May, 1953 
OCHRIS 
INTERCALARIS 
Burks: THE Mayr ties oF ILLINOIS 123 
PHOEBUS 
PALLIDULUS 
Fig. 285.—Baetis intercalaris, anteroapical area of fore wing of male. 
Fig. 286.—Baetis ochris, anteroapical area of fore wing of male. 
Fig. 287.—Baetis pallidulus, anteroapical area of fore wing of male. 
Fig. 288.—Baetis phoebus, anteroapical area of fore wing of male. 
the upper faceted portion of the eye is 
widely separated from the lower portion, 
figs. 255, 256. The width of the vertex sep- 
arating the compound eyes in the females is 
usually three times as great as the width of 
one eye. Each fore tarsus in the males varies 
from slightly shorter to slightly longer than 
each fore tibia. Each fore wing has rela- 
tively few crossveins, and the marginal inter- 
calaries are paired, figs. 31, 220. Each hind 
wing is relatively long and narrow, with or 
without an acute costal projection, some- 
times with two but usually with three longi- 
tudinal veins, of which the third is always 
shortest, virtually or quite without cross- 
veins, and with intercalary veins commonly 
present, figs. 270-284. The male genitalia, 
figs. 289-297, consist of a pair of four- 
segmented forceps and a penis cover. This 
cover varies from a well-developed, flaplike 
lobe to a small, extremely inconspicuous, 
membranous papilla. In some specimens, it 
is quite dificult to demonstrate the penis 
cover. The saclike and amorphous penis 
is membranous and internal, but can be seen 
extruded on an occasional specimen. The 
cerci are longer than the body. 
The nymphs, fig. 298, are streamlined, 
and live in shallow running water. They 
are most commonly found under stones and 
among debris or emergent vegetation along 
the banks of brooks or creeks. The maxil- 
lary palp has two or three segments; the 
labial palp has three. The legs are relatively 
long and slender, with long, narrow claws, 
each of which bears minute denticles on the 
inner ventral surface. In all known species, 
the gills are single and platelike, fig. 228. 
The median caudal filament is either shorter 
than the cerci or vestigial. 
In order to study the species in this genus, 
it is necessary to make dry mounts of the 
wings, especially the hind wings, and to 
clear, stain, and make slide mounts of the 
male genitalia. The latter operation must 
be done with extreme care, as these geni- 
talia are quite fragile and, also, easily dis- 
torted while being manipulated. 
The genus Baetis is one of the largest and 
most difficult genera of the mayflies. It in- 
cludes about 50 Nearctic species, of which 
17 are at present known to occur in I]linois; 
25 species are treated in this report. 
Ide (19375:219) has published complete 
descriptions and figures of the nymphs of a 
large number of species of Baetis. 
Reliable characteristics for the separation 
to species of adult females and female 
nymphs of this genus have not yet been 
found. 
KEY TO SPECIES 
Apvutt MALEs 
1. A prominent, pointed, mesal projection 
at apex of second segment of genital 
forceps arm, fig. 289..... 1. spinosus 
No apicomesal projection on second seg- 
ment of genital forceps arm, figs. 290— 
Nw 
Hind wing without a costal projection, 
ase SPAT ORD Al lebtes trated et oa mlene e ehA E 3 
Hind wing with a costal projection, figs. 
DU DED R Ava 2 ret oa ACN ae tea bac Mee oe. 5 
3. Abdominal tergites 2-6 white, or very 
faintly stained with yellow.......... 
FE UE ree eaten ae 2. propinquus 
Abdominal tergites 2-6 mostly or entirely 
darkibrownliee teeta Cee + 
4. Fourth segment of genital forceps arm as 
long as wide, fig. 290....3. elachistus 
Fourth segment of genital forceps arm 
three times as long as wide, fig. 291... 
PE Sea Tees erable ea 4. frivolus 
5. Hind wing entirely without a third longi- 
tudinal vein, figs. 272, 274, 276..... 6 
Hind wing with a third longitudinal vein, 
fips STS 2172 ake ete ie ee 9 
6. Hind wing relatively narrow and lacking 
marginal intercalary veins posterior to 
