186 ENTOMOLOGY 
Locomotion. 
tion are found in the common Hydrophilus triangularis (Fig. 
226). Its general form reminds one of a boat, and its long 
Excellent adaptations for aquatic locomo- 
legs resemble oars. ‘The smocthly elliptical contour and the 
polished surface serve to lessen friction. Owing to the form 
of the body (Fig. 227, A) and the presence of a dorsal air- 
HIG: 227; 
/ 
; 5 ie 
h 
y 
A B 
Transverse sections of (4) Hydrophilus and (B) Notonecta. e, elytron; hh, hemely- 
tron; 7, metathoracic leg. 
chamber under the elytra, the back of the insect tends to re- 
main uppermost, while in Notonecta (Fig. 227, B), on the other 
hand, the conditicns are reversed, and the insect swims with 
its back downward. ‘The legs of Hydrophilus, excepting the 
first pair, are broad and thin (Fig. 228, A) and the tarsi are 
fringed with long hairs. When swimming, the “ stroke” is 
made by the flat surface, aided by the spreading hairs; but on 
the “recover,” the leg is turned so as to cut the water, while 
the hairs fall back against the tarsus from the resistance of the 
water, as the leg is being drawn forward. ‘The hind legs, 
being nearest the center of gravity, are of most use in swim- 
ming, though the second pair also are used for this purpose; 
indeed, a terrestrial insect, finding itself in the water, instinc- 
tively relies upon the third pair of legs for locomotion. Hy- 
drophilus uses its oar-like legs alternately, in much the same 
sequence as land insects, but Cybister and other Dytiscide, 
which are even better adapted than Hydrophilus for aquatic 
locomotion, move the hind legs simultaneously, and therefore 
can swim in a straight line, without the wobbling and less 
economical movements that characterize Hydrophilus. 
