THE THORACIC EXO-SKELETON. 201 



The Sub-Epaulet (PI. X., e') is the most anterior of the distal sclerites 

 of the propterygiuni. It consists of a scale, usually concealed by the epaulet 

 and of a projecting process directed downwards, outwards and back- 

 wards. This process is connected with the coracoid by a slender rim of 

 chitin, which apparently acts as a spring, pressing the anterior margin of the 

 wing downwards when fully extended. 



The Coracoid (PI. X., cr) was described by Chabrier as the beak of 

 the humerus {bcc de P/uam'rus), a term applied by him to the base of the 

 nervures of the disc, my remigium. It somewhat resembles the coracoid 

 process of the human scapula, and it prevents the dislocation of the 

 remigium upwards. The coracoid is a perfectly distinct sclerite, wedged in 

 between the epaulet and ihe unguiculus. 



The Unguiculus (PI. X., Figs. 10 and 1 1 ) is a complex and important sclerite, 

 which, like the dens, is intimately concerned in the mechanism of flight. It 

 is the 'ongulaire' of Chabrier [72, vol. viii., p. 73] ; he confounds it with the 

 remigium, and terms it the base of the humerus in his description of the 

 wing of the cockchafer (^Mdolontha). Jurine termed it ' petit cubitale.' 



In the Blow-fly it consists of a horizontal plate, a vertical plate, and afoot. 



The superior or horizontal plate is seen from above ; it is wedged in 

 betweeen the coracoid and the processus dentatus of the dens. It is sub- 

 quadrate, and has a small claw-like process at its anterior distal angle. 

 This process led me to term it the ' unguiculus.' 



The vertical plate descends behind the propterygium in front of the 

 dentate process of the dens ; it curves forwards and terminates in the stirrup. 



The stirrup of the unguiculus projects below the wing, and forms a hook, 

 which articulates with the hamulus of the episternum (PI. VIII., Figs. 7 and 9, 

 ha) ; its inferior surface rests upon the sacculus {^sac). 



The anterior surface of the vertical plate forms a socket, in which the head 

 of the remigium rotates. In function it may be compared to the lesser 

 sigmoid notch of the ulna, the head of the remigium representing the head 

 of the radius. The hamulus of the pleuron draws the wing downwards and 

 forwards when the wing is extended ; when the wing is flexed {i.e.^ its 

 anterior margin drawn back in the horizontal plane), the hamulus releases 

 the unguiculus, so that the wing can be freely elevated. The sacculus forms 

 an elastic cushion, on which the foot of the unguiculus rests when the wing 

 is depressed. 



The Costal or Marginal Nervure articulates with the sub-epaulet, and 

 this articulation permits of a certain amount of elevation and depres- 

 sion. In extension of the wing, the sub-epaulet is rotated forwards under 

 the epaulet by the agency of the parapteron. 



The Remigium (PI. X., Figs. 3-5) is the 'tige basilaire de I'humdrus ' of 

 Chabrier. It represents the united roots of the subcostal and hypocostal 

 nervures, and consists of two parts, an upper and a lower. 



The upper part is sub-cylindrical, convex above and concave below ; its 

 proximal end articulates by syndesmosis with the coracoid. The lower 

 part is conical, or peg-like, tapering towards its distal end. Its proximal 

 end is hollowed into a deep cup, which is surrounded by a thick chitinized 

 ring {r). This ring is free below, but its upper half lies in the lower wing 



