18 Ittinois NaturAL History Survey BULLETIN 
is called Ry, in this report. The branches 
have been named M, and M, by Ulmer 
(1933, pls. I, IL) and 6 and 6! by Eaton 
(1883, pls. I-X XIII). The vein Ry; is of 
greatest importance, as it is the best land- 
mark that can be used in identifying the 
Vol. 26, Art. 1 
veins of the mayfly fore wing. Once it has 
been located (except in the greatly special- 
ized Oligoneuriidae), all other veins anterior 
and posterior to it can quickly be located. 
Traver considered Ry,; to be a branch of 
R,, whereas Tillyard believed it to be a 
fore wing 
claw 
\ #--tarsus 
foreleg \ 
\ NITY “pe 
‘fLompound eye 
< Lal / pronotum ; 
‘( / stigmatic 
TRL ZA —S- 
Nites 
7k; veins 
oe fer fork 
a 
A 
hind leg 
aS 
hind wing . a , 
[acs] 
DA] 
ep 4-forceps 
f \-cercus 
4 F \ 
f H a 
} ! \ 
; \ 
NH \ 
f Ny 
\ 
f --median \ 
+ caudal \ 
ff 1 filament \ 
Y \) \\ 
1 
iJ 
Fig. 7—Generalized mayfly adult male, showing structures used in classification. 
