WATER BEETLES IN RELATION TO PONDFISH CULTURE, 



245 



SNAILS. 



Cooke (1895), in his ^'Molluscs/* of the Cambridge Natural History series, 

 noted on page 34 that Lillian's pond snail {Lymnsea stagnalis lillianse) feeds on 

 dytiscid larvae, snails, and minnows. This fact, of course, suggests that it may 

 also eat the larvae of other water beetles, and that other carnivorous snails may 

 also feed upon them. 



PARASITES. 



The parasites do not usually get a chance to infest the larva until it comes 

 out of the water to pupate, and the injury that they inflict is not manifest until the 

 pupal stage. For that reason this enemy belongs to the pupal stage and will be 

 discussed there. 



ENEMIES OF PUPi«. 



WATER. 



Strangely enough the worst enemy of the pupa is undoubtedly water, although 

 both the larva and the adult live in the water. The pupa possesses none of the struc- 

 tural modifications found in the larva and adult which enable them to swim and 

 breathe while in the water, and hence it is absolutely helpless in such a medium. 

 Furthermore, it is confined within the narrow pupal chamber, whose diameter is 

 often less than its own length. If this pupal chamber becomes filled with water 

 the pupa is quickly drowned, and this is also true of the larva after it has burrowed 

 into the ground and before it has transformed into a pupa and of the beetle imago 

 after emerging from the pupa and before leaving the pupal chamber. The devices 

 possessed by the larva and adult for maintaining a supply of fresh air will not 

 function within the narrow confines of the pupal chamber. There is thus often 

 two or three days previous to pupation and two or three days after emergence 

 as well as the entire pupal stage during which the presence of water in any amount 

 is fatal. When the larva comes out of the pond and burrows into the ground, he 

 stakes his very life upon the weather conditions during the next two or three weeks. 

 If it is pleasant or with only a normal amount of rain, the chances are all in the 

 larva's favor; but if there comes a severe storm and the pupal chamber is filled 

 with water, either from surface drainage or from the raising of the surface of the 

 pond, it results in the death of the larva, pupa, or imago, whichever is caught in the 

 chamber. In digging for pupae along the shores of the fishponds in 1920 many pupal 

 chambers were opened that had been flooded in this manner. Four larvae of 

 Hydrous triangularis ^ 19 pupae, and 2 imagos were found dead; 2 larvae of Tro- 

 pisternus lateralis, 8 pupae, and 1 imago; 2 pupae and 1 imago of TJiermonectes 

 ornaticoUis. The absence of smaller larvae and pupae was probably due to the fact 

 that they had been carried off bodily by ants. All the larger ones had been thor- 

 oughly decimated by the same scavengers and much of the softer tissues removed. 

 48790°— 23 2 



