MUSCLES AND VISCERA OF THE IMAGO. 223 



which in some cases also surround the membranous tracheae, 

 and to certain muscles closely related to the spiracles, to be 

 described hereafter, which I regard as inspiratory. 



6. The Great Thoracic Muscles. -These are the largest muscles 

 in the insect, and occupy the greater part of the thoracic 

 cavity. They are the dorsales and sterno-dorsales. 



The dorsales are six large muscular bands on each side of the 

 median line, which extend from the mesophragma and post- 

 scutellum to the anterior three-fourths of the dorsal shield ; 

 they occupy the whole dorso-central region. These muscles 

 are regarded by Straus Durckheim as representing the longi- 

 tudinal dorsal muscles of the larva of the corresponding 

 segment, and Van Rees believes that he has traced the direct 

 conversion of one into the other in the fly nymph. I shall 

 hereafter give reasons for differing from Van Rees in this 

 respect. 



As Hammond [78] has pointed out, the dorsales in the fly 

 are entirely mesothoracic. In many insects both a meso- and a 

 metathoracic set are developed. In the Bees the latter are 

 quite rudimentary, and occupy the interior of the post-scutellum. 

 In the Coleoptera the meta- and not the mesothoracic set 

 are the only ones developed. 



The sterno-dorsales are three large bundles of muscle fibres, 

 which extend from the lateral regions of the dorsum to the 

 meso-sternum, and one which extends from the dorsum to the 

 ridge above the posterior spiracle, on each side. These muscles 

 are external to the dorsales in position ; their direction is from 

 above downwards, and from before backwards. They are 

 usually regarded as the antagonists of the dorsales (see p. 187). 

 In the Dragon-flies (Libellula) there are eight sets of these 

 muscles, four in the meso- and four in the metathorax, and 

 they are inserted directly into the wing-roots by cupules, or 

 cup-shaped apodemes, which terminate in rods attached 

 directly to the wing-sclerites. In all other insects they are 

 inserted into the dorsum, and not into the wing-roots. As in 

 the case of the dorsales, it is apparent, therefore, that all these 

 muscles in the Blow-fly belong to the mesothorax. 



