LIFE HISTORY OF HYDROUS ( HYDROPHILUS ) TRIANGULARIS. 



29 



FRONT TARSUS OF MALE. 



The last joint of this tarsus is as long as the other four joints, oval in outline 

 and strongly flattened. The dorsal claw is very long and stout, while the ventral 

 one is much reduced in size. Along the inner margin, behind the bases of the 

 claws, is an elliptical area whose ventral surface is covered along its margins with 

 rows of tiny suckers, like those on the tarsi of the male Dytiscus, but much smaller. 

 Those at the distal end of the area are tipped with stiff bristles, the others are naked. 

 A long papilla extends outward from the distal end of the area between the bases of 

 the two claws and is tipped with a tuft of stiff bristles. 

 Another papilla, also tipped with bristles, is found on the 

 ventral surface of the joint near its outer margin, behind the 

 base of the small ventral claw (fig. 22) . 



FOOD. 



Miall stated on the same page quoted above (1895, p. 81), 

 the food of the adult Hydrophilus is largely vegetable, but 

 it will prey upon small aquatic insects." He kept four speci- 

 mens in an aquarium mth plenty of vegetation but no animals 

 larger than Daphnia, and they remained perfectly healthy. 

 Similarly, the present author placed in an aquarium with 

 plenty of vegetation but no animal food two of the beetles that 

 had been reared from pupae. They lived for three weeks and 

 appeared perfectly healthy, eating up the water plants every 

 two or three days. Under natural conditions, however, the 22.— Tarsus of front 

 adults also eat animal food and sometimes kill and eat fish. 



The female that spun the egg case described on page 10 celebrated the com- 

 pletion of her task by killing and eating a small buffalo fish that happened to 

 be in the aquarium. It is quite possible that out in the pond where other animal 

 food was abundant she might have chosen something else; but since there was an 

 abundance of fresh plant food in the aquarium, which she passed by, she evidently 

 craved something containing proteids. The adults sometimes congregate in large 

 numbers and would then become a serious menace if they should choose to attack 

 the yoimg fish. In fact, the adult, although it feeds largely upon vegetation and 

 may live and apparently thrive for a long time without other food, nevertheless 

 stands as a constant potential menace to the breeding of fish. 



RESPIRATION. 



The mode of respiration of the adult Hydrous is peculiar. It was first cor- 

 rectly described by Nitzsch (1811, pp. 440-458; PI. 9) and afterwards in greater 

 detail by Miall (1895, p. 75). This latter account agrees in every detail with that 

 observed by the present author in the American species and may be summarized 

 as follows: , 



In the adult beetle there are two pairs of thoracic spiracles and seven pairs of 

 abdominal spiracles. Those on the meso and metathorax and the anterior abdomen 

 segments are considerably larger than the others, and it is through these that 

 respiration chiefly takes place. The space between the elytra and the dorsal surface 



