98 ILtinois NATURAL History SuRVEY BULLETIN 
as Cloeon and Baetis. The median caudal 
filament is vestigial. 
All the nymphs are streamlined, rather 
fishlike forms, and typically vigorous swim- 
mers. The head is not flattened dorsoven- 
trally, as in the Heptageniidae, and the com- 
pound eyes are lateral. Usually, the ab- 
dominal gills are single and more or less 
platelike, but, when they are double, the 
lower element of the pair is not composed of 
a mass of fibrillae, except in the genus 
Isonychia. In various genera, the median 
caudal filament may be either well developed 
or vestigial. 
TO SUBFAMILIES 
ADULTS 
KEY 
1. Vein Me of fore wing detached at base 
from stem of M, figs. 31, 220-222; 
hind wing greatly reduced or wanting 
entirely; ‘hind tarsus with only three 
clearly defined segments, fig. ae. 
ee MP Deere ss Baetinae, Pls 
Vein Mbp of fore wing not detached at base 
from stem of M, figs. 217-219; hind 
wing well developed; hind tarsus with 
four clearly defined segments, figs. 18, 
SL pS SEN aD ine Me eae SA Ree aE 2 
Gill remnants present at base of rudi- 
mentary maxilla and at base of fore- 
COKR Seema chs Isonychiinae, p. 108 
Gillemnsntssabbent -:.2 totus nee aes 
RTE fare ee Siphlonurinae, p. 98 
tro 
Marure Nympus 
1. Each abdominal gill composed of a plate- 
like dorsal element and a ventral fibril- 
lar tuft, fig. 225; fore coxa and maxilla 
with gill tufts. . .Isonychiinae, p. 108 
All abdominal gills platelike; gills usually 
single, but, when double, both elements 
of each gill platelike, figs. 223, 224, 
226-228; fore coxa and maxilla without 
SS feraay aed ae ietae Mects ae helt oar ee ea 2. 
Posterolateral angles of each apical ab- 
dominal tergite prolonged as thin, flat 
spines, figs. 240B, 247, 254; labrum 
with anterior margin entire or with a 
broad, median, V-shaped notch, fig. 
105. DO OL AR Siphlonurinae, p. 98 
Posterolateral angles of apical abdominal 
tergites not prolonged as spines, figs. 
266, 298; labrum with a median, square 
MOtCH, Ge ea Loe ahs Baetinae, p. 113 
nm 
SIPHLONURINAE 
The subfamily Siphlonurinae, as here de- 
fined, corresponds very closely to Ulmer’s 
family Siphlonuridae (1933: 209). 
In the Siphlonurinae, each compound eye 
in the adult males is made up of an upper 
portion composed of large facets and a lower 
Vol. 26, Art. 1 
portion of smaller facets, but the two por- 
tions of the eye are not distinctly separated, 
fig. 241. The fore tarsus in adult males is 
always much longer than the fore tibia. Gill 
remnants are wanting on the head and 
thorax of adults of both sexes. The fore 
wing in this subfamily is readily dis- 
tinguished from the fore wing in all other 
mayflies, in that the cubital intercalary veins 
form a series of parallel, often sinuate but 
usually not branched veins extending from 
vein Cu, to the anal margin of the wing, and 
in that vein Cu, is straight throughout its 
length, fig. 219. In the hind wing, vein M is 
either not forked or forked in the basal half 
of its length. In many species, the wings are 
wholly or partly shaded with brown, yellow, 
or tan, with prominently colored veins and 
crossveins. In the adults, the median caudal 
filament is always vestigial. 
The vigorous, fishlike nymphs, figs. 240B, 
247, of the members of this subfamily are 
strong and rapid swimmers, almost always 
living in rapidly flowing water. The tarsal 
claws of the nymphs are long and slender, 
but are always shorter than the tibiae, fig. 
26. Gills are borne only by the abdomen, 
and these gills are platelike and usually 
single, but when they are double both parts © 
of the individual gill are platelike. There 
are always three well-developed caudal fila- — 
ments. Each cercus has long setae on the 
inner side only. The nymphs of Siphlonurus 
have been shown to be at least in part pre- 
daceous (Morgan 1913:386). The struc- 
ture of the mouth-parts of the nymphs of 
another genus, Parameletus, indicates that it 
also may be predaceous. 
KEY TO GENERA 
ADULTS 
1. Abdominal segments 5-9 with broad, 
flat, lateral expansions, fig. 233; median 
ventral spine present on mesosternum ~ 
and metasternum ...24. Siphlonisca 
Abdominal segments without broad, later- — 
al expansions; no median ventral spines — 
present on thorax.) 7.) Zi 
2. Hind wing with an acute costal angulation, 
and vein M forked near base, fig. 237. 
stare BSGAE tte (CRs on Re Se 25. Ameletus 
Hind wing with a blunt, or with no, costal — 
angulation, and vein M either not 
forked or forked well distad of the base, 
HES. 2Uoo Lorn 3 
3. Vein M of hind wing simple, not forked, 
hes 2NB os eae eee 26. Parameletus 
Wer M of hind wing forked, fig. 219. 
27. Siphlonurus— 
