i82 THE IK TEGUMENTAL SKELETON OF THE IMAGO. 



front by the posterior edge of the lateral plate ; below by the 

 sternopleural suture ; and behind by the hypopleural suture, 

 the tympanic bulla and the attachment of the great winglet or 

 squama. It is a complex region, which is far larger in the 

 Syrphidai and Volucella than in the heavy-flying Muscidae. It 

 contains the following sclerites, which form part of the thoracic 

 wall : The episternum (Plate VII., 28), the epimeron {26), the 

 costa {2§), the epicosta (Plate VIII., Fig. 7, cpc), and the 

 parapteron (Plate VII., //). 



The Pleuron (Plate VIII., Fig. 8) is sub-triangular, divided into two by a 

 vertical suture, which projects as a distinct ridge on its inner surface. This 

 ridge is united below with the great entopleuron. 



The part in front of the suture is the episternum ; that behind it is the 

 epimeron below, and the costa above. 



The Episternum is an irregular quadrilateral plate, terminating above in 

 two curved processes. These processes are sub parallel ; the anterior sup- 

 ports the pre-epaulet of the wing. The posterior process has a hemispherical 

 swelling behind, and below it, the great ampulla (Mihi). The process curves 

 over the great ampulla, and ends in a strong hook, the hamula, which articu- 

 lates with one of the sclerites of the propterygium. The inner surface of the 

 great ampulla gives origin to a powerful muscle, which acts on the wing root 

 —the ampullar muscle (Mihi). 



The episternum is bounded in front by the mesopleural syndesmosis, 

 below by the sternopleural suture, behind by the epimeron, and above by 

 the great ampulla. 



The Epimeron is much thinner than the episternum ; it is irregularly cor- 

 date, with the emargination above. Seen from the interior of the thorax, it 

 exhibits a series of radiating ridges, which commence at its margin and form 

 the sutures between the adjacent plates of the thoracic wall ; the strongest 

 extends to the external angle of the post-scutellum. 



The epimeron articulates — below with the meso- and metasternum, from 

 the latter of which it is separated by the hypopleural suture ; behind with 

 the tympanic bulla, and above with the costa. 



The Costa is a thin shell-like plate. The outer surface is convex, and 

 looks downwards and outwards ; it is covered with fine bristles. Its upper 

 edge is continuous with the syndesmosis of the wing, and adjacent to a small 

 sickle-shaped sclerite— the epicosta. 



The Epicosta presents a strong, chitinized, hollow, curved protuberance in 

 front, which forms a kind of handle to the sickle. I term this projection the 

 'lesser ampulla'; it gives origin to the lesser ampullar muscle. Kiinckel 

 d'Herculais [25, p. 98] incorrectly names the epicosta, the ' parapteron,' a 

 term already applied by Audouin to a very different part. 



In Volucella and the Syrphida- the epicosta is prolonged behind as a long 

 cylindrical flexible process, covered by fine setae. This tail-like process 

 was first described by Chabrier [73, tom. viii., p 398], who fancifully com- 



