60 MEDICAL ENTOMOLOGY 



Newstead, it is actually connected to the lateral extremities of the 

 labrum by a fine membrane, so that the food canal in the haustellum 

 becomes a closed tube. The organ lies for the most part of its length in 

 a small median recess in the labial gutter. 



The mentum in the bulb presents no special characters, but in the 

 narrow part of the proboscis the chitin is very thin and semi-membraneous, 

 and bears a number of peculiar spines on the ventral surface, arranged in 

 two parallel rows. It is connected to the labial gutter by a narrow 

 membrane divided into polygonal areas. The labial gutter is composed 

 of thick chitin, and contributes much more to the rigidity of the 

 proboscis than the mentum. In the middle of the haustellum it forms 

 an almost complete tube, rather broader in the transverse diameter than 

 in the vertical, and with a deep and narrow pocket in the middle ventral 

 line for the reception of the hypopharynx. The lateral portions are 

 thinner and pointed, and overlap the labrum, to which they are connected 

 by the row of interlocking teeth mentioned above ; at this level the sides 

 of the mentum are also thickened, so that there is very little space left 

 between the anterior and posterior walls of the labium. Distally the 

 walls of the gutter become thinner, and are directly continuous with 

 the inner walls of the labella, no discal sclerite being differentiated in this 

 form. At the upper end of the labium the sides of the gutter are more 

 widely separated from one another, and the labrum appears on the 

 dorsal surface of the proboscis. 



The biting apparatus (Plate XIII, fig. 9) consists of prestomal teeth 

 evidently homologous with those of Musca, and in addition the ' rasps ', 



and certain sharp spines on the outer wall of the 

 101 labellum. The arrangement at the prestomum is 



as follows. On each of the internal labellar walls 

 there are two fissures, which divide it into three equal portions, 

 dorsal, middle, and ventral. The main part of each division consists 

 of a ' rasp ', an oblong plate, longest in the antero- posterior diameter, 

 and traversed by about ten ridges running from the dorsal to the 

 ventral side ; each of these is in turn being divided into minute ridges 

 running in the long axis of the proboscis. At the distal end of each 

 rasp there are two large conical teeth, the internal ones of the dorsal and 

 ventral sets being larger than the external. These large teeth arise 

 from a small chitinous eminence, at the sides of which there are 

 two smaller teeth. Between the two larger teeth of each division, 

 and also on the dorsal and ventral surfaces of the inner wall, there 

 is a set of excessively fine scales, arranged in a fan -shaped manner, 



