238 
BULLETIN OF THE BUREAU OF FISHERIES. 
interrogatus, common; Tropisternus glaber, two specimens ; Berosus striatus, abundant; 
B. perigrinus, one; PTiilydrus ochraceus, one; Peltodytes edentulus, a few; Laccophilus 
maculosus, one. Yet Berosus striatus and Peltodytes edentulus were two of the 
beetles that were not found in pond 9D, although they were common in 8D, next 
to it. 
DISTRIBUTION ACCORDING TO SIZE. 
Needham and Williamson, in the paper already referred to, while discussing 
the distribution of the Dytiscidse 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 Hydrophilid8e 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 Laccobius agilis and Hydrovatus pus- 
tulatus. Another sweep will bring up Dytiscus Tiybridus and Bidessus lacustris, 
with beetles of various intermediate sizes. Hydrophilus obtusatus and Tropis- 
ternus nimbatus 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 Dytiscidas 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 pro thorax in front, to each other along the mid 
line, and to the sides of the abdomen along their lateral margins. 
