500 MEDICAL ENTOMOLOGY 



from before backwards and also from side to side, so as to form a 

 covering for the constituent parts of the biting apparatus as they emerge 

 from the head. The basal and largest of the two joints is heart-shaped, 

 and is directly continuous with the chitin of the head wall ; it may 

 represent the clypeus. This flap can be raised and lowered with the rest 

 of the proboscis. Both joints are covered on their external surface with 

 small serrated hairs. 



The proboscis is a long cylindrical and pointed organ, which, though 

 it points backwards in the resting position, can be erected until it points 

 downwards and forwards. It consists of a pair of mandibles, a pair of 

 first maxillae, and a labium, which, as in the simpler Diptera, serves as 

 a sheath for the piercing parts. There is no epipharynx or hypopharynx, 

 the food channel being formed by the two mandibles, which also serve to 

 convey the saliva to the wound. 



The labium (Plate LXII, fig. 7) is the only part of the proboscis which 

 can be seen without dissection, as the other appendages are concealed in 



an almost closed groove on its anterior surface. It 

 The labium . ... . . . . 



consists of four joints, the first of which is the broadest 



and shortest, has a semi-membraneous wall, and lies in a V-shaped notch 

 on the ventral side of the proboscis aperture ; the second and third joints 

 are each about twice the length of the first and rather narrower, have 

 well chitinized walls, and are separated from one another and from 

 the basal joint by loose membraneous areas. The distal joint is narrow 

 and terminates in a blunt point. The method of articulation of the 

 joints with one another permits of the shortening of the whole organ 

 which takes place when the piercing stylets enter the skin. 



The labium is a hollow organ with a chitinous wall, the internal space 

 being continuous with the haematocoele of the body. It contains a pair 

 of longitudinal muscles, passing between the several joints and serving 

 to effect the necessary retraction, a pair of tracheae, and many hypoder- 

 mal cells, which are larger than one usually meets with in such situations, 

 arranged around the wall. The dorsal surface of the organ inferior 

 when in the position of rest is invaginated throughout its whole length 

 to form a groove for the reception of the mandibles and maxillae. The 

 two lips of the groove meet, or even overlap, in front. The walls of the 

 groove are composed of chitin as thick as that of the rest of the labium, 

 and there is no membraneous interval separating the labial gutter from 

 the posterior part of the wall. 



The mandibles have entirely lost what may be supposed to have been 

 their original function, and have become modified to form two channels, 



