CHAPTER 9 



Muscles and Movements 



All the muscles of insects, whether they be 'visceral muscles' sur- 

 rounding the alimentary canal or the heart, or 'skeletal muscles' 

 actuating the appendages, are made up of striated muscle-fibres. The 

 histology of these fibres varies widely. They may consist of bundles 

 of fibrillae with inconspicuous striations, enclosed in lobulated 

 masses of cytoplasm ; fibres like those of vertebrates with highly de- 

 veloped striations and peripheral nuclei; 'tubular muscles' in which 

 the nuclei form an axial core along the fibre ; or the peculiar 'fibrillar 

 muscles' which compose the indirect flight muscles of Hymenoptera 

 and Diptera. Each giant 'fibre' in this last type of muscle consists 

 of a bundle of very large fibrils or sarcostyles 2-5-3 y. in diameter, 

 with rows of giant mitochondria or 'sarcosomes' lying between them. 

 Where the muscles are attached to the body wall it is usually possible 

 to see fibrillae ('tonofibrillae') passing through the epidermal cells 

 and running into the substance of the cuticle. In most of the skeletal 

 muscles it is possible to observe a double innervation : one nerve is 

 responsible for rapid muscular contractions, the other for the slow 

 contractions of muscle tonus. In a few insects a third axon with an 

 inhibitory effect is present. 



The physiological properties of insect muscles 



Some insects are able to lift weights of greater mass than their own 

 bodies, and leaping insects can project themselves great distances 

 through the air. But these achievements are a simple result of their 

 body size. For the power of a muscle varies with its cross-section, 

 that is, with the square of one linear dimension ; while the volume 

 or mass of the body varies with the cube of the linear dimensions. 

 Consequently, as the body becomes smaller the muscles become rela- 

 tively more powerful. The absolute power of a muscle is defined by 



