BURROWING MAYFLIES. 275 
The mayflies are the chief herbivores among the insects of the stream bed. That 
they are not more numerous there is doubtless due to the remarkable abundance of 
carnivorous dragonflies and stoneflies always associated with them. In this population 
both pursued and pursuers fall into two principal ecological groups, according as they 
burrow in the sand and gravel of the stream bed or live in the water above it. The 
mayflies of the genera Hexagenia and Pentagenia and the dragonflies of the genus 
Gomphus are all true burrowers, possessed of flattened and more or less shovel-like, 
digging, front feet (see Pls. LXXII and LXXIV), have the hind legs appressed to 
the body and adapted for pushing, and have the front of the head sloping forward 
and somewhat pointed. There are also many special adaptations to burrowing, among 
which none is more remarkable than the development in these mayflies upon the 
front of the mandibles of a pair of long, strong, upcurving, and pointed tusks that are 
driven forward into the soil and upon which the roof of the burrow is lifted, opening a 
subterranean passageway. 
When a burrowing mayfly nymph is thrown out upon the surface of the sand, it digs 
in again more quickly than a mole. A few thrusts of the tusks forward, a few tosses of 
its head upward, a few side sweeps with its broad front feet, and it disappears from view 
beneath the sand. 
The mayflies of the genera Polymitarcys and Chirotenetes and the stoneflies of the 
genus Acroneuria live above the stream bed and do not burrow. They prefer the 
shelter of stones or of timbers but occur occasionally in more open places. In Dr. 
Howard’s collections these two mayflies are more frequently associated with one another 
than with any other species, and neither is once taken in association with either of the 
burrowing mayflies named above. 
Polymitarcys and Chirotenetes have little more in common, however, than a 
habitat. They are very different in form and also in manner of life. Polymitarcys is 
a bottom sprawler, depressed, flat, hairy, protectively colored, and inactive. Chiro- 
tenetes is an agile swimmer and an artful dodger, with body compressed, smooth, and 
of beautiful stream-line form. It gathers its food from the passing current by means 
of plancton-retaining fringes of hairs that margin the fore legs (Clemens, 1917). Poly- 
mitarcys is a member of the subfamily Ephemerine and has for its nearest allies the 
burrowing mayflies above discussed. Its mandibles are tusked (PI. LX XVIII, fig. 41), 
but the tusks are not upcurving and are not used for burrowing; they are laid out 
flat upon the bottom as are also those of the allied Euthyplocia (PI. LX XIX, fig. 48). 
Chirotenetes is a member of the subfamily Betine, a group in which there are no bur- 
rowers. This is the only member of the group that appears abundantly in the collec- 
tions from the Mississippi River, and, beyond the figures of Plate LX XXII illustrating 
Chirotenetes siccus, the group receives no further treatment in this paper. 
In January A. F. Shira, director of the Fairport station, sent me the insect con- 
tents of nine stomachs of river herring that had been collected during the two preceding 
seasons and observed to contain mayflies, so far as determinable, as indicated in the 
following table: 
