LIFE HISTORY OF HYDROUS ( H YDROPHILUS ) 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 Haliplids 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 sufiicient 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 



