MUSCLKH AND VISCERA OF THE IMAGO. 223 



which in some cases also surround the membranous trachea, 

 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 si.\ 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 (Libellulas) 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. 



