Family Baetidae 
Subfamily Oligoneurinae 
Subfamily Ametropinae 
Subfamily Metretopinae 
Subfamily Siphlonurinae 
Subfamily Baetiscinae 
Subfamily Ephemerellinae 
Subfamily Leptophlebiinae 
Subfamily Caeninae 
Subfamily Baetinae 
Subfamily Prosopistomatinae 
The supergeneric classification of the 
mayflies which I have adopted here does 
not coincide exactly with any of the previous 
classifications, although it more closely 
agrees with the classification of Banks 
(1900) than with that of any other author. 
I do not believe that all mayflies can be 
divided into only three main categories with- 
out introducing an unjustifiably large num- 
ber of exceptions into the characterization 
of each of those three main divisions. Al- 
though the mayflies are an extremely archaic 
group, we still have living representatives 
of many of the diverse branches that have 
arisen within the order during its long 
history. As has been shown by Tillyard 
(1925, 1932), the mayflies reached their 
maximum abundance in the Permian Period 
and have declined since. In the long period 
from the Permian to the present, many quite 
distinct types of mayflies have arisen, all of 
which evidently represent considerable di- 
vergence from the Permian mayfly prototype. 
Characteristics of this ancestral mayfly type 
are discussed by Tillyard (1932) and 
Carpenter (1933). 
In our present-day mayfly fauna, we re- 
tain representatives of many of these diverse 
lines of mayfly evolution. The family classi- 
fication should, as much as possible, reflect 
these degrees of divergence from the an- 
cestral mayfly prototype. Certainly there 
are more than 3, and, in my opinion, there 
are at least 11 distinct lines of descent. I 
have accordingly distributed our [Illinois 
mayflies among 10 families; the eleventh, 
Prosopistomatidae, is not represented in 
North America. This family classification 
has been arrived at through an evaluation of 
all available characteristics in nymphs and 
adults. The conspectus of the supergeneric 
classification followed here is given below: 
Order Ephemeroptera 
Family Ephemeridae 
Subfamily Campsurinae 
22 Ittinors Narurat History Survey BULLETIN 
Vol. 26, Art. 1 
Subfamily Potamanthinae 
Subfamily Ephoroninae 
Subfamily Ephemerinae 
Subfamily Palingeniinae 
Family Neoephemeridae 
Family Caenidae 
Family Ephemerellidae 
Family Baetiscidae 
Family Prosopistomatidae 
Family Oligoneuriidae 
Family Leptophlebiidae 
Family Baetidae 
Subfamily Siphlonurinae 
Subfamily Isonychiinae 
Subfamily Baetinae 
Family Ametropidae 
Family Heptageniidae 
It must be admitted that varying degrees 
of relationship are indicated among some of 
these families. “The Caenidae almost cer- 
tainly arose from an ephemerid ancestor, 
as is shown by the still existing but ex- 
tremely rare interstitial forms which are 
placed here in the Neoephemeridae. The 
point of divergence, however, of the Cae- 
nidae from the ephemerid stem must have 
been quite remote. The Ephemerellidae, 
Oligoneuridae, Baetiscidae, and Prosopisto- 
matidae apparently have no near relatives 
in the recent fauna. The Leptophlebiidae 
possibly arose from the same stem which 
produced the Baetidae, although the simi- 
larity between the two is slight. On the 
other hand, the Heptageniidae and the 
Baetidae must have arisen from an ancestor 
common to both of these families; the rare 
but still existing interstitial forms between 
these two families indicate this probable 
relationship. 
The forms here included in the Baetidae 
admittedly represent rather widely divergent 
types. The Siphlonurinae contain quite 
archaic forms that (Spieth 1933: 329) prob- 
ably arose very early from the Permian 
mayfly prototype, while the Baetinae con- 
tain greatly reduced adult forms that prob- 
ably arose much later. The baetine and 
siphlonurine nymphs are, however, quite 
similar, and most adult baetine structures 
can be derived by simple reduction from 
corresponding adult structures in the Si- 
phlonurinae. The divided compound eyes of 
the adult male baetines, on the other hand, 
seem to be strikingly different from the eyes 
of the siphlonurines, but actually the begin- 
nings of the development of this divided eye 
