LIFE HISTORY OF HYDROUS ( HYDROPHILUS) TRIANGULARIS. 



21 



2. The young larvse up to 30 mm. in length eat nothing beside these two foods 

 except one another, for they are confirmed cannibals, and when first hatched, 

 unless there is plenty of acceptable food, and sometimes even when such food is 

 abundant, they eat one another voraciously until all but one are gone ; and some of 

 them keep up this cannibalism when they are fully grown, in proof of which witness 

 the accompanying records of Nos. 19, 30, 46, and 51. By this means their numbers 

 are greatly reduced and they are naturally held in check. On reaching a length of 

 30 mm. they begin to seek larger prey and attack other beetle larvae, dragonfly 

 nymphs, tadpoles, and small fish. By the time they reach 40 mm. in length their 

 diet has become quite varied. Nos. 19, 30, 39, 42, 50, 51, and 52 show a very 

 mixed diet and, especially the last two, probably ate everything they could capture. 



3. They do eat fish and in considerable quantity; 10 of the 52 had eaten fish 

 to the extent of 30 to 75 per cent of their total food, and there were a few fishbones 

 in the intestine of another larva. 



In judging the damage thus inflicted, however, we must remember several 

 facts. First, all the larvae examined were taken from this one pond which contained 

 only young buffalo fish {Ictiohus cyprinella), the hatch of the year. These little 

 buffalo fish ate some of the smaller Hydrous larvae, and possibly these constituted as 

 large a percentage of their food as they themselves contributed to the food of the 

 larger larvae; but there were no fish to keep down the full-grown beetle larvae, as 

 there would be in a pond containing both adults and young. Consequently, the 

 contest between the fish and the beetle larvae was one-sided and very much in favor 

 of the larvae. Again, the presence of thousands of these young fish in the confined 

 area of a small pond gave the beetle larvae another advantage. The fish were 

 comparatively easy to catch, and there was abundant opportunity to prey upon 

 them. Accordingly, the percentages here obtained are probably above the average 

 and are certainly higher than would be expected in a pond containing adult fish 

 as well as young. Furthermore, when the larva does eat fish there is enough for 

 it to fill up on without eating anything else. 



But after making due allowance for all these considerations the fact still remains 

 that the Hydrous larva does eat fish when it has an opportunity, and therefore 

 becomes a menace to successful fish raising in ponds. The methods to be used in 

 guarding against this menace will be discussed later. 



THE PUPA. 

 PUPATION. 



When the larva is fully grown, it assumes a uniform brownish-black color, as 

 already stated, there being no difference in shade between the dorsal and ventral 

 surfaces. It then crawls out of the water and searches for a place to bury itself. 

 The distance to which it travels varies considerably and may be from 2 to 8 or 10 

 feet from the water's edge. The selection of a spot apparently depends upon the 

 consistency of the soil as well as upon its moisture content. Black clayey loam is 

 preferred to sandy soil, but under compulsion almost any kind of soil will answer. 

 Two larvae were kept in an aquarium where nothing but clear sand was available, 

 and in this they buried, pupated, and finally emerged without apparent incon- 

 venience. 



