THE ORDER DIPTERA 35 



the mouth-parts and even the mouth are vestigial. The 

 proboscis is a composite structure, and the exact homologies 

 of its component parts are somewhat problematical. It is 

 generally assumed that the proboscis, as seen without dis- 

 section, is the labium ; it forms a sheath for the other mouth- 

 parts, which lie concealed in a deep groove that runs along 

 its dorsal surface. The parts so ensheathed by the labium 

 are not constant, as may be observed by comparing the 

 proboscis of a female mosquito with that of a blow-fly. 



In the female mosquito (Fig. 2) the labium is long and 

 slender, and ends in a pair of small stiffish lobes, the labella. 

 In the groove of the labium lie (i) the long pointed upper- 

 lip, or labrum-epipharynx ; and (2) the slender hypopkatynx. 



FiQ. 2.— Head of temale Mosquito. 



which is traversed in all its length by the common salivary 

 duct. The labrum-epipharynx is grooved all along its ventral 

 surface, and the groove is converted into a tube by the 

 apposition of the epipharynx : up this tube the blood is 

 drawn when the " mosquito " bites a victim. Besides these 

 two tubes — an efferent salivary tube tunnelled in the hypo- 

 pharynx, and an afferent suctorial tube formed by the 

 apposition of the hypopharynx to the epipharynx — there 

 exist two pairs of organs which are regarded as homologous 

 with the mandibles and maxillce of insects, such as the cock- 

 roach (Fig. 112) that feed on solid food. Both mandibles and 

 maxillae are long, slender, flexible rods slightly broadened 



