56 Ittinois NatrurAt History Survey BULLETIN 
for example, has only a few dozen adults 
but hundreds of nymphs. 
In the Nearctic region, the family Ephem- 
erellidae includes one genus, Ephemerella. 
13. EPHEMERELLA Walsh 
Ephemerella Walsh (1862:377). 
Drunella Needham (1905:42). 
Chitonophora Bengtsson (1908: 243; 1909:6). 
Torleya Lestage (1917:366). 
Timpanoga Needham (1927:108). 
Eatonella Needham (1927:108). 
This genus now includes more than 80 
Nearctic species, only 11 of which have, so 
far, been taken in Illinois. Quite a few more 
species of Ephemerella might, however, 
Vol. 26, Art. 1 
basal three of which are short and approxi- 
mately equal in length; the apical segment 
is usually as long as the basal three com- 
bined. Each arm of the male genital forceps 
has three segments, the basal one short, 
broad, and usually indistinctly set off from 
the distal ones; the second segment is long 
and somewhat bowed; and the third segment 
is usually minute, although it is fairly long 
in a few species. The penis lobes invariably 
are fused at the base on the meson; in most 
species, this mesal fusing extends almost to 
the tips of the penis lobes. There are three 
long caudal filaments. 
The nymphs, figs. 172 and 173, are more 
or less flattened dorsoventrally. In each, the 
Fig. 115.—Ephemerella argo, head of adult male, anterior aspect. 
Fig. 116.—Ephemerella lutulenta, hind wing of adult. 
Fig. 117.—Ephemerella ora, head of adult male, anterior aspect. 
eventually be found to occur in the smaller 
streams in the northern tier of Illinois 
counties. For that reason, a number of 
extralimital, northern species of Ephemerella 
have been included in the keys and discus- 
sions given below. 
In each fore wing of the Ephemerella 
adults, the costal crossveins are entirely 
wanting or a few apical ones are very 
weakly indicated, the stigmatic crossveins 
are weak and usually slightly anastomosed, 
and all other crossveins are weak, fig. 29. 
All the longitudinal veins are well indicated. 
Vein Cu,, at mid-length, is bent at right 
angles toward the wing margin. The hind 
wing, fig. 116, has a subangulate costal 
projection and weak crossveins. 
In adult males, the large compound eyes 
are almost or quite contiguous on the dorsal 
meson and each is rather indistinctly di- 
vided, figs. 115 and 117, with the ventral 
portion smaller and darker than the dorsal 
portion. In both sexes, the hind tarsus has 
four clearly differentiated segments, the 
head is relatively small, and the body is 
often as broad across the middle abdominal 
segments as across the thorax. The integu- 
ment in the Ephemerella nymphs is more 
heavily sclerotized than it is in most mayfly 
nymphs. In many species of Ephemerella, 
the nymphs are provided with prominent 
dorsal spines and tubercles. In some western 
species, the nymphs have each an ingenious, 
ventral, abdominal sucker-disc that permits 
them to cling to rocks in the rapid streams 
in which they live. In various species, the 
abdomen bears either four or five platelike 
gills; the second segment is invariably with- 
out gills. The first pair of platelike gills 
sometimes forms a sort of operculum which 
covers the more caudal pairs of gills. Struc- 
tures of the individual gills of two species 
of Ephemerella are shown in figs. 170 and 
171. There are three well-developed caudal 
filaments. 
In the figures of male genitalia included 
here, ventral spines borne by the penis lobes 
are shown by broken lines; dorsal spines are 
; 
