SUCTORIAL ACTION. 115 



it will be seen that posteriorly it has two walls, with a narrow — 

 or in the resting position with no — space between them, and the 

 anterior of these walls is the thinner. The lateral wall (c) is 

 thick, and anteriorly coalesced with the thickened part of the 

 epistoma (d). In reality the pharynx is a part of the oesophagus, 

 which has been firmly chitinised, widened, and strongly depressed, 

 with the anterior wall slightly concave and the lateral margins 

 produced into very broad wings (6), which turn forward and 

 coalesce with the epistoma. The large space between the lateral 

 wings, the anterior of the two posterior walls and the firm part 

 of the epistoma is nearly filled with enormous muscles (m) ; in 

 the distal part of the pharynx the fibres run very obliquely 

 upwards and forwards to the epistoma (Fig. 21); in the upper 

 part of the pharynx they run horizontally, or in front even a 

 little downwards (Fig. 21). The contraction of these muscles 

 produces a space between the two posterior walls of the pharynx, 

 and the whole organ is a pumping-engine. It must be supposed 

 that the distal fibres of the muscle contract, raising the corre- 

 sponding part of the front wall, and thus producing a space void 

 of air into which the fluid is sucked from the tube formed by the 

 external mouth-parts. Next, the more proximal fibres contract, 

 while the most distal ones relax, and the result of a successive 

 contraction of all the fibres from below upwards, the lower ones 

 relaxing simultaneously with the contraction of the upper ones, 

 and so on, will force the blood of the victim upwards through 

 the phai'ynx into the ojsophagus itself, and also produce a suction 

 from the wound at the end of the labella. The oesophagus itself 

 (Fig. 21, /) is a membranous tube (having in Glossina a small 

 sub-basal dorsal plate. Fig. 13, e), which is at first as broad as 

 the pharynx (Fig. 13), then narrower, and is bent backwards in 

 a curve to the occipital foramen (Fig. 21). Between the lower 

 end of the solid pharynx and the attachment of the labrum to 

 the hypopharynx is found a tube, which in Glossina is rather 

 short and wide, firmly chitinised in the posterior half (Fig. 13, c) 

 and membranous in front. In Stomoxys it is longer and narrower, 

 with irregular sub-circular linear thickenings in the wall 

 (Fig- 22, g). 



From the postero-superior process of the pharynx a strong 

 muscle (Fig. 21, w^) proceeds downwards to the cheek at the side 

 of the epistoma. The thickened part of the epistoma, with which 

 the wings of the pharynx are coalesced, is united with the solid 

 chitin below the antennae by a narrow transverse membranous 



