32 
BULLETIN 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 LARVAE. 
The Hydrous larva is its own worst enemy, and a very generous percentage of 
larvae 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 
bullfrog, which contained “a larva over 2 inches long of a water beetle.” T his 
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 probability it does so occasionally. 
Several of our common fishes, notably the green sunfish ( Lepomis cyanellus ) 
and the bluegill ( Lepomis pallidus) 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 larvae 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, 
