52 MEDICAL ENTOMOLOGY 



altered to form a duct with a wide lumen, connecting the lower end 

 of the pharynx with the food canal in the haustellum. The wall of this 

 duct, which is of uniform calibre throughout, is composed of two layers 

 (Plate XII, fig. 4). The internal one is membraneous in the lower 

 part, and composed of thin chitin in the portion lying between the two 

 sides of the fulcrum, which are produced downwards in a funnel-shaped 

 manner. The outer layer is composed of a series of thick chitinous rings 

 set side by side in the same manner as those of a pseudotracheal channel. 

 The rings are open in front, and much thicker posteriorly than at the 

 sides, approaching one another so closely that when seen from the side 

 they appear as if united in the middle line. Such a channel can be 

 bent without its lumen becoming occluded, and will not collapse when 

 the pressure within it is reduced during the sucking action.* In the 

 resting position the rostrum and haustellum are in the same relation to 

 one another as is the case in Musca, and the buccal cavity is acutely 

 flexed (Plate XI, fig. 1). 



The labium is almost entirely chitinous, the membrane in its wall be- 

 ing confined to a narrow strip which connects the mentum and the labial 

 gutter (Plate X, fig. 3). The mentum is spindle-shaped, narrow in 

 its distal part and expanded at the upper end into the 'bulb', in which 

 the muscles are contained; it extends throughout the sides as well as the 

 posterior surface, leaving only a narrow interval in front. The degree to 

 which the mentum is narrowed distally and expanded above differs in 

 the different genera, and is a good index of the degree of specialization. 

 The labial gutter is not so conspicuous in these forms as it is in Philae- 

 matomyia, for its place in supporting the biting apparatus is to a great 

 extent taken by the mentum. It is, however, thick and strong in the 

 lower part of the haustellum, in the region of the anterior joint. The 

 'keel' is not so well developed, and is restricted to the lower half or 

 third of the gutter, this being due to the fact that the anterior joint does 

 not play such an important part in the mechanism, and that con- 

 sequently its muscles are not so well developed (Plate X, fig. 3). 



The labella are small oval lobes, which, when the proboscis is 



in the position of rest, are little if at all broader than the end of the 



labium. Each is composed, as in Musca, of an inner 



6 a m and an outer wall enclosing a space which is a part 



of the haematocoele. The outer wall is continuous 



with the wall of the haustellum and is composed of membrane, in 



*A ringed wall of a similar nature is commonly found in the salivary duct, where it has 

 to resist the action of the salivary pump. 



