390 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 Tabanide, 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. 137). 
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 undulatory 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 
