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, Hydrophilidae, 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 himger 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 imder 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 larvae of 

 Hydrous triangularis, and fish remains were found in the stomachs of 12 out of 

 50 larvae examined (Wilson, 1923). In the present investigation the stomach 

 contents of 50 larvae of two species of Tropistemus 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 hydro philids 

 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 larvae as fish eaters, 

 therefore, narrows down to answers to the following questions: What kinds of 

 beetle larvae 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 larvae 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 larvae are commonly called water tigers. Kellogg, 

 in his American Insects (1908, p. 257), said of the Dytiscidae: 



Both larvae 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 2 J inches in length. As soon as they felt 



