104 Intinors NaturAL History SurvEY BULLETIN 
marked mayflies. Each compound eye in 
living males and females has contrastingly 
colored stripes extending across the outer 
surface. The head and thorax usually are 
mostly dark brown, while the abdomen al- 
ways has a conspicuous color pattern of con- 
trasting light and dark areas which are 
somewhat annular in arrangement. The 
fore wing, fig. 219, is long and relatively 
narrow, with membrane usually hyaline and 
veins dark. There are numerous, irreg- 
ular, marginal intercalary veins, and the 
stigmatic crossveins are usually anastomosed. 
The cubital intercalary veins are typical 
for this subfamily. The hind wing has a 
broadly rounded, inconspicuous costal angu- 
lation, and vein M is forked at a point 
midway between the base and the outer 
margin of the wing. 
The male genitalia in the various species 
show the most strikingly distinct structural 
differences to be found in any Nearctic 
mayfly genus, figs. 242-246. The median 
caudal filament is represented by a minute 
vestige in the adults of either sex. 
The vigorous, streamlined nymphs, fig. 
247, typically inhabit quiet pools along the 
edges of streams. They also occur commonly 
in shallow pools filled by seepage water on 
rock ledges, as well as in small, shallow pools 
fed only intermittently with fresh water. 
They are not rheophilus, except in the early 
instars of some species. The mature nymphs 
are found invariably in quiet water. As 
Morgan (1913: 386) has shown, Siphlonurus 
nymphs are in part predaceous. 
In the Siphlonurus nymphs, the compound 
eyes are lateral, and the head is hypogna- 
thous. The tarsal claws are slender and 
pointed, and considerably shorter than the 
tibiae. Abdominal segments 1-7 bear plate- 
like gills, the first two pairs of which in- 
variably are double, while the more posterior 
pairs are single, except in alternatus. The 
posterolateral angles of the abdominal ter- 
gites are produced and spinelike. There are 
three well-developed caudal filaments; each 
cercus bears a fringe of long setae on the 
mesal side only. 
This genus includes 18 Nearctic species, 
4 of which are known to occur in Illinois; 
1 other species may be taken here eventually. 
Reliable characteristics for the separation 
of the females of these species have not yet 
been found. 
Vol. 26, Art. 1 
KEY TO SPECIES 
Apvu.tt MALEs 
1. Membrane of hind wing completely 
shaded with brown; male genitalia 
relatively simple, fig. 245............ 
SKS atsld Vi ne ee 5. marshalli 
Membrane of hind wing hyaline; male 
genitalia relatively complex, figs. 242— 
244, 246 ) 
2. Each abdominal sternite with a pair of 
dark brown, lateral, triangular marks, 
these spots connected on meson by a 
large V-shaped mark with its apex at 
median point of anterior margin of 
sternite; male genitalia with a promi- 
nent, serrated bulge on median margin 
of each inner process, fig. 242........ 
Shake abt otis ne 3. quebecensis 
Abdominal sternites not with lateral, 
triangular spots connected on meson b 
V-shaped marks; male genitalia wit 
no serrated bulge on mesal margin of 
each inner process... . +... 4a 3 
3. Each abdominal sternite with a median, 
anterior, brown spot, a pair of oblique, 
lateral marks, and a pair of submedian 
dots; male genitalia with a pair of 
broad, dorsal flaps, fig. 246........... 
Sap Shotts Gi Une cee 2. alternatus 
Abdominal sternites not with such mark- 
ings; male genitalia without broad, 
dorsal flaps... ......5 4.) 4 
4. Abdominal venter with an interrupted, 
longitudinal, median stripe; inner proc- 
esses of male genitalia finger-like at 
apexes; fig. 243 125° 2 eae 1. rapidus 
Abdominal venter unmarked or with only 
faint, lateral, triangular marks on an- 
terior sternites; inner processes of male 
genitalia nipple-like at apexes, fig. 244.. 
eee Une tt atten 4. typicus 
Marure Nympus 
1. Each gill borne by abdominal segments 
3-6 with a dorsal, recurved flap, fig. 
2). SERRE Y Lene eS 2. alternatus 
Each gill borne by abdominal segments 
3-6 simple, without a dorsal, recurved 
flap, fig. 223C......)...6 Gee 2 
2. Abdominal sternites 4-8 each with a pair 
of broad, longitudinal, lateral, brown 
bands and four submedian dots...... 
pos Unie er 5. marshalli 
Abdominal sternites almost entirely 
brown, light yellow only on median, 
triangular area at posterior margin and 
on small area at anterolateral angles 
of each sternite...... 3. quebecensis 
1. Siphlonurus rapidus McDunnough 
Siphlonurus rapidus McDunnough (1924c:75). 
Ma.e.—Membrane of fore wing faintly — 
stained with tan, almost hyaline, costal and 
subcostal crossveins well developed, and 
