20 Ittinoris NarurAL History Survey BULLETIN 
that the median caudal filament is wanting, 
but actually this filament is simply reduced 
to a minute, unsegmented or partly seg- 
mented vestige. In all adult mayflies there 
is at least some indication of it. The outer 
caudal filaments are the cerci. 
An arbitrary system for the designation 
of sizes of specimens has been followed in 
this report. The size of an adult specimen 
is taken to be the length of the body with- 
out the head or fore legs and without the 
caudal filaments. Specimens having such 
a body length of 5 mm. or less are called 
small, those from 6 to 10 mm. long are 
called medium, and those over 1 cm. long 
are designated as large. The length of the 
body in nymphs is without the head or 
caudal filaments, and the sizes given refer 
to mature nymphs, that is, ones with dark 
wingpads. 
Identification of Sexes 
The adult male mayfly is easily recog- 
nized by the presence of a pair of forceps, 
or claspers, near the apex of the ninth 
sternite, as well as by two shorter structures, 
the penes, between the arms of the forceps. 
The forceps, except in the subfamily Camp- 
surinae, are segmented, and both forceps 
and penes vary greatly in size and form, figs. 
60-67. These same structures can be seen 
through the pellicle of the subimago and 
of the mature nymph, although in these two 
stages they are soft and less sclerotized 
than in the adult. They are not fully ex- 
panded in the subimago, and much less de- 
veloped in the nymph. 
In the female, the posterior margin of the 
ninth abdominal sternite is rounded, simple, 
and without prominent processes or addi- 
tional structures. In a few genera, the 
female possesses a rudimentary ovipositor. 
Classification 
The order Ephemeroptera is one of the 
most archaic of winged insect groups. It 
is not closely related to any other order but, 
as Tillyard (1917) points out, certain points 
of resemblance give some slight evidence of 
a very ancient connection with the Odonata. 
These points of resemblance, not common to 
other orders, are the presence of only one 
wing axillary, the inability to fold the wings 
backward or downward over the abdomen, 
and the retention of abdominal gills in 
Ephemeroptera and in a few _ primitive 
Vol. 26, Art. 1 
nymphs of Odonata (Cora and Pseudo- 
phaea). The fossil record indicates that 
the two orders were already differentiated 
in the Upper Carboniferous Period but 
were then more closely related than they are 
today (Tillyard 1917:6). 
At the time Eaton wrote his Revisional 
Monograph (1883-88), the mayflies were 
considered to constitute the single family 
Ephemeridae. However, several years 
earlier Eaton (1869:132) had indicated 
that the family Ephemeridae could be sub- 
divided into three major divisions, based on 
the habits and structures of the nymphs: 
(1) the burrowing forms with tusked man- 
dibles, (2) the flat, crawling forms, and (3) 
the rather long, slender, free-swimming 
forms. In the Rewisional Monograph, he 
divided the adults into three groups; these 
groups somewhat paralleled the divisions he 
had previously suggested, based on the 
nymphs. He further subdivided these three 
groups into 13 generic types; the three 
groups were not named as taxonomic cate- 
gories. 
Banks (1900: 246) published a classifica- 
tion of the mayflies, considering them to 
represent but the one family Ephemeridae, 
but dividing this family into seven tribes: 
Baetiscini, Polymitarcini, Leptophlebini, 
Siphlurini, Ephemerini, Baetini, and Caenini. 
Needham (1901:419) published a key to 
the nymphs of the family Ephemeridae and 
indicated that this family could be divided 
into three subfamilies, the Ephemerinae, 
Heptageninae, and Baetinae. Needham 
mentions (1905:29, footnote) that this key 
indicated subdivisions of the Ephemeridae 
into subfamilies which were very similar 
to those given in a manuscript key prepared 
earlier by C. A. Hart for use by students 
at the University of Illinois. 
A few years later, Needham (1905: 22) 
published a revised key to the family 
Ephemeridae in North America. In this 
key, which included both nymphs and adults, 
he again divided the Ephemeridae into the 
three subfamilies Ephemerinae, Hepta- 
geninae, and Baetinae. These three sub- 
families corresponded only very roughly to 
the three groups into which Eaton had di- 
vided the family Ephemeridae in his classi- 
fication. 
Klapalek (1909) then published a greatly 
expanded classification of the mayflies, based 
on German species, dividing the order into 
