56 MEDICAL ENTOMOLOGY 



interdental armature are rotated outwards and upwards on their 

 bases, which are moved with the rest of the internal wall, and as this 

 occurs their cutting points are pulled upwards through the wound, cut- 

 ting as they go. It is by repeated contractions of these muscles, 

 producing eversion of the teeth, that the wound is made. 



In the position of action thus produced (Plate VIII, fig. 7) the 

 interdental armature is swept upwards in advance of the teeth, and as 

 each contraction occurs the blades sweep the surface of the wound clear. 

 When the wound is made, and the fly begins to suck up the blood, they 

 surround the prestomum like a hollow brush, and so prevent the ingress 

 into the canal of particles too large for its lumen, a function of peculiar 

 interest when considered in relation to the homology of the parts, for the 

 interdental armature in the Stomoxydinae is developed from the remains 

 of the rings of the pseudotracheae as seen in Musca. 



The contractions of the posterior set of muscles are presumably 

 repeated with very great rapidity, as is the case in the biting apparatus of 

 the Orthorraphic flies, and it is necessary to consider by what mechanism 

 the parts can be replaced in position for the next contraction. To 

 understand how this occurs one must revert for a moment to the struc- 

 ture of the labella. In the resting position the prestomum is at some 

 distance from the distal end of the proboscis, being in fact separated 

 from it by the total depth of the inner walls. This may be regarded as 

 an invagination, the distal borders of the sclerite, which bound the 

 prestomum, being the apex. Now when the external wall is displaced 

 upwards and the inner wall everted, this invagination is undone, and the 

 blood which was between its inner and outer walls is driven up- 

 wards. The return of the parts to the resting position is brought about 

 by the pressure of blood in the proboscis, in exactly the same way as is 

 the case in the distension of the oral lobes of Musca. This is due partly 

 to the respiratory 'movements of the fly, and partly to the contraction of 

 the transverse muscles which pass between the mentum and the labial 

 gutter, in the lower end of the labium. 



The muscles of the labium are divided into the same bundles as in 

 Musca. The posterior set, however, which has to pull the teeth through 

 the tissues, and has therefore to act against considerable resistance, is 

 very much larger than the others, and occupies practically the whole of 

 the bulb. In the narrow part of the proboscis the muscle fibres are 

 replaced by tendons, which spread out at their insertions so as to gain 

 attachment to the external wall of the labellum, as well as to the furca. 

 The anterior set, which in Philaematomyia erects the labellar rods, is 



