i66 THE IXTEGUMEXTAL SKELETON OF THE IMAGO. 



In the Diptera the dorsal arch of the prothorax forms a 

 narrow rim, which is not visible without dissection. 



It would, perhaps, be hazardous to give a more detaile4 

 description of the thorax applicable to insects generally, 

 and any attempt to describe the various modifications of 

 the exo-skeleton would be foreign to my purpose. I shall, 

 however, in the special description of the thorax of the Blow- 

 fly, give such comparative details as are necessary to the 

 correct interpretation of the homologies of its several parts. 



b. General Description of the Thoracic Skeleton of the Blow-fly. 



The nomenclature I employ is a modification of that pro- 

 posed by Osten-Sacken [81] for the sutures, and, as far as 

 possible, that of Audouin [38 and 39] for the sclerites of the 

 meso- and metathorax. 



The Thoracic Skeleton forms a subovoid capsule, with a 

 cervical opening in front, an abdominal opening behind, and 

 three pairs of ventral foramina with which the basal joints of 

 the legs articulate. There are two large spiracles on each 

 side, the anterior and posterior. It presents for examination 

 a dorsal, a lateral, a ventral, an anterior, and a posterior aspect. 



The Dorsal Aspect (Fig. 35) exhibits two transverse sulci, 

 prescutal and postscutal. The anterior or prescutal sulcus 

 is faint and separates the prescutum (j*) from the scutum 

 {2) ; the posterior or postscutal sulcus is deep and well marked, 

 and divides the scutum from the scutellum (/). 



The Prescutum is very convex from before backwards ; its 

 anterior part, which cannot be seen from above, descends 

 vertically to the border of the cervical opening. 



The Scutum [2) exhibits a lateral projection, the scutal spine 

 {Milii) {4), behind the anterior root of the wing (j). In front 

 of the spine is a depression in which the anterior wing root lies, 

 the pre-alar fossa ; behind the spine there is a smaller fossa, 

 the post-alar fossa ; and externally to this a deep triangular 

 fissure, the supra-tympanic fissure. 



* The numbers and letters refer to all the figures. 



