I02 



The Fly of Chironomus 



towards the hinder end of the large segment to which it 

 belongs. 



The chief part of the prothorax of the fly consists of 

 an obliquely placed ring, which encloses the muscles 

 of the fore-leg. On the dorsal surface it appears as 

 a narrow band, with a median incisure and suture. 

 The ring is thicker below, and defined by consjDicuous 

 grooves (fig. 72). At first sight it would appear that 

 this ring formed the whole prothorax. But the tracheal 

 gill of the pupa is certainly prothoracic (p. 142), and the 



scar left upon the 

 thorax of the fly by 

 this tracheal gill 

 must be prothoracic 

 also. We have there- 

 fore to extend the 

 prothorax of the 

 pupa or fly, so as to 

 include the tracheal 

 gill or its scar. The 

 extension has been 

 called the humerus, 

 an unfortunate name 

 for a part of the 

 thorax ; it is the paratreme of Lowne. This part of 

 the prothorax has no clear boundary in Chironomus, but 

 thins away gradually, and passes into the conjunctival 

 membrane ; in some other Diptera it is clearly defined. 



The mesothorax is enormous, and chiefly occupied by 

 the powerful muscles which are directly or indirectl}'' 

 concerned in flight. On its fore edge the anterior thoracic 

 spiracle can be easily made out. The humped dorsal 

 surface shows a prominent semi-cylindrical transverse 

 ridge ; this is the scutellum ; the wings are attached on 

 either side of it. Behind the scutellum the dorsal surface 



???f. 



Fig. 73. — Mesothoracic muscles of fly. ?.hj, lon- 

 gitudinal muscles, v.m, vertical ditto mt, meta- 

 thorax. I'" hind-leg:. 



