242 
BULLETIN OF THE BUREAU OF FISHERIES. 
FOR SEX FUNCTIONS. 
Certain structural modifications that are evidently designed for sex functions 
are found in the males of these beetles. Amongst these is the enlargement of one 
or both of the anterior pairs of tarsi, which is carried much farther than in terrestrial 
beetles. Attention has already been called to the smooth and polished surface of 
the adult beetle as a help in locomotion through the water; but such a surface is 
very difficult to grasp and hold by any ordinary means. Accordingly, the tarsi 
of the males are modified in many of the genera by the dilation of the basal segments, 
by the formation of palettes or suckers on the ventral surfaces* of these expanded 
segments, and by the secretion of a peculiar viscid fluid. This fluid is secreted by 
glands inside the tarsus and is carried by ducts to the base of the columella or stalk 
on which the palettes are borne. The stalk is hollow and conveys the fluid to the 
disk of the palette whenever the latter is pressed against any surface. The fluid 
spreading over the surface of the disk affords a secure attachment on any smooth 
surface to which the disk may be applied. The arrangement of the dilated segments 
and their disks will be described under the separate species. 
FOR SIGHT. 
Another remarkable structural modification is concerned with sight and 
furnishes the most distinctive characteristic of the Gyrinidae. Their eyes are 
completely divided by the lateral margins of the head, leaving half of each eye 
on the upper surface of the head with which to look into the air and half on the 
lower surface with which to look into the water. They are thus admirably suited 
for their whirligig mode of life on the surface of the water. No such modifications 
are found in any of the other families of water beetles. 
ENEMIES OF LARVAE. 
THE LARVA. 
The larva itself is its own worst enemy. All the larger larvae among the 
Dytiscidae, Hydrophilidae, and Gyrinidae are cannibals, and when kept together 
eat one another relentlessly until only one remains. Hydrous triangularis spins a 
silken cocoon in which are deposited about 100 eggs. When these hatch, the 
tiny larvae begin their warfare on one another as soon as they are born, and many 
of them are eaten before they ever leave the cocoon. Not even the presence of an 
abundance of suitable food affects their cannibalism in the slightest. The first 
question for settlement with them is the survival of the fittest, and as long as 
another of their kind is present and the two can find each other, this question 
demands an immediate answer. Some of the smaller species of these three families 
and all the haliplid larvae are free from cannibalism. In consequence they may be 
safely kept together in an aquarium and their entire life history worked out in 
detail. This has actually been accomplished by Matheson (1912) for some of the 
Haliplidse, by Needham and Williamson (1907) for Hydroporus, and by the writer 
for both Hydroporus and Enochrus. 
