39 THE ALIMENTARY CANAL OF THE IMAGO. 



1. THE MOUTH AND PHARYNX. 



The Mouth, like that of the Diptera generally, is very highly 

 modified ; it is a cylindrical tube extending from the discal 

 sclerites (p. 143) to the prepharyngeal tube, which may 

 be regarded as its posterior limit or isthmus faucium. The 

 comparative immobility of the prelabrum, owing to the over- 

 lapping of the edges of the haustellum and the fusion of the 

 base of the ligula with it to form the prepharyngeal tube, 

 prevent the insect from opening the mouth, like the blood- 

 sucking Tabanidae, so that food can only pass into it through 

 the oral cleft and prestomum. 



The Pharynx. The term pharynx is applied to the space 

 between the two plates of the fulcrum ; it is dilated by the 

 contraction of the dilator pharyngis muscle, and is entirely 

 closed by the elasticity of the fulcrum, which brings its dorsal 

 and ventral plates into apposition when the muscle is relaxed 



(P- '37)- 



The dilatation of the pharynx serves to draw food through 



the long tubular mouth, and its contraction forces it back into 

 the oesophagus. Although there are no valves, the fulcrum 

 acts as an effective pump, probably by a rhythmic contraction 

 of the muscles producing an undulating movement of the 

 anterior (superior) plate. Food is frequently, probably 

 normally, regurgitated and reswallowed, so that it is evident 

 that the unclulatory movement is capable of effecting a flow 

 of fluid either from or towards the mouth. The mouth and 

 pharynx have a chitinous cuticular lining, and with the ex- 

 ception of a double row of fine bristles, which project back- 

 wards in the pharynx, there is no evidence of the existence of 

 any sensory structures in either. 



2. THE SOFT PARTS AND MECHANISM OF THE PROBOSCIS. 



The proboscis is a highly complex accessory to the digestive 

 apparatus. The anatomy and morphology of its skeleton has 

 already been discussed, the present section only deals therefore 

 with a description of its soft parts and with those structures 



