533 



leads posteriorly into the oesphagus by its narrow end. The walls of the 

 pharynx are chitinized, but are not so thick as those of the buccal cavity. 



At the anterior end of the buccal cavity there is the circlet of minute 

 chitinous teeth previously mentioned as surrounding the mouth opening. 

 These are attached to the membraneous wall of the head, and in the 

 normal resting position are turned inwards and retracted. As will appear 

 later, they can be turned outwards on their bases so as to transfix the 

 skin of the host during the act of feeding. 



The structure and mechanism of the sucking apparatus is best studied 

 in series of transverse and longitudinal sections of the head, which reveal 

 a complex musculature. In the distal part of the buccal cavity the 

 lumen is a transverse slit, bounded by dorsal and ventral plates of 

 approximately equal thickness (Plate LXVII, fig. 1). At this point 

 the ventral wall is connected to the dorsal wall of the proboscis sheath 

 by means of a membraneous expansion. The thickness of the walls 

 increases gradually towards the posterior end of the chamber, and at the 

 same time the lateral thickenings become pronounced. They are not 

 mere thickenings of the chitinous wall, as they appear to be when 

 the organ is examined in cleared preparations, but are lateral pockets 

 running on each side of, and slightly dorsal to, the main portion of the 

 lumen ; at the extreme posterior end of the buccal cavity they become 

 solid and are slightly separated from the rest of the wall when seen in 

 transverse section. 



The muscles inserted into the buccal cavity are numerous, and are 

 adapted for two purposes, the protraction of the whole structure and the 

 dilation of its lumen. Their precise attachments are 

 very difficult to determine, as they are far too small to 

 be isolated w 7 ith precision by dissection, and some of 

 them are so short that a slight error in the angle at which a section is 

 taken might give a false impression as to the course. Four pairs can be 

 distinguished. One pair, which consists of four separate strands on each 

 side, lies in the middle line dorsally, and runs directly backwards from 

 the front of the head to the distal end of the cavity (d. pr. in figs. 1 

 and 2, Plate LXVII). Contraction of this pair must evidently bring 

 about a forward displacement of the buccal cavity.. On each side of 

 this pair there is a larger muscle, having only a short course, which runs 

 obliquely inwards from the dorso-lateral angle of the head to the middle 

 part of the dorsal plate of the cavity. None of its fibres pass to the 

 lateral thickenings of the wall, and it must, therefore, be regarded as a 

 dilator, acting, as in the case of Tabanus, by the withdrawal of the 



