PROBOSCIS OF GLOSSINA : MECHANISM 61 



which represent the interdental armature. These are termed by Stephens 

 and Newstead the ' fans ' . 



The mechanism of the biting apparatus (Plate XIII, fig. 8), is a 

 modification of that described in the Stomoxydinae. On account of the 

 disappearance of the discal sclerite there is no anterior joint, all the 

 movements taking place on the posterior one. The furca is represented 

 by a broad but rather thin transverse bar, the lateral arms of which turn 

 forwards and upwards on to the sides of the labella, and are there 

 received into shallow pits in the external wall. The fork of the mentum 

 is represented by a thickening of the chitin immediately posterior to 

 the furca, and consists of two lateral rods united by their broad distal 

 ends, and so closely opposed to the furca that they appear to form with 

 it a T-shaped piece. Both the furca and these forks are simple thicken- 

 ings of the chitin, not nearly so well demarcated as the corresponding 

 parts in Stomoxys, and not raised above the surface. 



The external walls of the labella are partly membraneous and partly 

 chitinous. On the posterior surface there is a circular area, situated 

 about midway between the distal end and the furca, in which the wall is 

 thickly chitinized and pigmented. This part, referred to by Stephens and 

 Newstead as the 'dark area ', is readily recognized under a low magnifica- 

 tion. Posterior to this the wall is membraneous as far as the furca. The 

 lateral walls are mainly chitinized, and are produced at the posterior ends 

 to a sharp angle on each side. Distal to these angles, and considerably 

 removed from them in the position of rest, there is a pair of small pits at 

 the end of longitudinal thickenings on the lateral wall of the mentum. 



The muscles of the bulb, which cannot readily be separated into 

 bundles, terminate in long tendons which traverse the whole of the 

 narrow part of the mentum, and are inserted partly into the lateral 

 arms of the furca and partly into the thick and pigmented area anterior 

 to it. When they contract the furca, which is fitted into a groove on 

 the distal end of the fused rods, is rotated in the usual manner, so 

 that its lateral arms come to point in the reverse direction, that is, 

 towards the base of the proboscis. As the tendons are also inserted into 

 the chitinized area distal to the furca, the posterior wall as a whole is 

 drawn upwards together with it, so that there is a complete change in 

 the relations of the parts. The dark area is approximated to the middle 

 portion of the furca, and the posterior angles of the external walls are 

 pulled upwards till they fit into the pits on the lateral wall of the men- 

 tum. The teeth and rasps are everted and pulled upwards through the 

 wound in the usual manner, following the movement of the external wall. 



