May, 1953 
generic names Heptagenia and Ecdyonurus 
(or the emended form Ecdyurus). As 
Traver (1933a: 173) pointed out, however, 
these American species actually represent 
a discrete generic unit, differing from either 
Heptagenia or Ecdyonurus both in type of 
male genitalia and in nymphal characters. 
Stenonema is the most difficult genus in 
the order Ephemeroptera. This is due to 
the fact that the male genitalia are quite 
similar throughout the genus, and the geni- 
talic differences between closely related spe- 
cies are, therefore, obscure. The principal 
bases for the separation of species are the 
colors of the males and the relative propor- 
tions of the male first and second fore tarsal 
segments. Unfortunately, both vary consid- 
erably. Specimens collected early in the 
season usually are much darker and larger 
than specimens of the same species collected 
in midsummer. In the large male specimens, 
the first fore tarsal segment tends to be 
relatively longer than it is in smaller speci- 
mens. However, a decision as to the proper 
limitation of a given species can be made 
when all available characters of the adult 
males and the mature nymphs are consid- 
ered together. Fortunately, most of our 
species of Stenonema have been reared, and 
good series of nymphs and adult males are 
available for study. The females are usually 
separable at best to species groups only. 
In the adult males, the large eyes, never 
contiguous on the meson, are usually sepa- 
rated on the vertex by a space at least one- 
half as wide as one eye. Each fore leg is, in 
all species except integrum, as long as, or 
slightly longer than, the body. In integrum, 
the fore leg is slightly shorter than the body. 
In all species, the fore tarsus is longer than 
the fore tibia and the first fore tarsal seg- 
ment varies from one-third to four-fifths 
as long as the second segment. The wing 
venation in both sexes, fig. 317, is typical 
for the family; the species of Stenonema 
and Heptagenia cannot, unfortunately, be 
distinguished generically by the characteris- 
tics of the wings. The male genitalia, figs. 
331-356, consist of a pair of four-segmented 
forceps, the second segment being longer than 
the other three combined, and a pair of 
[T-shaped penis lobes. 
Use of these generic characters will re- 
| quire the transfer of the species Epeorus 
| modestus Banks (1910:202) to the genus 
Stenonema. 
Burks: THE MAyr ties oF ILLINOIS 
155 
The nymphs, fig. 360, are greatly flat- 
tened, the legs sprawling laterally; the an- 
terior margin of the head is entire or only 
very slightly emarginate on the meson, and 
the eyes are dorsal and just touching the 
posterior margin of the head. Each tarsal 
claw is single, short, slightly hooked at the 
apex, and has a fairly large ventral tooth 
near the base; in many species, each claw 
also has two or three minute ventral denti- 
cles near the tip. The abdominal gills are 
of three types: those on segments 1-6 in the 
members of the interpunctatum group are 
pointed at the apexes, fig. 358, and each gill 
of the seventh pair has one longitudinal tra- 
chea; in the members of the tripunctatum 
group, the first six pairs of gills are rounded 
at the apexes, fig. 357, and each gill of the 
seventh pair has one or two longitudinal 
tracheae; in the pulchellum and bipunctatum 
groups, the gills on segments 1—6 are truncate 
at the apexes, fig. 359, and the gills of the 
seventh pair are without tracheae. The three 
caudal filaments are equally long, uniformly 
clothed with short setae, and, often, with 
alternating pairs of segments dark and light 
in color. 
All Illinois species of Stenonema develop 
under stones in the shallower parts of creeks 
and rivers. Some, such as tripunctatum, can 
tolerate a great deal of silt; in many of the 
small, sluggish, heavily silted streams in 
central Illinois, tripunctatum is the only 
mayfly now found. The later instar nymphs 
of most species of Stenonema can be reared 
through to maturity in stagnant water. 
Adult specimens of Stenonema should be 
pinned for preservation and study. Most 
specimens, if collected and preserved in alco- 
hol, cannot reliably be determined to species, 
as the necessary color characters, at best im- 
permanent, are quickly lost in alcohol; dry 
specimens retain their colors for several 
years if stored out of the light. Correct 
specific determinations can seldom be made 
from the male genitalia and fore tarsal char- 
acters alone. The color of the eyes in adult 
males should be recorded at the time of 
collection, as these eye colors are helpful in 
placing specimens in the proper species or 
species group. 
My views as to specific limits in this genus 
most closely coincide with those of Traver 
(1935a:297). 
The adult females can be distinguished 
only to groups. 
