THE ANATOMY OF THE ADULT MOSQUITO 85 



retractor muscle, the base of the labium can be invaginated 

 within the bulb, so that its point is withdrawn behind those 

 of the other mouth parts, while by that of the transverse 

 fibres, it can be forced out and extended once more to their 

 level, or be3^ond them. 



Firmness is imparted to this arrangement by a pair of 

 strong spines, which project from the lateral trabecular of 

 the front of the head into the bulb. Beyond the bulb, the 

 labrum becomes a strong, rigid style of a form not unlike 

 that of a surgeon's " director," but with a more complete 

 groove ; the transverse section forming at least three parts 

 of a circle ; and this incomplete canal is converted into a 

 tube by the hypopharynx, which, throughout its length, 

 lies in contact with its lower surface. It is through this 

 channel that blood or other food reaches the mouth cavity. 

 The hypopharnyx springs from the anterior part of the floor 

 of the buccal bulb, and is a tubular organ with, at first, 

 a very broad but shallow lumen. This, however, rapidly 

 contracts to a minute tube, of about the same diameter as 

 the salivary duct, of which it is virtually a continuation, and 

 on either side of the tube are developed the membranous 

 wings, which serve to convert the groove of the labrum 

 into a tube. Into the floor of the broad cul-de-sac of the 

 commencement of this tube opens the salivary duct, the 

 mouth of the latter being guarded by a remarkable arrange- 

 ment, which serves as a valve. The floor of the buccal 

 valve at this point is strengthened by a strong chitinous 

 plate ; and this, though of course continuous with the 

 thinner membrane near it, forms a veritable articulation 

 with a separate plate in the floor of the base of the hypo- 

 pharynx, in which is placed the termination of the salivary 

 duct. As the rest of the lining of the cul-de-sac is delicate 

 and flexible, this plate can be moved forward and back, and 

 is provided with two special muscles for the purpose. In 

 the latter position the mouth of the duct is effectually 

 closed, while it is freely open when drawn forward by the 

 anterior muscle, which springs from the chitine of the 

 ventral wall of the labium. Grouped round the lower sur- 

 face of the hypopharynx and the sides of the labrum are 



