44 



ENTOMOLOGY 



ported by the other three legs, as on a tripod. The front leg, having been 

 extended and its claws fixed, pulls the body forward by means of the con- 

 traction of the tibial flexors; the hind leg, on the contrary, pushes the 

 body, by the shortening of the tibial extensors, against the resistance 

 afforded by the tibial spurs; the middle leg acts much like the hind one, 

 but helps mainly to steady the body. Different species show different 

 peculiarities of gait. In its analysis, the walking of an insect is rather 

 intricate, as Graber and Marey have shown. 



The mode of action of the principal leg muscles may be gathered from 

 Fig. 65. Here the flexion of the tibia would cause the tibial spur (s) 

 to describe the line 5 i; and the backward movement of the leg due to the 



. r 



FIG. 65. Mechanics of an insect's leg. a, axis of coxa; r, coxa; cl, claw; e, extensor of 

 tibia; cc, extensor of claw; et, extensor of tarsus; /, flexor of tibia; fc, flexor of claw; //, flexor 

 of tarsus; r, r, rotators of coxa; s, spur; /, trochanter muscle (elevator of femur) ; //, tibia. 

 After GRABER. 



upper coxal rotator r would cause the spur to follow the arc s 3. As the 

 resultant of both these movements, the path actually described by the 

 tibial spur is 5 2; then, as the leg moves forward, the curve is continued 

 into a loop. 



Caterpillars use their legs successively in pairs, and when the pairs of 

 legs are few and widely separated, as in Geometridae, a curious looping 

 gait results. 



The leg muscles of a cockroach are shown in Fig. 66. 



Leaping. The hind legs, inserted nearest the center of gravity, are 

 the ones employed in leaping, and they act together. A grasshopper 

 prepares to jump by bending the femur back against the tibia; to make 

 the jump, the tibia is jerked back against the ground, into which the tibial 



