SUCKING APPARATUS OF MOSQUITO 33 



separated from the buccal cavity by a considerable distance, the interval 

 being bridged over by a chitinous tube, which merges at the two ends 

 with the sucking chambers. This tube is arched with its concavity down- 

 wards, the amount of curvature depending upon the angle at which the 

 proboscis is held. In the Anopheline mosquitoes it is only slightly curved, 

 while in Culex, in which the buccal cavity and the pharynx are inclined 

 to one another at a considerable angle, the curvature is very marked. 

 The pharynx, when seen from the side, is a pear-shaped organ, 

 continuous by its narrow end with the above tube. It is composed of 

 three chitinous plates, two lateral and one dorsal, attached to one 

 another by membraneous intersections. Each plate is curved in cross- 

 section, with the convexity directed inwards, and in the resting condition 

 the three are in contact with one another, so that the lumen is 

 triradiate. To each of the plates a strong dilator muscle is attached 

 (Plate V, fig. 2), those of the lateral plates arising from the sides of the 

 head cavity behind the eyes, and those of the dorsal plate from the 

 epicranium in the region of the vertex. When the muscles contract the 

 plates are pulled apart, and the lumen made more or less circular. 



At the point where the buccal cavity ends and the tube which connects 

 it with the pharynx begins there is, in Anopheles, a distinct angle, and at 

 this point there are on the internal aspect of the ventral plate a. number 

 of stout hairs of peculiar form, described in detail by Annett, Button, 

 and Elliot in Anopheles costalis. It is stated that these structures, aided 

 by the angling of the tube, function as a valve. 



At the posterior end of the pharynx the chitinous plates are marked by 

 numerous longitudinal ridges, which project on the internal surface as 

 small tubercles. These are adorned with clusters of fine hairs, which, 

 according to Nuttall and Shipley, act as strainers in separating the coarse 

 particles from the food. Where the pharynx is narrowed to become 

 continuous with the oesophagus there is a well developed sphincter 

 muscle. 



It will be noted that the buccal cavity in the mosquito is so much 

 smaller than the pharynx that it cannot possibly draw up enough blood 

 to fill the later, and that there is no sphincter muscle between them. 

 Probably the chambers dilate together, communication with the aliment- 

 ary tract being shut off by the sphincter muscle posterior to the pharynx. 



The reader may remark, in the foregoing account of the mouth parts of 



the Orthorrapha, and especially with regard to the 



, f , . . Mechanism of the 



mosquito, that the account of the mechanism is some- 



what different to that usually put forth. The motive 

 5 



