238 



BUIJ^TIN OF THE BUREAU OF FISHERIES. 



interrogatusj common; Tropisternus glaher, two specimens; Berosus striatus, abimdant; 

 B, perigrinus, one; PMlydrus ochraceus, one; Peltodytes edentulus, a few; Laccophilus 

 maculosuSj one. Yet Berosus striatus and Peltodytes edentulus were two of the 

 beetles that were not found in pond 9D, although they were common m 8D, next 

 to it. 



DISTRIBUTION ACCORDING TO SIZE. 



Needham and Williamson, in the paper already referred to, while discussing 

 the distribution of the Dytiscidae in the ^^Gym" pond at Lake Forest, made the 

 following statement (1907, p. 478) : 



The shoreward distribution of these beetles corresponds roughly with their size; the largest are 

 found in the deepest water, the smallest nearest shore. 



On the following page this statement is somewhat modified. 

 It must not be understood that there is any such definite and sharply limited zonal distribution 

 as aquatic plants on such a sloping shore often exhibit. That is not to be expected in animals possessed 

 of such excellent powers of locomotion. We have meant to indicate merely the favorite haunts of 

 each species, and the general correspondence between the size of the beetle and the depth of the water. 



If the beetle list be extended to include the Hydrophilidse and Haliplidse, the 

 modified statement probably still holds good, but it does not apply at all to the 

 Gyrinidae. Moreover, it is profoundly modified by the breeding habits of the 

 various species. During the breeding season beetles of all sizes gather indiscrimi- 

 nately in the shallower waters near the shore for the purpose of depositing their 

 eggs. In June, July, and August, the months during which the present investiga- 

 tions have been conducted, there is practically no distribution according to size. 

 The same sweep of the net will capture Hydrous triangularis, the largest beetle in 

 the ponds, and such microscopic forms as Laccohius agilis and Hydrovatus pus- 

 tulatus. Another sweep will bring up Dytiscus Tiyhridus and Bidessus lacustris, 

 with beetles of various intermediate sizes. HydropMlus ohtusatus and Tropis- 

 ternus nimhatus are nearly always associated, and yet the former is fully twice the 

 size of the latter. After the breeding season is over, however, the beetles probably 

 separate more or less according to size, the same as the other denizens of the pond. 



STRUCTURAL ADAPTATIONS. 

 FOR LOCOMOTION. 



Doctor Sharp (1882) in his monograph on the Dytiscidse called attention to 

 certain structural modifications which help in adapting the beetles to their aquatic 

 life. These were afterwards explained in greater detail by Needham and William- 

 son (1907) and may be briefly summarized as follows: The three principal adapta- 

 tions that are concerned in locomotion are an increased body rigidity, a diminished 

 resistence to the water, and an increased swimming efficiency of the hind legs. 

 Body rigidity has been secured by compacting together the various parts of the 

 body, thereby greatly reducing, and usually entirely eliminating, the flexibility 

 found in land beetles. The head is immersed in the front of the thorax; the three 

 divisions of the thorax and the abdomen segments are tightly telescoped together; 

 the elytra are closely joined to the prothorax in front, to each other along the mid 

 line, and to the sides of the abdomen along their lateral margins. 



