38 [Luinois NATURAL 
Marquette State Park, Poplar Bluff, Quincy 
(Mississippi River), Rock Island, Rockton 
(Rock River), Rosiclare, Shepherd, Urbana, 
and Waukegan. 
2. Pentagenia robusta McDunnough 
Pentagenia robusta McDunnough (1926: 185). 
All crossveins in wings tan; no pigmented 
dots in fore wing. Abdominal sternites 
marked with longitudinal, brown lines, and 
terminal two sternites mostly brown; caudal 
filaments brown, with yellowish articula- 
tions. 
Known only from Ohio. 
7. HEXAGENIA Walsh 
Hexagenia Walsh (1863:197). 
The various members of this genus are 
the commonest Illinois mayflies, as well as 
the largest. The eyes in the adult males are 
large, but never quite so large as in Penta- 
genia vittigera, and are always separated on 
the meson by at least a small space. Each 
fore leg in the males is approximately as 
long as the body. In the fore wing, the 
crossveins near the bulla are not crowded, 
fig. 27, as they are in Ephemera, fig. 46. The 
median caudal filament is reduced to a mere 
vestige in both sexes. 
In the nymphs, fig. 2, the frontal process 
is dome-shaped, conical, or truncate; the 
lateral angles are mever produced an- 
teriorly as spines or prongs. The upcurved 
mandibular tusks are long, slender, and 
smooth, entirely lacking rasplike teeth. The 
apical segment of the labial palp is broad, 
with a median apical point. 
My views of specific limits in this genus 
correspond in the main with those of Spieth 
(19414: 233); I have not, however, followed 
him in recognizing subspecific segregates 
within the species limbata and munda. 
It is not advisable to attempt to name 
single female specimens, unassociated with 
males; the characters given below for fe- 
males apply to typical specimens only. 
KEY TO SPECIES 
ADULTS 
Se a a a i 2 
LATE SS ane ese ee aa 7 
2. Apex of each penis lobe reflexed, fig. 73; 
first segment of forceps shorter than 
SRUMMIOOE ie es ktce caes 6. recurvata 
History Survey BULLETIN 
6. 
10. 
Vol. 26, Art. 1 
Apex of each penis lobe not reflexed, figs. 
74-78; first segment of forceps longer 
than penis lobe... .....:.....+.smeeeee 3 
An anteapical protuberance on mesal 
margin of each penis lobe (dorsal view), 
few] 8% 6 cawee wt. ken 2. bilineata 
No anteapical protuberance on mesal mar- 
gin of each penis lobe, figs. 74-77... .4 
Apexes of penis lobes sharply incurved, 
ADS V5 be as Pte ee ee 3. limbata 
Apexes of penis lobes arcuate, figs. 74, 
Penis lobes short and blunt, fig/7 deen 
RSPR e ye Tren sigty Gee aS 1. atrocaudata 
Penis lobes elongate, figs. 74, 76....... 6 
Penis lobes tapered gradually to apexes, 
fipi 74.0087... 52) Oa 5. rigida 
Penis lobes abruptly constricted at about 
midlength, fig. 76......... 4. munda 
Membrane of wings uniformly stained 
with brown pigment, and lacking darker 
spots or areas at costal margin of fore 
wing, posterior margin of hind wing, or 
in disc of either of these wings. 
SE ere TMA tors ee eS 6. recurvata 
Wing membrane hyaline, yellowish, or 
very light tan, and always with some 
darker areas or spots in Wings: imaoe 
Hind wing with a biased reddish or pur- 
plish-brown band at posterior margin, 
2 aL 2 PARR aii Aa Be 1. atrocaudata 
Hind wing either without darkened pos- 
terior margin or with a relatively nar- 
oo 
row, sometimes discontinuous, dark 
brown band at posterior margin, figs. 
82,.84-87... cs gee 9 
Hind wing with membrane hyaline, veins 
hyaline, and discal crossveins black, 
figs. 86, 87; abdomen white or cream 
colored, with dorsal color pattern of 
dull red markings. . 5. rigida 
Hind wing with membrane hyaline, 
yellowish, or light tan, with veins partly 
or entirely yellowish or tan, and cross- 
veins black or brown both in discal and 
marginal areas of wing; abdomen yel- 
low, tan, or light brown, with brown 
color pattern... .."..... dy. 10 
Hind wing with a continuous, dark brown 
band at posterior margin and two rela- 
atively large, brown spots in disc of 
wing; each discal crossvein surrounded 
by a small, brown cloud, fig. 82....... 
Ee eee ea a cr 2. bilineata 
Posterior margin of hind wing without 
brown band, fig. 85, or often with an 
irregular, discontinuous one, fig. 84; 
no large, brown spots in disc of wing... 
MESS SAT 3. limbata, 4. munda 
Mature Nympus 
Each gill of first pair composed of a single 
filament. G62 yn, ve eae t 6. recurvata 
Each gill of first pair composed of two or 
more filaments. 7 ..3..2/) eee 
Frontal process of head truncate at apex 
and with a small mesal indentation, 
HE.) ees prtah it ee cence Pe 1. atrocaudata 
Frontal process of head rounded or angled 
on meson, figs. 68, 69, 71, 72........ 3 
