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BULLETIN OF THE BUREAU OF FISHERIES. 



abundant and seven common, in the latter only one was abundant and six common. 

 Here, again, we must seek an explanation in the physiological differences of the two 

 ponds, due to the dissimilar character of the vegetation. Pond 9D contained an 

 abundance of the blue-green alga (Aphanizomenon jlos-aquse) . Dr. Moore stated 

 (1920, p. 12) that when first called to her attention '^It was so abundant that the 

 water appeared blue-green and oily." Multiplication continued to take place 

 with great rapidity until the maximum was reached in the interval of July 24 to 

 30, when the algae could be rolled up from the bottom like mush." The beetles 

 seem to have been as much prejudiced against mush under foot as they were 

 against an algal blanket overhead. 



In series E pond 2 was manured in 1918 and pond 3 in 1919. As a result they 

 proved very attractive to the beetles, and pond 2 had the highest number of dytiscid 

 species and the largest aggregate of hydrophilid individuals of all the ponds ex- 

 amined. It is significant that these two ponds in 1920 were covered with a heavy 

 blanket of Hydrodictyon, which greatly reduced their beetle fauna both in numbers 

 and variety. The dytiscids and gyrinids practically all removed to other ponds, 

 leaving only the haliplids and a few hydrophilids. Furthermore, in 1920 and 1921 

 the blue-green algae in pond 9D almost disappeared, and as a result the beetle fauna 

 noticeably increased. The influence of the algae, therefore, is only temporary, 

 and once they are removed the beetle fauna quickly recovers. 



RESTRICTION OF SPECIES. 



Species are often restricted to one or two ponds and are not found in any of 

 the others. In the table here presented 8 species are confined to a single pond and 

 10 others are found in only 2 or 3 ponds, usually contiguous in the same series. 

 This is a third of the entire number, and there are also some of the others whose 

 distribution is far more limited than the table would seem to indicate. 



In contrast with these there are certain other species that may be classed as 

 cosmopolitan, for the Haliplidae {Peltodytes edentulus), for the Dytiscidae (Lac- 

 copMlus maculosus), for the Gyrinidae {Dineutes americanus) , and for the Hydro- 

 philidae {Tropisternus lateralis and T. gldber) are found in all the ponds except one 

 or two. Hydrous triangularis is also reported from every pond except two, but 

 it can not be included with these others because it is restricted in its breeding and 

 the larvae rarely appear in more than a single pond during any one season. 



In 1920 Thermonectes ornaticollis was present and breeding in large numbers in 

 pond 12B but was found only rarely in any of the other ponds. Two specimens 

 of this species were all that were obtained in 1916 — one in pond 8D and one in 

 pond IE. Similarly, Thermonectes hasilaris was confined to pond 5D in 1919, to 

 pond 8D in 1920, and to pond 7D in 1921. Cyhister fimhriolatus, although the 

 adults have been found in a dozen ponds, including some of every series, has never 

 been known to breed anywhere except in 5D, with the exception of a single pupa 

 found on the shore of 13B in the summer of 1921. In general, ponds 2, 3, 4, 5, 

 and 7, D, and 2 and 3, E, are the richest both in variety of beetle species and in 

 nimibers of individuals, while the B series of ponds are the poorest. 



