32 



BULLETII^ OF THE BUREAU OF FISHERIES. 



an abundance reserved for future use. It would seem, then, that if fertilizers are 

 used to increase the food supply the fish breeder must be prepared to find also an 

 increased number of these obnoxious beetles. 



ENEMIES OF THE EGGS. 



The eggs are so well protected in their floating silken case that they are seldom 

 destroyed. Sometimes, however, the egg case gets overturned by being driven into 

 the rushes and other water plants along the shore during a high wind. Apparently 

 this is fatal to the hatching of the eggs. Two such overturned egg cases, with 

 dead eggs partially developed, were found on the shores of pond 2, series E. It 

 is not probable, however, that this accident occurs with any frequency, the cases 

 being so well balanced that it is difficult to overturn them. 



Again, the Ichneumon flies, which work such havoc with the eggs of many 

 other water insects, seem unable to get at the eggs of Hydrous. This is apparently 

 due to the density of the triangular plate that covers the upper, exposed part of 

 the open end of the case and also to the thick layer of porous silk that lines this 

 plate and covers the top of the egg mass, the only points from which the flies can 

 get access to the eggs. 



ENEMIES OF THE LARVi^. 



The Hydrous larva is its own worst enemy, and a very generous percentage of 

 larv9B hatched are eaten by their brothers and sisters. This is especially true if 

 an abundance of attractive food is not forthcoming at the time of hatching, and it 

 continues to operate during the entire life of the larva. Referring to the table 

 already discussed (p. 21), the fact may be emphasized again that they often eat 

 one another voraciously until all but one are gone. By this means alone, therefore, 

 they will be kept within certain bounds, for as soon as there is any danger of over- 

 production other food becomes scarce and they fall to eating one another. It is 

 hardly too much to say that this cannibalistic habit is the salvation of the other 

 insect denizens of the fishpond. Otherwise they would have very little chance in 

 the struggle for existence. 



Certain of the dragon-fly nymphs, especially those of Anax, Aeschna, and 

 the larger Libellulids, eat Hydrous and Dytiscus larvae. This has been discussed 

 in a previous paper upon the economic relations of the dragonflies (Wilson, 1920, 

 p. 201). 



The common bullfrog must be reckoned as a possible enemy of the Hydrous 

 larva. Dyche (1914, specimen No. 129, p. 153) examined the stomach of a large 

 bulKrog, which contained " Si larva over 2 inches long of a water beetle.^' This 

 may have been either Dytiscus or Hydrous, it is not stated which, but if the frog 

 can eat one it can eat the other, and in all probabflity it does so occasionally. 



Several of our common fishes, notably the green sunfish (Lepomis cyanellus) 

 and the bluegill {Lepomis paUidus) are fond of Hydrous larvae and frequently eat 

 them. This has already been noted by Forbes (1888), and his observations have 

 been confirmed at Fairport, the exact data being reserved for future publication. 

 On many occasions during the summer seasons of 1917 to 1921 the author threw 

 full-grown Hydrous larvse either into pond 6, series D, which is stocked exclusively 

 with adult large-mouthed black bass, or into one of the tanks in the tank house, 



