ENTOMOLOGY 



FIG. 66. 



Among adult insects, female scale insects are exceptional in 

 being legless. 



Walking. An adult insect, when walking, normally uses 

 its legs in two sets of three each ; thus the front and hind legs 

 of one side and the middle leg of the other move forward 

 almost simultaneously though not quite, for the front leg 

 moves a little before the middle one, 

 which, in turn, precedes the hind leg. 

 During these movements the body is sup- 

 ported by the other three legs, as on a 

 tripod. The front leg, having been ex- 

 tended and its claws fixed, pulls the body 

 forward by means of the contraction of 

 the tibial flexors; the hind leg, on the 

 contrary, pushes the body, by the short- 

 efning 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 peculiari- 

 ties 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 ^ i; and the backward movement of 

 the leg due to the upper coxal rotator r 

 would cause the spur to follow the arc s j. 

 As the resultant of both these movements, 

 the path actually described by the tibial 

 spur is s 2; then, as the leg moves forward, the curve is con- 

 tinued into a loop. 



Caterpillars use their legs successively in pairs, and when 

 the pairs of legs are few and widely separated, as in Geomet- 

 ridae, a curious looping gait results. 



Muscles of left mid 

 leg of a cockroach, pos- 

 terior aspect. abc, ab- 

 ductor of coxa; adc, 

 adductor of coxa; ef, 

 extensor of femur; et, 

 extensor of tibia; if, 

 flexor of femur; ft, 

 flexor of tibia; fta, 

 flexor of tarsus; rt, re- 

 tractor of tarsus. After 

 MIALL and DENNY. 



