534 MEDICAL ENTOMOLOGY 



dorsal plate from the ventral one (d.b.c. in figs. 1 and 2). On the 

 ventral side of this muscle there are two others of considerable size ; the 

 upper one lies immediately external to the lateral thickening of the wall 

 of the buccal cavity, and may, therefore, be termed the lateral protractor 

 (l.pr. as above), while the other is ventral and slightly internal to the 

 thickening, and may be termed the ventral protractor. The first runs 

 directly backwards, the second obliquely backwards and upwards ; both 

 are inserted into the cornua at the posterior end. 



Of the four pairs of muscles, then, three are protractors, whose func- 

 tion it is to thrust the buccal cavity forwards. As to how far out of the 

 head the chamber is thrust, and what purpose the protraction serves, it 

 is not possible to say. Only a small amount is ever observed in the 

 living insect, or can be brought about in dissections of fresh material. 

 When protraction occurs the teeth at the mouth opening are rotated on 

 their bases so that their points are turned outwards, the rotation, which 

 can often be seen to have occurred in fixed preparations, being a result 

 of their position and method of attachment. It should also be noted 

 that when the chamber is thrust forwards the origin and insertion of 

 the dilator muscle will be approximated, so that the course of the 

 muscle will be vertical. 



The membraneous portion of the canal, which connects the buccal 

 cavity and the pharynx, is bent into a short loop in the resting position, 

 passing upwards, backwards, and again downwards, so that in some 

 sections it may be cut across twice. Its wall is thick and tough, but it 

 has no dilator muscles attached to it. The presence of this loop allows 

 for the necessary extension when the buccal cavity is thrust forwards. 

 The loop is situated about the level of the insertion of the antennae. 



The pharynx appears in sections as an irregularly tri-radiate structure, 

 composed of one dorsal and two lateral plates of chitin, with a smaller 

 plate ventrally in the anterior part of the wall. The separate plates are 

 united to one another by short bands of membrane, the whole structure 

 closely resembling the corresponding organ in the mosquito. Posteriorly 

 the lumen contracts gradually, and the wall is reduced in thickness, until 

 a very narrow and thin-walled oesophagus is produced. 



To each of the dorsal and lateral plates of the pharynx there is 

 attached a strong dilator muscle, which passes inwards from the 



wall of the head. The dorsal muscle is divided into 

 The pharynx and its , , ,, , . , .. , . xl _ 



musculature several bundles, and is situated a little anterior to the 



lateral ones (Plate LXVII, fig. 3). In addition there 

 are well-defined sphincter muscles, which are arranged in two sets, one 



