42 OX THE ANATOMY OF THE FLY. 



anteriorly with the mouth, and posteriorly with the oesophagus. 

 The dilatation of this cavity draws fluid from the mouth, and its 

 obliteration which is entirely effected by the elasticity of its walls, 

 injects it into the oesophagus and sucking stomach or crop, the 

 absence of valves being made up for by the peculiar rythmic 

 action of its muscles, the anterior fibres contracting and again 

 relaxing before the posterior, so that an undulating motion is 

 given to the superior wall of the cavity. Although I believe this 

 organ is homologous to the fulcrum in bees, I prefer the term 

 pharynx, from its double connection with the mouth and oesopha- 

 gus, as well as its peculiar function. ( It is represented in Plate II, 

 Figs, 11, 12, 13, 14 and Plate III. a.) 



The pharynx or fulcrum consists of a body and four processes, and the 

 body of a dorsal and ventral arch ; the dorsal arch is continuous with the 

 integument, and is united above with the trapezoidal plate, which I have 

 called the epistoma, of which it probably forms a part ; the ventral arch 

 is more properly pharyngeal, being probably developed in the walls of the 

 alimentary canal and not belonging to the integument ; it consists of two 

 layers, between which the cavity connecting the mouth and the oesophagus 

 is situated. The superior, or dorsal layer, is thickened in the mesial line, 

 forming a rod of chitine, which extends beyond the inferior plate at its 

 anterior extremity. On either side of this a row of fine seta) project into the 

 interior of the cavity, their points being turned towards the oesophagus ; these 

 are probably sensory organs by which the act of swallowing is controlled. 

 The inferior layer is thicker and stronger than the superior, and gives 

 strength to the whole organ. The hollow between the ventral and dorsal 

 arches is occupied by a pair of sets of muscles, which arise from the inferior 

 part of the dorsal arch, and from the dorsal edges of the ventral arch, and 

 are inserted into the mesial line of the superior layer of the pharyngeal 

 tube, which they separate from the inferior by their contraction. 

 They are remarkable in containing a large number of branched mus- 

 cular fibres. 



The posterior processes give insertion to the exsertor muscles of the 

 proboscis, (Plate II, Fig. 13 a) which arise from the interior of the 

 cheeks. In order to understand their action, we must consider the position 

 of the pharynx when the proboscis is retracted. It is then withdrawn into 

 the cavity of the head and lies with its posterior processes against the upper 

 part of the facial plate, and its anterior or distal end against the inferior 

 (posterior) margin of the oral cleft, so that its direction is almost 

 horizontal, the epistoma being folded back on the facial plate. The 

 contraction of the exsertor muscles draws down the pharynx and causes it 

 to revolve upon the junction of the epistoma and facial plate. 



