, fig. 174, very 
similar throughout the genus, are composed 
of a pair of three-segmented forceps and a 
pair of subconical penis lobes which are 
fused on the median line almost to the tips. 
The apical margin of the terminal abdominal 
segment in the females has a pair of sub- 
median, triangular projections with a V- 
shaped notch on the meson between them. 
The median caudal filament is vestigial in 
both the male and female adults. 
In the subimagoes, the wings are heavily 
shaded with dark brown or black; white 
spots surround the crossveins and often two 
vague, white bands extend obliquely across 
each wing. 
The nymphs, the first one of which was 
described by Walsh (1864), are among the 
most unique and distinct of all mayfly naiads, 
fig. 181. In these nymphs, the integument is 
more heavily armored than in any other 
Nearctic species. The head is small and 
hypognathous; a pair of small frontal horns 
is usually present between the bases of the 
antennae; and the genae are produced above 
the bases of the mandibles as a pair of 
spines or small, flat ledges. These projec- 
tions of the head vary in size and shape 
among the different species. The distal 
margins of the labium and labrum meet to 
close completely the mouth opening ante- 
riorly; the buccal cavity is closed laterally 
by the mandibles. The labial and maxillary 
palps each have three segments; the second 
segment of the labial palp has an apicolateral 
projection which forms, with the third seg- 
Vol. 26, Art. 1 
Fig. 174.—Baetisca obesa, male genitalia. — 
Fig. 1754.—Baetisca bajkovi, gill of first 
abdominal segment. q 
Fig. 175B.—Baetisca bajkovi, gill of fourth 
abdominal segment. 
Fig. 175C.—Baetisca bajkovi, gill of sixth 
abdominal segment. 
Fig. 176.—Baetisca obesa, hind wing. 
Fig. 177.—Baetisca lacustris, head of mature 
nymph, dorsal aspect. 
Fig. 178.—Baetisca bajkovi, head of mature 
nymph, dorsal aspect. 
Fig. 179.—Bacetisca laurentina, head of ma- 
ture nymph, dorsal aspect. 
Fig. 180.—Baetisca obesa, head of mature 
nymph, dorsal aspect. 
