THE MANDIBLES 439 



distinguished in them. Proximally the chitinous portion of the labrum 

 ceases at the peri-oral ring, to the margin of which it is attached by a 

 narrow band of membraneous chitin. The epipharynx, or ventral 

 lamina of the compound organ, has no attachment to the wall of the head 

 cavity, but is continued inwards in direct chitinous continuity with the 

 dorsal wall of the pumping organ, as in the Diptera. The distal end 

 of the labrum -epipharynx is blunt and rounded, and is not provided with 

 teeth or tubercles. 



The hypopharynx (Plate LIV, fig. 11) is rudimentary, and can only 

 be seen in dissections, and even then it is by no means easy to find. It 

 consists of a small but thick triangular plate, continuous behind with 

 the salivary receptacle, and projecting forwards between the epipharynx 

 and the mandibles at the base of the proboscis. It is pierced through 

 its centre by the salivary duct, which opens at its pointed apex, permit- 

 ting the saliva to flow into the grooves of the mandibles immediately 

 distal to it. 



The mandibles (Plate LIV, figs. 7 and 8) are the most important parts 

 of the mouth apparatus, as they are the only cutting weapons ; they also 



take part in the formation of the food canal, and convey 



, . . . . . The mandibles 



the saliva to the wound. In their method ot action and 



articulation the}' correspond more closely to the first maxillae in the 

 Diptera. Each mandible is a broad blade, rounded in contour at the 

 proximal end, but flattened for the most part of its length, the surfaces 

 of the two blades being inclined backwards and inwards from the externa 

 border. At the proximal end the internal surfaces are in contact with the 

 hypopharynx, but where this short piece ceases they come to lie in contact 

 with the epipharynx, forming the lateral sides of the food canal ; from 

 this point the flattened blades are inclined more vertically. On the 

 inner side of each mandible there is a groove, commencing at the point 

 where the hypopharynx ceases, and narrowing as it proceeds distally, 

 until in the distal half of the blade it is almost closed by the approxi- 

 mation of its edges. The grooves receive the saliva from the canal 

 in the hypopharynx, and convey it to the distal end of the proboscis 

 (Plate LVIII, fig. 9). 



The distal two-thirds of the external surface of the mandible is beset 

 with minute hook-like teeth (Plate LIV, fig. 7), arranged in three or four 

 rows, and directed towards the base of the blade. They are not set on 

 the extreme edge, as this is very much attenuated and soft, so much 

 so that it is difficult to define under the microscope. The purpose of 

 this membraneous expansion is of course to ensure that the canal formed 



