WATER BEETLES IJ^T RELATION TO PONDFISH CULTURE. 



265 



also treated alike the previous year; that is both were wintered full and not dry. 

 Yet pond 2 contained more than twice as many beetle species as pond 1, and while 

 in the former seven species were abundant and eight were common, in the latter 

 only two were abundant and four common, and all six were small species and 

 mostly vegetable eaters. 



A partial explanation of such a radical difference may be found in the state- 

 ment of Dr. Moore (1920, p. 11), which was as follows: 



Physiologically, however, they are more or less distinct because of the dissimilar character of the 

 vegetation in them. Pond ID has been richly stocked with floating algse, which at times have covered 

 the surface. Few of the larger-rooted aquatics are present. Pond 2D has no algal mats or blankets, 

 but fully one-tenth of the surface area has been covered by the large-rooted aquatic Potamogeton illino- 

 ensis, interspersed in places with the nonrooted Ceratophyllum, or hornwort. 



These two ponds furnish such striking differences in their beetle faima that 

 they have been photographed as illustrations of the presence and absence of floating 

 algae and are shown in Figures 6 and 7 as they appeared in 1921. In pond 

 ID (fig. 6) the algal blanket described by Dr. Moore has persisted from year 

 to year and has covered practically the entire surface. As a result the beetle fauna 

 of 1916 has steadily declined in numbers and variety until in 1921 there were only 

 straggling specimens left, outside of the haliplids. The dytiscids were reduced to 

 the minute species of Bidessus and Hydroporus, and the hydrophilids to those of 

 Berosus and Philydrus. One of the species that was abundant in 1916 (LaccopUlus 

 maculosus) had entirely disappeared, and the other {Tropisternus lateralis) was 

 reduced to a minimum and would have to be classed as rare. 



In contrast with this the photograph of pond 2D (fig. 7) shows no algse but 

 a limited area of Potamogeton. This also has persisted from year to year and has 

 increased in extent until in 1921 it covered about a third of the entire surface. 

 The beetle fauna has steadily increased in number and variety, particularly of the 

 dytiscids and hydrophilids. The seven species that were abundant in 1916 have 

 remained so, and five of the eight species that were then common have become 

 abundant. 



A similar result has been observed to follow the presence of duckweed on the 

 surface of a pond. The httle pond, 13B, only 25 feet square, was faMy swarming 

 with hydrophilid beetles in 1919. The latter part of that summer, however, it 

 developed a blanket of duckweed which remained through 1920 and 1921. 

 In consequence every beetle species except the haliplids disappeared. We may 

 reasonably conclude, therefore, that the presence of a blanket of any sort that 

 covers the entire surface of a pond is extremely unfavorable to beetle development. 

 The diving beetles require open water in order to obtain their an- supply, and 

 even the scavenger beetles are seriously hindered by such a blanket in securing 

 sufficient oxygen. The gyrinids are, of course, absolutely banished, since there 

 is no water surface left upon which to perform their whirligig movements. The 

 little haliplids are the only ones that can successfully compete with such restrictions. 



Ponds 8 and 9, D, are also contiguous, are practically the same size and depth, 

 and are surrounded with identical vegetation. Pond 8D, however, contained more 

 than twice as many beetles as 9D, and whereas in the former six species were 



