The Mayfrlies, 
or Ephemeroptera, 
of [llinots 
AYFLIES or shadflies are a group 
of insects constituting the order 
Ephemeroptera. In the young or 
nymphal stages, they live in the water of 
ponds, lakes, or streams, where they can be 
found under rocks or logs, in the mud at the 
bottom, or occasionally swimming about. 
When the nymphs are full grown, they come 
to the surface of the water and transform 
into free-flying aerial insects. As such, they 
are familiar to many fishermen and nature 
lovers. 
In Illinois, a few large, conspicuous forms 
come to general attention every year when 
they emerge on warm midsummer evenings 
in enormous numbers from our larger lakes 
and rivers. However, these constitute only 
a relatively small part of the mayfly fauna 
of the state. Other forms are to be found 
emerging at various times of the year from 
all the relatively permanent and unpolluted 
bodies of water, including ponds, lakes, 
brooks, creeks, and rivers. 
The mayfly may be distinguished readily 
from all other aquatic insects. The nymph 
has a definite head, thorax, and abdomen. 
It has three pairs of well-developed legs, 
a pair of gills on each of the middle ab- 
dominal segments, and either two or three 
long “tails” (called caudal filaments) ex- 
tending from the posterior end of the body. 
It more closely resembles the stonefly nymph 
than any other nymph but differs from it in 
having gills on the middle abdominal seg- 
ments. 
Unlike most insects, the mayfly typically 
has two winged stages. It is the only exist- 
ing insect that molts after getting functional 
wings. The first winged stage is called 
the subimago, which is actually a subadult 
stage; soon after it is formed this subimago 
Bi 2D) BUCKS 
(in most species) molts to form the true 
adult or reproductive stage, sometimes called 
the imago. In a very few species, noted 
later, which never develop to the adult, the 
female lays her eggs while in the subimago 
stage. 
The subimago is very similar to the adult 
in appearance, but the body and all append- 
ages are incased in a transparent skin or 
pellicle. The adult has its mouthparts and 
alimentary system represented by only mi- 
nute, distorted vestiges; it usually has two 
pairs of extremely thin and papery, trian- 
gular wings (the posterior pair being much 
smaller, or lacking in a few species), which 
are held upright and not folded above the 
back when the insect is at rest. As in the 
nymph, the adult has two or three long, 
well-developed caudal filaments; if the 
median one appears to be lacking, it will be 
found on close examination to be represented 
by at least a small rudiment. 
There are over 550 different species of 
mayflies known for North America north 
of Mexico. This report includes 48 genera 
and 222 species, with Illinois records of 126 
species, 15 of which are described as new. 
Importance of Mayflies 
Although, at times of unusual abundance, 
the adults of a few species may swarm to 
lights and become an expensive nuisance 
in towns and cities near rivers and lakes, 
mayflies are, on the whole, harmless and 
gentle creatures. Some species apparently 
do not even indulge in the activity of swarm- 
ing. The nymphs are likewise innocuous 
except in two exotic species of Povilla. One 
species of this genus, found in the East 
Indies, bores into wood submerged in fresh 
water, often seriously weakening or de- 
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