32 MEDICAL ENTOMOLOGY 



readily when the muscles which traverse the labium in its long diameter 

 contract. In a mosquito in the act of feeding the labrum-epipharynx can 

 be distinguished with the aid of a lens, passing directly from the head 

 into the wound, and widely separated from the labium, the upper part of 

 which is bent sharply backwards. 



The sucking apparatus of the mosquito (Plate V, fig. 5) differs from 

 that of any of the flies so far considered in several important particulars. 

 The first sucking chamber is very small, and takes only a minor part in 

 the mechanism, while the pharynx is very well developed, the two being 

 connected by a stout chitinous tube of considerable length, instead of a 

 short membraneous duct. It should be noted that the pharynx of the 

 mosquito is not, strictly speaking, homologous with that of Tabanus, for 

 it is situated mainly behind the brain, instead of anterior to it. Their 

 true relation will be evident from what has been said with regard to the 

 development of the sucking chambers. 



The buccal cavity lies below the clypeus, and commences by the fusion 

 of the two edges of the groove in the epipharynx, the upper lamina 

 of the hypopharynx uniting with the tube so formed ; it terminates at 

 the level of the posterior border of the clypeus, running upwards and 

 backwards, not quite in line with the food channel. Its lumen is 

 transverse in the anterior part, and circular posteriorly. The ventral 

 wall is the thicker of the two, and is chitinous throughout. The dorsal 

 wall is chitinous in front and behind, but has a membraneous area in the 

 middle portion. The muscles which dilate the cavity are inserted into 

 its dorsal wall, and arise from the internal aspect of the clypeus. 



There is a curious structure, a part of the salivary apparatus, attached 

 to the anterior end of the ventral plate of the buccal cavity. (Plate V, 

 fig. 5.) It consists of a small cup-shaped receptacle, with the open end 

 directed posteriorly, and covered by a tough membrane which is 

 pierced in its centre by the salivary duct coming from the thorax. The 

 closed anterior end communicates directly with the salivary duct in 

 the hypopharynx. Where the duct enters the membrane there are 

 several chitinous spicules, and to these are attached delicate muscle 

 fibres, which arise from a pair of prominent flanges at the posterior end 

 of the ventral surface of the buccal cavity. According to Nuttall and 

 Shipley the function of this structure is to pump the saliva from the 

 thorax. The contraction of the muscles produces a negative pressure in 

 the receptacle, the elastic rebound of the membrane forcing the saliva 

 outwards. 



The pharynx of the mosquito is mainly posterior to the brain, and is 



