WATER BEETLES IN RELATION TO PONDFISH CULTURE. 
249 
BEETLES AS FISH EATERS. 
The food of the various larvae and adults will be given under the separate 
species, and it will be sufficient here to make a few general statements. On the 
whole, the larvae of the Dytiscidae, Hydrophilidse, and Gyrinidae are carnivorous, 
while those of the Haliplidae are strictly vegetarian. Of the adults the Dytiscidae 
and Gyrinidae eat animal .food, whereas the Hydrophilidae and Haliplidae eat veg- 
etable food of various kinds. Neither the plant-eaters nor the animal-eaters are 
strictly confined to their own kind of diet, however. Under stress of hunger or 
when for any reason the natural food supply fails they both show great ability to 
change their diet. Some species probably combine the two kinds under normal 
conditions. The thing that concerns us most in connection with their food is the 
fact that some of them eat small fish, and this is worthy of careful consideration. 
In considering this problem it will be convenient to separate the larvae and 
adults and discuss them separately. 
In searching for the food of the larvae no information can be obtained from an 
examination of the contents of the stomach and intestine if the larva belongs to 
the Dytiscidae, the Gyrinidae, or the Haliplidae. In these three families the larval 
mandibles are either grooved or pierced for the passage of fluids only. Hence, 
the fluid contents of the food are all that is swallowed, and obviously they furnish 
nothing that could identify the species of plant or animal from which they came. 
The hydrophilid larva, on the contrary, chews up its prey and swallows the 
solids as well as the liquids. Instead of being pierced or grooved its mandibles 
are toothed on the inner margins to fit them for chewing. Accordingly, in this 
family we can obtain the very best information by an examination of the con- 
tents of the stomach and intestine. This has already been done with the larvse of 
Hydrous triangularis, and fish remains were found in the stomachs of 12 out of 
50 larvse examined (Wilson, 1923). In the present investigation the stomach 
contents of 50 larvae of two species of Tropisternus have been examined, but no fish 
remains were found in any of them. The larvae of Berosus are too small to 
yield much that is of value from such examinations. For these small hydrophilids 
and for all the larvae in the other families actual observation is the only source of 
information in reference to the food. The problem of beetle larvse as fish eaters, 
therefore, narrows down to answers to the following questions: What kinds of 
beetle larvse have been seen eating fish and under what conditions ? What kinds 
are likely to eat fish either under normal conditions or from stress of hunger ? Of 
the Dytiscidae the larvse of various species of Dytiscus and Cybister have been 
observed eating fish even when an abundance of other food was available. By 
reason of their voracity these larvse are commonly called water tigers. Kellogg, 
in his American Insects (1908, p. 257), said of the Dytiscidae: 
Both larvse and adults are fierce and voracious, and the larger species attack and kill small fish. 
In the middle States these beetles actually do much damage in carp-ponds. 
When stationed at the U. S. Fish Hatchery in Homer, Minn., H. L. Canfield had 
occasion late in August, 1913, to draw down one of the ponds containing small- 
mouthed black bass fingerlings from 2 to 2J inches in length. As soon as they felt 
