May, 1953 
rare circumstances, have interfered with 
the normal rhythm of successive-day emer- 
gence. A few of the larger species have 
Fig. 2—Nymph of Hexagenia limbata. 
been known to form dense clouds and to 
settle in great drifts over roads, bridges, and 
streets. These fragile insects die within a 
few hours, and when occurring in such 
hordes their dead bodies may clog ventilator 
Burks: THE Mayr.tts oF ILLINoIs 3 
ducts and sewers and may also cause 
temporary trafic difficulties, fig. 1. On 
July 23, 1940, the Associated Press carried 
the following dispatch: 
“Sterling, Ill—Shadflies that in some 
places piled to a depth of four feet blocked 
trafhe over the Fulton-Clinton highway 
bridge for nearly two hours last night. 
“Fifteen men in hip boots used shovels 
and a snow plow to clear a path. The 
bridge appeared to be covered with ice and 
snow. ‘Trucks without chains were unable 
to operate until most of the flies had been 
shoveled into the Mississippi river.” 
In both aquatic and terrestrial stages, 
figs. 2 and 3, mayflies achieve their chief 
importance as food for other animals. They 
are preyed upon by birds, fish, amphibians 
(frogs and salamanders), spiders, and many 
predaceous insects. It is as a natural food 
for fish that they are of primary economic 
value. 
The first extensive observations on the 
role of mayflies as fish food were made by 
Forbes (1878-1888). They were based on 
examinations of the stomachs of Illinois 
fishes. Since the observations by Forbes, 
many contributions to this subject have 
been made by many authors, but no attempt 
is made here to collate the material in the 
limnological literature. 
In general, it has been found that the 
diet of fishes consists of the most readily 
available suitable food. Consequently, fish 
of the same species in a body of water will 
be found to have quite different organisms 
in their stomachs at different seasons of the 
year. At certain times of year, mayflies 
are abundant in lakes or streams, and at 
these times are readily eaten by fish. Both 
adult and nymphal mayflies are eaten by the 
fish, the adults either when molting at the 
surface of the water, or when alighting 
later to lay eggs. 
Mayflies have been found in the stomachs 
of most species of the larger Illinois fishes, 
including all the sport fishes, such as crap- 
pies, bass, and various other types of sun- 
fish. There is little doubt that fish of many 
species feed extensively on mayflies, and 
that, at times of great mayfly emergence, 
the fish of a considerable number of species 
subsist chiefly on these insects. 
That fish will consume mayfly nymphs 
readily was shown in an interesting way 
in New Zealand. The New Zealand may- 
