PIEECING ORGANS. 113 



inner side of the labella, and the extremely strong muscles, the 

 tendons of which are attached to the outer wall of the labella, 

 must be the chief factors in the necessary movements. By the 

 contraction of these muscles the inner sides of the labella are 

 turned so much outwards that their teeth are able to act upon 

 the skin of the victim ; I am unable to decide whether the 

 muscles described above as the third pair in the labium of 

 Stomoxys can assist by pushing the plate in the groove outwards, 

 and thereby affecting the plates on the inner side of the labella 

 in a similar manner. In Glossina the teeth are inserted rather 

 near the end of the labella, and it must be these six pairs of 

 teeth which are at least the chief instruments in piercing a hole. 

 But I am bound to confess that I can form no opinion as to the 

 significance of the thi-ee pairs of rasp-shaped plates — whether 

 they assist during the piercing or are useful in another inspect. 

 In Stomoxys the five strong teeth are situated near the base of 

 the labella ; but in this insecfc the labella are moi-e movable, and 

 can be turned outwards so that the teeth can act on the skin of 

 a victim.* The blades in the two rows are very sharp, but 

 rather thin ; they probably have an accessory function in the act 

 of piercing. 



It is, in my opinion, highly probable that the hole produced 

 by Stomoxys is not very deep, the labella being proportionately 

 rather thick, and the essential teeth being placed so far from the 

 end that the labella must be greatly shortened and much turned 

 outwards when the fly bites. But in Glossina the labella are 

 very slender, and more strongly chitinised on the outer side, with 

 their teeth near the end, and it is therefore probable that the 

 proboscis can be sunk rather deeply in thick skin. The saliva is 

 introduced into the wound from the end of the hypopharynx, 

 and — as in the case of mosquitoes {Culex, Anopheles) — is certainly 

 mixed with the blood which is to be pumped through the 

 proboscis. 



D. Lower Portion op the Head in Both Genera. 



As in the blow-fly and the common house-fly, the lower part of 

 the head, between the outer mouth-parts and the firmly chitinised 

 portion of the head, is completely membranous and can be forced 



* In a specimen of Stomoxys preserved m spirit I found tlio labella 

 turned outwards to such an extent that the big teeth occupied their distal 

 extremity, and were directed almost vertically to the sides, and only 

 slightly downwards. 



I 



