Muttkowski—The Fauna of Lake Mendota. 
443 
larger fish have difficulties in breaking and swallowing them be¬ 
cause of their rather pointed ends and the considerable stiffness 
of the tubes proper. 
As for repellant secretions, most of the red Hydrachnida are 
refused by the general macrofauna, but not by the red Hydrach¬ 
nida themselves. Apparently, therefore, they are obnoxious or 
distasteful; yet individuals of Atax turgidus, Eylais, Hygro- 
bates, and other red genera are too frequently present in fish 
diet to permit a sweeping statement as to their offensiveness. 
This applies equally well to other types, such as the various 
Corixae and Gyrini, especially to the adults of the latter. Fish 
of various species frequently gorge themselves on Corixae, and 
adult of Gyrinus are not uncommon in fish stomachs. Yet these 
are all well-known instances of supposedly well-protected 
species,—at least that has been the assumption of the 
purpose of the emanations from these species. The facts of 
fish food would tend to force a change or modification of these 
assumptions. 
As a whole, the place of insects in the fish diet is a variable 
one, and fiuctuates with the habitat and the species of fish. In¬ 
sects compose about 60% of the macrofauna of the lake, exclud¬ 
ing fish, and their bulk percentage in fish food is equal, or near, 
to the same figure. Taking the insects as a unit, the Trichoptera 
form 30%, and the Corethridae and Chironomidae 65% of the 
supply. The actual use, however, appears to be Trichoptera 
5%, Corethridae and Chironomidae 90%. 
The Lake Toll .—As an irregular source of fish food must be 
mentioned the thousands of insects which are blown from the 
land, and such that venture too near the surface in the chase 
of food and become water-trapped. 
In the first category can be placed myriads of Lachnosterna, 
mayflies, Corethra, bees, ants of various species, wasps, and 
many other, less common insects. In the second category be¬ 
long primarily Odonata and Asilidae. Although expert fliers, 
Odonata frequently become water-trapped in a too eager quest 
of prey, often of their own kind. Yet they are often able to free 
themselves, especially if the surface is somewhat disturbed; it 
seems that they wait for an opportune wave-crest to lift them 
slightly above the surface and this is sufficient to give them the 
initial freedom of wing movement. 
