LIFE HISTORY OF HYDROUS ( HYDROPHILUS) TRIANGULARIS. 35 
impressed with the magnitude of this agent of destruction. It must cause the death 
of almost unbelievable numbers of these beetles every year. 
Under certain conditions, particularly in early summer, large numbers of 
insects are sometimes found along the shores of the Great Lakes. Needham has 
published a paper on this subject in The Canadian Entomologist for 1904, but 
he failed to mention any of the large water beetles as being destroyed in this 
way; but Carl L. Hubbs, of the Field Museum at Chicago, noted May 20, 1917, 
on the beach near Winnetka, 111., amongst a similar lot of insects a hundred or more 
Hydrous triangularis (unpublished records). These insects had been washed ashore 
after a storm, but it was not ascertained definitely whether they were all dead, 
although it is probable that most of them were. Evidently this is another way in 
which numbers of these beetles may be destroyed yearly. Cinnamon teal, pintail, 
and wood ducks were recorded by D. C. Mabbott (1920, p. 62) as eating species of 
adult Hydrous beetles, but not in sufficient numbers to make them serious enemies. 
GENERAL SUMMARY. 
Both sexes of Hydrous triangularis, in the larval as well as the adult stage, 
feed upon fish fry whenever conditions are favorable, and thus may become a 
serious menace to pondfish culture. If adult fish are kept in the same pond with 
the fry, it is not probable that the beetle larvse will more than hold their own. 
Many adult fish, notably the sunfishes, eat Hydrous larvse and thus keep them 
within due bounds; but if the fry are placed in a separate pond, and especially 
if any effort is made to increase the supply of natural food, precautions against 
the beetles may become necessary. 
There are several ways in which the beetles may be checked. If the pond is 
thickly carpeted with such water plants as Naias or Potamogeton, or is plentifully 
supplied with blanket algse, duckweed, or Elodea canadensis, the danger from these 
water beetles will be greatly diminished; but this_ kind of vegetation is not alto- 
gether desirable in such quantity. 
The mating season of the adults comes about the last of May, and at that 
time both sexes fly about at night in considerable numbers. If a trap lantern be 
constructed according to any of the standard patterns close to the shore of the 
pond and be furnished with a light for two or three weeks at that season, it is 
probable that many of the adult females could be caught before they began to 
lay eggs, and in this way the larvse could be reduced in numbers. 
Again, if the pond be raised 15 or 20 inches for two or three hours once a 
week during July and August when the larvse are pupating, the pupal chambers 
will be flooded and the pupse will all be drowned. This reduces the number of 
adults, and there are not as many to produce larvse; but it is a precaution against 
the future rather than a remedy for the present. 
When full-grown larvse actually appear in sufficient numbers to menace the 
fish fry, they must be removed at once, if the remedy is to be effective. The best 
and practically the only way to do this is to drain the pond, removing the fish fry 
temporarily to another pond or to large aquaria. After the water is out the large 
beetle larvse become very conspicuous and may be quickly and entirely removed. 
