THORAX OF MUSCID FLIES 75 



plate of the metathorax, and should be termed the metanotum. The 

 point is of importance from the systematic point of view, as most writers 

 have followed the nomenclature of Theobald when describing species. 



In the Muscid flies the thorax is much rounder and more compact 

 than it is in Tabanus, and is without a definite ' keel ' to the ventral 



surface. The various parts are so welded together, 



. .. ... ... . Stomoxys (Plate XVI) 



and the various authorities hold such divergent views 



regarding the homology of the different sclerites, that it would be 

 hopeless to attempt to give an acceptable account of the subject within 

 a short space. The main divisions are, however, easily recognized. 

 The whole of the dorsal wall as seen externally belongs to the meso- 

 thorax. It is divided about the middle by a deep transverse fissure, 

 the anterior part, as in Tabanus, being the praescutum, and the poster- 

 ior the scutum. Posterior to this there is the scutellum, like that of 

 Tabanus, but generally more conspicuous and pointed. The ventral 

 surface is almost entirely occupied by two large quadrilateral plates, the 

 mesosterna ; behind these there are several irregularly shaped plates be- 

 longing to the metathorax. On the anterior aspect of the thorax there 

 are two prominent convexities between the praescutum and the anterior 

 end of the sternum, which represent the pronotum. On the lateral 

 aspect there are two plates between the dorsal wall and the sternum ; 

 the anterior of these is the episternum, and the posterior the epimeron, 

 of the mesothorax. These are 'separated from one another by a 

 conspicuous vertical suture. In the space anterior to and below the 

 wing base, and in the angle between the episternum and the epimeron, 

 there are several small sclerites, one of which is specially distinguished 

 as the tegula. * 



The internal structure of the thorax .resembles in general that of 

 Tabanus. The thoracic inlet has, however, more definite boundaries, 

 and the fibrous bands which unite the median and lateral walls behind 

 the neck are well developed, and are here termed the hypotremes. 

 There is a medi-furca resembling that of Tabanus, a small post-furca, 

 and a mesophragma, though this is not nearly so well developed as in 

 the Tabanidae. 



-From the foregoing it will be evident to the reader that the ter- 

 minology of the thorax is at present in an unsatisfactory state, and 

 must remain so until a great deal of attention has been devoted to 

 the subject. Nevertheless, some settled method of nomenclature is a 



* This term is applied by some writers to a division of the wing base. 



