THE THORACIC EXO-SKELETON. 199 



metapterygium, from which all its nervures arise. Behind the 

 patagium in the Blow-flies a small segment is separated from 

 the rest of the wing by a notch (I term this the ' lobulus '), and 

 behind the lobulus are the squamula and the squama. 



The nomenclature of the nervures which I have adopted will 

 be sufficiently understood by the figure (PI. X.). 



The Wing-Roots. The wing of the Blow-fly is apparently 

 supported by only two roots ; an anterior, from which the 

 nervures of the marginal and discal areas arise, and a posterior, 

 which supports the nervures of the patagium. 



The anterior wing-root consists, however, of the united pro- 

 and mesopterygium. It presents five carpoid sclerites ; these 

 support the marginal and the common root of the subcostal 

 and discal nervures, which I term the ' remigium.' Its move- 

 ments are quite independent of those effected by the meta- 

 pterygium. 



The carpoid sclerites are arranged in a proximal and a distal 

 series. I term the proximal sclerites the epaulet and the 

 dens ; they articulate with the dorsum and with the sclerites 

 of the distal row. The latter are the sub-epaulet, the coracoid, 

 and the unguiculus (Mihi) ; they articulate with the pleuron, 

 with the proximal sclerites, and with the marginal nervure and 

 the remigium. 



The Dens (PL X., Figs. 7-9) consists of a body and three processes. The 

 body is irregular in form, narrowed in front, forming the neck, which supports 

 an oval convex scale, seen between the epaulet and the coracoid. The 

 neck terminates in a rounded head concealed by the scale, this articulates 

 with a socket formed by the uncinate process of the anterior parascutal 

 plate. Behind, the dentate process projects downwards and backwards, 

 and lies upon the posterior surface of the unguiculus. Internally, there are 

 two processes, an anterior, which projects inwards and forwards and receives 

 the insertion of the accessory elevator muscle of the wing. The second or 

 posterior process is far larger ; its distal extremity articulates by a cup-shaped 

 cavity with the head of the great alar apophysis ; a line joining the head of 

 the dens with this cup-shaped cavity may be termed the axis of the dens it 

 is the axis on which the wing is raised and depressed. The direction of this 

 axis undergoes rotation about the head of the dens with increased convexity 

 or flattening of the dorsal valve. Increasing convexity of the dorsum 

 produces the descent of the wing, pushes the alar apophysis forwards 

 beneath the head of the dens, and renders its axis nearly vertical, so that the 



