LIFE HISTORY OF HYDROUS ( HYDROPHILUS) TRIANGULARIS. 
31 
The four ponds of series E are as nearly alike in all their relations as possible. 
However, No. 1 was covered during the summer of 1918 with an abundance of 
blanket algae, largely Hydrodictyon, while there were only floating patches along the 
shores of the other three ponds. No Hydrous beetles, larvae, or pupae were found in 
or around pond No. 1, although diligent search was made for them; in the other three 
ponds they were very abundant. The banks of earth separating ponds 1 and 2 
and 2 and 3 are as nearly alike in size, height, and vegetation as could be desired. 
And yet when pond No. 2 was drained and the adult beetles were compelled to seek 
shelter elsewhere, almost without exception they went to pond No. 3 and not to 
pond No. 1. 
Again, in series F pond No. 1 was not disturbed in 1918, while ponds 2 and 3 
were drawn and cleared of water plants. In consequence pond 1 was carpeted 
with a dense growth of Elodea canadensis, excluding almost everything else, while 
ponds 2 and 3 contained only small amounts of vegetation. Repeated searching 
failed to discover a single hydrophilid or dytiscid beetle in pond 1, although there 
were plenty of haliplids, but they were all abundant in ponds 2 and 3, although 
only a few of the genus Hydrous itself were obtained. 
In series D, ponds 1, 3, and. 6 were filled with water plants in 1918, 1 and 6 
containing abundant blanket algae, while 3 was entirely carpeted with Naias and 
Potamogeton. Beside the Haliplidae almost no beetles were obtained from these 
three ponds, while in ponds 2, 4, 5, and 7 other families were abundant both in 
species and in actual numbers. 
From these data it would seem fairly conclusive that the adult beetles both 
possess and exercise a definite choice of habitat; and though a moderate amount of 
water vegetation may prove attractive it is possible to have too much, and thereby 
repel the beetles. Hence, if the fishpond be kept abundantly supplied with water 
plants there will be little danger from Hydrous ; it will not breed in sufficient numbers 
to become a menace. 
RELATION TO FERTILIZATION. 
In order to secure the propagation of small Crustacea, insect larvae, and other 
minute organisms in great numbers as food for young fishes, various organic fertilizers 
have been used. Our knowledge of the economic importance of these fertilizers, and 
especially of the relative value of the different kinds, is still very rudimentary and 
incomplete. We do know, however, that by placing well-rotted manure in a limited 
amount over the bottom of a pond that has no appreciable current running through 
it the supply of animal food for young fishes will be greatly increased. An experi- 
ment of this nature was conducted in pond No. 2E during the season by H. W. 
Clark, of the Fairport staff. The pond was drained, manured, and refilled in May. 
The fact we wish to note here is that the presence of the manure proved attractive 
to beetles, especially the Hydrophilidae, as well as to other insects and Crustacea. 
When the pond was drained in July, it proved to be literally swarming with beetle 
larvae and adults, while the pupae were equally abundant around the shores. In 
all, 15 species of beetles were obtained from this one small pond, an eighth of an 
acre in area, with the larger genera, Hydrous, Dytiscus, and Hydrophilus in especial 
evidence. All the material for the present paper was secured here, and in addition 
