160 COMPARATIVE ZOOLOGY. 
and second left together; and, lastly, the third left and 
second right together. The front and hind pairs are, 
therefore, moved like those of a quadruped. The Insect 
has six legs, each of 
five joints: the coxa, 
or hip; trochanter ; 
Jemur; tibia, or 
shank; and tarsus. 
The last is subdi- 
vided usually into 
five joints and a 
pair of claws. Such 
as can walk upside 
down, as the Fly, 
have, in addition, 
two or three suckers 
between the claws.” 
While the leg-bones 
Fia. 126.—Feet of Insects: A, Bibio febrilis; B, of Vertebrates are 
House - fly (Musca domestica); C, Water - beetle 
(Dytiscus). covered by the mus- 
cles which moved them, the limbs of Insects are hollow, 
and the muscles inside. The fore legs are directed for- 
ward, and the two hinder pairs backward. In motion, 
the fore and hind feet on one side, and the middle one 
on the other, are moved simultaneously, and then the re- 
maining three. 
The four-legged animals have essentially the same ap- 
paratus and method of motion. The Crocodile has an 
awkward gait, owing to the fact that the limbs are short, 
and placed far apart, so that the muscles act at a mechan- 
ical disadvantage. The Tortoise is proverbially slow for 
a similar reason. Both swim better than they walk. Liz- 
ards are light and agile, but progression is aided by a wrig- 
gling of the body. 
The locomotive organs of the mammalian quadrupeds 
