48 



THE BIOLOGY OF INSECTS 



fibres — slender contractile threads extending in the direction 

 of the length of the muscle, which may thus be considered 

 as composed of many bundles of fibres. It has already been 

 mentioned (p. 2), that the muscles of insects resemble the 

 skeletal muscles of vertebrates in being composed of striped 

 or striated fibres (Fig. 16). With a comparatively low degree 

 of magnification it is seen that these fibres exhibit alternating 

 darker and lighter transverse bands ; they are striated. On 

 studying the structure of a single fibre with a high degree 

 of magnification, it is seen in places to split lengthwise into 



Fig. 16. — Microscopic structure of mandibular Muscle of Hornet 

 ( Fespa crahro). A, part of fibre treated with potash ; X 425. B, the same 

 stained with haematoxylin. Sarc, sarcolemma ; Id., Dobie's Hne ; 

 da, thickenings of fibrillae ; T, air-tube ; N, nerve end-plate. C, 

 partial cross-section at Dobie's line showing radiating fibrils ; D, partial 

 cross-section through median region of a sarcomere showing fibrillae 

 and nuclei (nuc), X 850. From C. Janet, Etudes sur les Fourmis les 

 Guepes et les Abeilles, xii, 1895. 



excessively fine threads or fibrils, while the cross-striation 

 becomes so definite as to suggest that the fibre is built up 

 of an innumerable series of discs arranged one on the other ; 

 a very rough model of its apparent structure might be made 

 with a pile of pennies and half-crowns arranged alternately, 

 and such transverse dissociation of a fibre can be brought 

 about by treatment with gold chloride. But the fibre never 

 spHts naturally into discs as it does into fibrils, and the 

 alternating darker and lighter transverse regions are indica- 

 tions of physical differentiation in the substance of the fibre. 



