MOUTH PARTS : MOSQUITO 31 



projecting as minute ' teeth '. There is usually a well marked central 

 split. At the base the epipharynx and the labrum separate, the former 

 fusing with the buccal cavity, the latter projecting within the cavity of 

 the clypeus, which in the mosquito is always prominent. 



The hypopharynx (figs. 1 and 4) is an extremely delicate flattened 

 slip, perforated throughout its length by the salivary duct. It falls a little 

 short of the other mouth parts, and has a soft distal end. (Plate VI, fig. 11.) 



The labium (fig. 1, and Plate VI, fig. 4) is a long, narrow, and flexible 

 chitinous tube, hollowed out on its anterior surface for the reception of 

 the piercing parts. The greater part of its wall is formed by the mentum, 

 which forms the posterior and lateral sides, and arches inwards across 

 the groove. The anterior surface is completed by membrane, forming a 

 labial gutter which is depressed below the lateral margins. In the space 

 enclosed by the mentum and the labial gutter there are several muscles, 

 some tracheae, and a few free cells ; some of the latter belong to the 

 haematocoele, for the cavity of the labium is in communication w r ith the 

 rest of the body cavity. At its distal end the labium is divided into two 

 small labella, which are articulated to the rest of the organ by a definite 

 joint formed by small chitinous rods. The distal margin of the labellum 

 has on it many fine sensory hairs. There is no pseudotracheal mem- 

 brane, but delicate longitudinal grooves can be made out on the inner 

 surfaces of the lobes. At the proximal end the wall of the labium 

 becomes continuous with the membraneous area at the inferior surface of 

 the head. 



The movements of the labium are the same as those in Tabanns. 

 The intrinsic muscles, which are easily seen in cross sections, are 

 inserted partly into the chitinous rods which form the joint between 

 the labium and the labella, and partly into the lower end of the mentum. 

 When the insect commences to feed the labella are applied to the surface 

 of the skin, and diverged from one another so that their oral surfaces are 

 in contact with it. The piercing stylets are then thrust into the skin 

 between the two labella, which act, to use Nuttall and Shipley's expres- 

 sion, like the fingers of a billiard player in guiding the cue. As the 

 chitinous stylets enter the skin, the labium is bent backwards, the labella 

 retaining their position. The bending is in consequence of the dispropor- 

 tion in the length between the labium and the free portion of the stylets, 

 but it is not produced by mere mechanical displacement. The upper end 

 of the mentum is not a uniform sheet of chitin, but is broken up into a 

 number of minute elongate transverse plates set closely side by side, 

 with membraneous interstices, an arrangement which enables it to bend 



