46 THE LARVA OF THE BLOW-FLY. 



The dilator muscles of the gullet are inserted into the 

 middle of the lower wall of the pharyngeal sinus, the epipharynx, 

 and by their contraction dilate the alimentary tube, so that 

 the food is sucked into it. When they relax it is depressed on 

 the hypopharynx by its elasticit}', like the plunger of a pump; 

 by this means the food is driven into the crop. As there are 

 no valves to the pharyngeal apparatus, I suspect the plunger 

 itself acts as a valve, its anterior portion rising and descending 

 before the posterior portion, so that the food is swallowed by a 



Fig. io. — A vertical and horizontal diagrammatic section of the larva, to show the 

 relative position of the internal organs and the segments. The segments are 

 numbered on the supposition that the three cephalic post-oral somites exist, as, 

 anterior spiracle ; pit, pharynx ; id, imaginal discs ; ivd, wing disc ; nc, nerve 

 centre ; c, crop ; pv, proventriculus ; cli, chyle stomach ; ?', hremal curve of the 

 intestine ; sg, salivary gland ; dv, dorsal vessel ; mliv, Malpighian vessels ; ps, 

 posterior spiracle ; r, rectum. The lower figure shows the general arrangement 

 of the larger trachea;. 



kind of peristaltic act. The action of this organ is exactly 

 similar to that of the fulcrum (pharynx) of the imago. The 

 suctorial action of the pharynx of the larva of CEstrus was 

 first pointed out by Scheiber [21]. The upper and back part of 

 the pharyngeal sinus of the young larva lodges the antennal 

 disc sac. In the adult larva this is placed behind the cephalo- 

 pharynx, but it sends a pair of processes into the pharyngeal 

 sinus, between the cornua and the inferior processes, in which 

 the imaginal rudiments of the fulcrum of the fly are developed. 



