114 InLinois NATURAL 
of development may appear quite unlike the 
female nymphs. 
The nymphs, figs. 266, 298, are stream- 
lined and fishlike in body form, each with 
a labrum having a square notch on the meson 
of the anterior margin, fig. 231, one or two 
pairs of wingpads, slender, denticulate, and 
single tarsal claws, figs. 264, 265, usually 
single and platelike gills, and two or three 
well-delevoped caudal filaments. 
This entire subfamily is very difficult to 
treat taxonomically, as really good structural 
characters for the separation of species have 
not yet been found. The various species are 
at present distinguished almost entirely on 
differences in the color patterns which, un- 
fortunately, in most species are subject to 
considerable variation. The differentiation 
of species throughout this subfamily is, thus, 
made on a rather insecure basis. It often 
is not possible to separate the females of 
this subfamily to genus. 
KEY TO GENERA 
Aputt MALEs 
1. Fore wing and hind wing with relatively 
numerous crossveins, figs. 221, 222.. 
Se Pte te ath Set ee ete 29. Callibaetis 
Fore wing with relatively few crossveins, 
fig. 220; hind wing with very few cross- 
veins or with none, figs. 270-284, or 
hind wing wanting entirely......... 2 
2. Hind wing present, although often greatly 
reduced 3 
Pid wine absent) teen. cries esas 5 
3. Marginal intercalary veins of fore wing 
single, as in fig. 221.30. Centroptilum 
Marginal intercalary veins of fore wing 
Mapas ness 220; 229 anette 4 
4. Hind wing greatly reduced and either 
without venation or with traces only of 
a single longitudinal vein............ 
Plat hte cero f 31. Heterocloeon 
Hind wing relatively well developed, with 
two or three longitudinal veins, figs. 
PID eee aero MUR ane cicpoaciate 32. Baetis 
5. Marginal intercalary veins of fore wing 
Me PAS AS NES 220 ee og else eee 
Pa rein muha Leah este 33. Pseudocloeon 
Marginal intercalary veins of fore wing 
Bitigles ARNON OL ee eee 6 
6. Second forceps segment of male with a 
prominent, angular projection on mesal 
margin, fig. 299. .34. Neocloeon 
Second forceps segment of male simple, 
without a mesal projection, fig. 300. . 
35. Cloeon 
Mature Nympus 
1. Gills single, platelike on all abdominal 
BECIDENIS ME D28 5A. ihn. bar it 
Gills double on at least some abdominal 
segments, or each gill a thin, somewhat 
History Survey BULLETIN 
Vol. 26, Art. 1 
irregular sheet with a recurved, dorsal 
or ventral flap, as in fig. 226 6 
2. Hind wingpad absent........,j0 ame 3 
Hind wingpad present...... 1). ee 4 
3. Maxillary palp with two segments; median 
caudal filament usually vestigial...... 
SOA alt pink rene - Pseudocloeon 
Maxillary palp with three segments; 
median caudal filament well developed 
rc Ae ee ce Pete ene 34. Neocloeon 
4. Median caudal filament as well developed 
as the cerci....... 30. Centroptilum 
Median caudal filament reduced or ves- 
tigial. 6... 0a oe ee 
5. Suture between second and third seg- 
ments of labial palp partly or com- 
pletely obliterated; second segment 
without an apicomesal projection, fig. 
261, median caudal filament vestigial 
a Glen pehee oe ae 31. Heterocloeon 
Suture between second and third segments 
of labial palp well marked; second seg- 
ment with an apicomesal projection, 
figs. 258, 259, 262, 263, median caudal 
filament reduced or vestigial See 
5 sof SSSA ERR a 32. Baetis 
6. Hind wingpad absent....... 35. Cloeon 
Hind wingpad present... . ..j.0eee 7 
7. Maxillary palp with three segments; each 
abdominal gill with an inconspicuous 
dorsal. flap soins 30. Centroptilum 
Maxillary palp with two segments; each 
gill borne by abdominal segments 1 and 
2 with two well-developed laminae, 
each gill borne by more posterior seg- 
ments with only a relatively small, re- 
curved ventral flap, fig. 226...... 2 a8 
Satine ie 2 oa ete 29. Callibaetis 
29. CALLIBAETIS Eaton 
Callibaetis Eaton (1881:196). 
In Callibaetis, the upper portion of each 
compound eye in the males is stalked, but 
this stalk is relatively low, fig. 257. The 
width of the vertex separating the compound 
eyes in the females is about twice as great 
as the length of one eye. In the fore wing 
in the males, the basal costal crossveins are 
weak or wanting, but these crossveins are 
well developed in the fore wing in the 
females. The fore wing in the males usually 
is not pigmented, but in the females it is, 
at least in the costal and subcostal inter- 
spaces in Nearctic species. Two general 
types of arrangements of the crossveins of 
the fore wing are to be found in this genus: 
in one type, there are relatively few cross- 
veins, with none very near the posterior 
wing margin, and the crossveins form a 
single irregular row across the wing, fig. 
221; in the other type, there are relatively 
numerous crossveins, some of them located — 
near the posterior wing margin, and the — 
