242 



BUIXETIN 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 Gyrinidse. 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 LARVi^:. 



THE LARVA. 



The larva itself is its own worst enemy. All the larger larvse among the 

 Dytiscidse, Hydrophilidse, and Gyrinidas are cannibals, and when kept together 

 eat one another relentlessly until only one remaius. Hydrous triangularis spins a 

 silken cocoon in which are deposited about 100 eggs. When these hatch, the 

 tiny larvsB 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 larvse 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 accomphshed by Matheson (1912) for some of the 

 Haliplidse, by Needham and Williamson (1907) for Hydroporus, and by the writer 

 for both Hydroporus and Enochrus. 



