V THE THORACIC EXO-SKELETON. 169 



pleural, and mesopleural sutures and the wing, there is a 

 complex region, the pleuron, consisting of the episternum {28), 

 the epinieron {26), and the costa {25). Behind this region and 

 above the posterior spiracle a convex protuberance is seen. 

 This I term the tympanic bulla («), and between the tympanic 

 bulla and the scutellum there is a large convex plate {22), the 

 lateral plate of the post-scutellum. 



All these sclerites, except the nietasternum, belong to the 

 mesothorax. Behind the metasternum are the small plates 

 of the metapleuron (41 and 42), with the halter (0) above them. 

 These parts are better seen from behind and below. 



In front of the anterior spiracle there is a curved plate, which 

 appears in the figure somewhat like a pistol handle. Bur- 

 meister [8] called it the scapula, and identified it correctly 

 with the tegula of the Hymenoptera and the scapula of Straus 

 Durckheim. In the Diptera, Fabricius described it as ' punc- 

 tum callosum ante alam.' It was also called the humerus by 

 the earlier writers. I shall term it the paratreme. It is the 

 representative of the tegula in the Hymenoptera, the operculum 

 of Chabrier. Hammond [78] recognised the fact that this is 

 part of the prothorax. Below it the epitrochlear plate, also 

 part of the prothorax {32), is easily made out. 



The Ventral Surface of the Thorax. — The greater part of the 

 ventral surface of the thorax looks forwards and downwards. 

 It is formed by the great mesosternal plastron with the 

 prosternal area in front of it and the intermediate coxcc 

 behind it. The posterior part of the ventral surface looks 

 backwards and downwards ; it is hidden to a great extent 

 by the intermediate and posterior coxae. 



To complete the examination of the ventral surface the 

 posterior and intermediate coxa; should be removed ; the parts 

 exposed will be described in the description of the meso- and 

 metasterna. The suture between the meso- and metasterna I 

 term the transverse ventral suture ; it separates the meso- and 

 metathoracic somites, but is not very obvious except on the 

 inner surface of the thoracic wall. 

 ;, The Anterior Surface of the Thorax.— (Plate VIII., Figs, i 



