ANATOMY AND PHYSIOLOGY 



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FIG. 109. 

 Striated muscle 

 fiber of an insect. 



unlike that of the abdomen, and in the head it is decidedly different; 

 though future studies will doubtless show that the thoracic and cephalic 

 kinds of musculature are only modifications of the simpler abdominal 

 type modifications brought about in relation to the 

 needs of the legs, wings, mouth parts, antennas and other 

 movable structures. 



The muscular system has been generally neglected 

 by students of insect anatomy; the only comprehensive 

 studies upon the subject being those of Straus-Diirckheim 

 (1828) on the beetle Melolontha; Lyonet (1762), New- 

 port (1834) and Lubbock (1859) on caterpillars; and 

 the more recent studies of Lubbock and Janet on Hy- 

 menoptera. 



The more important muscles in the body of a cock- 

 roach are represented in Figs. 106-108, from Miall and 

 Denny. The longitudinal sternals with the longitudinal tcrgals act to 

 telescope the abdominal segments; the oblique sternals bend the abdomen 

 laterally; the tergo-sternals, or vertical expiratory muscles, draw the 



tergum and sternum together. The 

 muscles of the legs and the wings have 

 already been referred to. 



Structure of Muscles. The mus- 

 cles of insects differ greatly in form and 

 are inserted frequently by means of 

 chitinous tendons. A muscle is a 

 bundle of long fibers, each of which 

 has an outer elastic membrane, or 

 sarcolemma, within which are several 

 nuclei; thus the fiber represents several 

 cells, which have become confluent. 

 With rare exceptions (" alary" muscles 

 and possibly a few thoracic muscles) 

 the muscle fibers of an insect present a 

 striated appearance, owing to alternate 



light and dark bands (Fig. 109), the former being singly refracting, or 

 isotropic, and the latter doubly refracting, or anisotropic. 



The minute structure of these fibers, being extremely difficult of 

 interpretation, has given rise to much difference of opinion. The most 

 plausible view is that of van Gehuchten, Janet and others, who hold that 

 both kinds of dark bands (Fig. no) consist of highly elastic threads of 



FIG. no. Minute structure of a 

 striated muscle fiber. A, longitudinal 

 section; B, transverse section in the 

 region of /; C, transverse section in the 

 region of 11. I, longitudinal fibrilke; 

 11, Krause's membrane; ;//, nucleus; 

 r, radial fibrilke; s, sarcolemma. After 

 JANET. 



