52 ENTOMOLOGY 



means of the contraction of the tibial flexors; the hind leg, on the con- 

 trary, 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. 67. Here the flexion of the tibia would cause the tibial spur 

 (s) to describe the line si ; and the backward movement of the leg due 

 to the upper coxal rotator r would cause the spur to follow the arc 

 53. As the resultant of both these movements, the path actually 



ec 



FIG. 67. Mechanics of an insect's leg. a, axis of coxa; c, coxa; d, claw; e, extensor of 

 tibia; ec, extensor of claw; et, extensor of tarsus;/, flexor of tibia ;fc, flexor of claw; ft, flexor 

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

 After GRABER. 



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. 68. 



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 spurs are driven, and the straightening of the leg by means of the 

 powerful extensors throws the insect into the air. At the distal end of 

 the femur are two lobes, one on each side of the tibia, which prevent 

 wobbling movements of the tibia. 



