50 The Form of Insects 



Having thus briefly noted the minute structure of j 

 insect muscle, we turn to consider the arrangement j 

 of the muscles in the cockroach. In our own bodies, i 

 as in those of all back-boned animals, the muscles i 

 surround the hard skeleton, to various parts of which j 

 they are attached. But in insects, as in all arthropods, ] 

 the hard skeleton is formed from the outer skin; con- i 

 sequently the muscles are fixed to the inner surfaces j 

 of the body and limb-segments, which they move. j 



If a Cockroach's body be opened (fig. 36), a sheet { 

 of muscle, divided transversely into sections, is seen 1 

 on either side of the nerve-cord, stretched length- | 

 wise along the lower wall of the abdomen, from the | 

 front edge of the second sternite to the front edge of j 

 the seventh ; these are the lotigitudi?ml sternal muscles ' 

 of the abdomen. Beneath the upper wall of the 

 abdomen, a series of longitudinal tergal muscles is seen 

 (fig. 37), each set connecting the front edge of a 

 segment with the front edge of that next behind ; it 

 is evident that the contraction of these longitudinal 

 muscles tends to telescope the segments one into the 

 other. Very short muscles {oblique sternals) connect 

 the adjacent edge of the sternites of the abdomen, 

 while corresponding ones above (oblique tergals) join 

 the edges of the tergites ; these probably serve to 

 bend the abdomen towards the side. Each tergite 

 is connected with its corresponding sternite by a 

 tergo-ster7ial or expiratory muscle ; it is clear that the 

 contraction of these must bring the upper and lower 

 walls of the abdomen closer together — that is, their 

 action lessens the capacity of the abdomen (see figs. 

 36 and 37). 



In the thorax, on account of the modification of 

 the segments and the presence of legs and wings, the 

 arrangement of muscles is more complicated. Sets 

 of longitudinal tergals are present, similar to those 



