MECHANISM OF PROBOSCIS: STOMOXYDINAE 55 



fit closely against the broad ends of the fork. On each side of and 

 behind the furca there is a small rod, closely pressed against the 

 furca but not fused with it, which represents the distal arm of the 

 fork of the mentum, displaced forward and separated from the proxi- 

 mal arm. 



It will be noted that there is a triangular space between the lateral 

 arm of the furca and the truncated end of the mentum, when the 

 proboscis is seen in profile (Plate XIII, fig. 7). This is filled in by 

 membrane continuous with the wall of the labellum and with the 

 mentum. In cleared preparations the labial gutter, which is thickest at 

 this point, can be seen at the dorsal side of the triangle. The membrane 

 is strengthened by the presence of numerous small oval plates of chitin, 

 and is thrown into several folds. 



The mechanism of the proboscis in the blood-sucking Muscidae is an 

 extremely interesting one, in view of the close similarity with Musca. 

 The rostrum and haustellum are extended and retract- 

 ed in the manner already described, the only differ- ^hanism of the 



3 , . . Proboscis 



ence being that on account of its increased length 



the haustellum remains conspicuous in retraction, projecting in front 

 of the head. When extended for use the rostrum and haustellum, 

 as in Musca, are brought into line with one another, the proboscis 

 being usually held pointing downwards and a little forwards. The 

 cutting apparatus is then brought into action by the posterior set of 

 muscles, which are attached to the lateral arms of the furca and to the 

 adjacent wall of the labellum. The contraction of these muscles rotates 

 the furca on the fixed point provided by the expanded ends of the mental 

 forks, and as the lateral arms pass backwards they carry with them, 

 because of their close attachments, the external labellar walls. When 

 this occurs the internal walls of the labella are pulled upon, and are 

 carried backwards as far as their attachment to the discal sclerite will 

 permit. As a result of a complete contraction, therefore, the lateral 

 arms of the furca are directed upwards and backwards until they make, 

 with the ventral border of the mentum, a narrow angle open towards the 

 head, the arms themselves lying in the membraneous triangle referred to 

 above ; the external wall of the labellum is displaced bodily upwards, 

 retaining its relation to the furca ; the internal walls of the labella 

 become external, being rotated through half a circle on their attachment 

 to the discal sclerite, the distal border of which, that is to say, 

 the prestomum, becomes the most distal point of the proboscis. 

 As a result of the eversion of the labellar walls, the teeth and the 



