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 Hydro philus is largely vegetable, but 
it will prey upon small aquatic insects.” He kept four speci- 
mens in an aquarium with 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* nuturah conditions , however, the 
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 young 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. 
Fig. 22. — Tarsus of front 
leg of adult male beetle. 
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 
