36 ELEMENTARY ENTOMOLOGY 



asphyxiate them through the well-developed tracheal system, though 

 occasionally the valves of the spiracles are so well developed that 

 an insect may keep them closed for a long time, so that fumiga- 

 tion, in order to be fatal, must be prolonged. 



Muscular system. Insects are well provided with powerful mus- 

 cles, a caterpillar having some two thousand. The muscles are 

 yellowish in color, and the fibers are striated as in the voluntary 

 muscles of vertebrates. The simplest type of muscles is found in 

 larvae and in the abdominal segments of adult insects, where the 



OS 



is 



FIG. 41. Muscles of cockroach, of ventral, dorsal, and lateral walls, respectively 



, alary muscle ; abc, abductor of coxa ; adc, adductor of coxa ; &, longitudinal sternal ; //; 



longitudinal tergal ; ///z, longitudinal thoracic ; os, oblique sternal ; ts, tergo-sternal ; /J 1 , 



first tergo-sternal. (After Miall and Denny) 



muscles of each segment are very similar, forming segmented 

 bands on the inside of the body wall. The longitudinal muscles 

 beneath the tergum and above the sternum are arranged so that, 

 when they contract, the body bends in that direction, and by their 

 rhythmic contraction the looping walk of the caterpillar is produced. 

 Oblique-sternal muscles bend the abdomen laterally, and vertical 

 muscles draw the tergum and sternum together in expiration. The 

 thorax of adult insects is filled with the strong muscles which 

 operate the wings and legs, and the muscles which operate the 

 mouth-parts occupy the back of the head. 



