WATER BEETLES IN RELATION TO PONDFISH CULTURE. 



251 



would mob together and seize the fish from different sides, and they kept up the 

 attack mitil the fish succumbed. Of course, when the pond was lowered all its 

 animal fauna was crowded together into a limited space, and the relations of the 

 various species to one another became somewhat abnormal. If these larvae attack 

 fish under such conditions, they might repeat the performance at another time, 

 especially if their food supply became scarce. So far as known no other gyrinid 

 larva has ever been reported as attacking fish. 



The haliplid larvae are all strict vegetarians and are so small and inactive 

 that they do not catch any living animal, much less attack small fish. 



To summarize, therefore, the larvae of Dytiscus, Cybister, and Hydrous are 

 known to eat small fish as a part of their regular diet. The larvae of Dineutes 

 have been observed kilhng fish fry under somewhat abnormal conditions and 

 might do the same thing whenever they had a chance. The larvae of Acilius and 

 Hydrophilus are large enough to overpower very small fish fry, but thus far there 

 is no positive proof that they actually do this. The other beetle larvae are too 

 small to be reckoned as a menace to fish culture. 



With reference to the attacks made by adult beetles upon fish the testimony 

 is much less satisfactory. Von Mentzschefahl in 1778-79 mentioned two species 

 of adult water beetles (Dytiscidae) that attacked the perch in a pond in Silesia. 

 Similar destruction of young fishes by adult water beetles was noted by EUes in 

 1830, by Dale in 1832, by Riley in 1885, and by Dimmock in 1886. 



EUes wrote as follows (1830, pp. 148-149) : 



I observe that one of your correspondents notices the probability of ponds in elevated situations 

 being stocked with fish through the agency of the water beetle. If this active and voracious little 

 creature were really useful in that way, it might in some measure atone for its other mischievous pro- 

 pensities; for I do not know a more destructive little insect to fish themselves, besides devouring the 

 spawn. A neighbor of mine lost several hundred of the fry of the gold and silver fish by this little 

 pest; and, to leave no doubt about the matter, he caught one and placed it in a large basin of water, 

 to which he shortly after added a little fish. The beetle immediately made a dead set at the fish, which 

 completely paralyzed the poor little animal, for it was soon after seized near the tail by the beetle 

 without making any effort to escape, and never left until it was a perfect skeleton similar to numbers, 

 that he had previously found. 



EUes gave no hint as to the genus and species of this beetle, although the 

 query was made by a correspondent in Volume IV of the same magazine, and 

 EUes's attention was called to it by Dale on page 668 of Volume V. 



Riley stated (1885, p. 311): 



The large water insect which attacks and kills young carp is evidently some species of Cybister 

 or Dytiscus of the coleopterous family Dytiscidse. 



Dimmock's testimony was quite different; he said (1886, p. 357) : 



I have seen a dead rat in a small pond surrounded by a great swarm of these beetles (Dytiscidae) 

 and they prefer such food to living food. 



Garman said (1890, p. 163) : 



Both adults and young (Dytiscidse) lead a predatory life, attacking and devouring whatever they 

 can master. They do not hesitate to attack animals many times larger than themselves and are very 

 destructive in fishponds to young fishes. They are in turn eaten by the larger fishes. 



