112 ACTION OF MOUTH-PARTS. 



margin of the labellum a number of hairs are inserted, some of 

 "which at least are branched. Below the terminal portion of 

 each of the big teeth one or two dark-coloured, rather long 

 chitinous rods (g) are inserted ; they are sub-cylindrical, slightly 

 curved, and hollow, and seem to possess an aperture on the 

 distal obtuse end. 



The labium contains three pairs of muscles. The first pair 

 is lamellar, with short fibres, and proceeds from the outer lateral 

 wall obliquely to the margin of the plate in the bottom of the 

 groove ; these muscles occupy the gi-eater part of the length of 

 the labium. The second pair is very large, filling most of the 

 lumen of the basal third of the labium ; each muscle terminates 

 in a very strong tendon, which is attached to the outer wall of 

 the labellum. The third pair is, so far as I was able to see, 

 very different from that in Calliphora and Tabamis ; the fibres 

 are very short, proceeding from the ventral side of the plate in 

 the groove obliquely forwards and downwards to the ventral 

 wall of the first segment, at least in its distal half. 



The hypopharynx, labrum, and labium are attached to each 

 other at their base much as in Glossina ; but the attachment of 

 the hypopharynx to the labium seems to be less solid than in 

 that genus. 



C. Mode op Action of the External Mouth-parts in 

 Glossina and Siomoxys. 



In both genera, the tube through which the blood of the 

 animal on which the fly may be feeding passes to the pharynx, 

 is constituted by the labium and the labrum together. The 

 labrum lies within the deep dorsal gi-oove of the labium, closing 

 it above and also forming the inner lateral walls of the tube ; 

 beyond the end of the labrum the sub-membranous margins of 

 the labium touch or perhaps cover each other above ; in specimens 

 of Stomoxys preserved in spirit, I have seen the margins of the 

 labium in contact for half the length of the proboscis. The 

 hypopharynx is completely enclosed in the tube mentioned. In 

 both genera a short terminal portion of the hypopharynx is 

 entirely membranous, and this organ is consequently useless for 

 the stabbing of victims ; the labrum is somewhat shorter than 

 the labium, and in Glossina at any rate its extremity is not 

 strong enough for piercing. The insects belonging to both 

 geuei'a perform this operation by means of the armature of the 



