40 MEDICAL ENTOMOLOGY 



The buccal cavity lies partly between the projecting lateral portions of 

 the lower end of the fulcrum. It is a small chamber, formed by a single 

 triangular plate of chitin, the apex of which points downwards, the 

 lateral angles being turned forwards and inwards towards one another 

 and connected by a membrane. Distally the cavity is in communication 

 with the food canal in the proboscis, in a manner which will be described 

 presently. The buccal cavity has no dilator muscles, and plays only a 

 passive part in the mechanism of feeding. 



The haitstellum consists of the labrum-epipharynx, the hypopharynx, 

 and the labium, the latter being divided distally into two labella. 



Mandibles and first maxillae are absent. The two 

 Haustellum , . , . , . , , . 



organs which form the sucking tube are concealed m 



the groove on the anterior surface of the labium, which alone can be 

 seen without dissection. 



The labrum-epipharynx (Plate IX, fig. 1), when seen from the front, is 

 shaped like a blunt arrow head. It is a much softer organ than that of 

 Tabanus, being composed of much thinner chitin, and having more 

 cellular tissue between the two parts, which are readily distinguished 

 from one another. The labrum gives its shape to the organ, and forms 

 the anterior and lateral sides, and the external thirds of the posterior 

 surface. Where it is deficient in the middle line of the posterior or 

 ventral surface there is a deep chitinous gutter, the epipharynx, connect- 

 ed to the edges of the labrum by a short strip of membrane. The 

 distal end of the organ is flattened and tongue-like, and has on its ventral 

 surface several conspicuous sensory tubercles. Between the two walls 

 there is a series of fan-shaped muscles, arising from the internal surface 

 of the labrum and inserted into the epipharynx. 



At the upper end the two parts separate from one another. The la- 

 brum becomes continuous with the anterior wall of the rostrum, which is, 

 of course, continuous above with the clypeus. The epipharynx projects 

 a little distance into the rostrum, and is connected with the anterior wall 

 of the buccal cavity by a short membraneous fold. 



The labrum is provided with a pair of apodemes, which lie within the 

 rostrum, and are articulated to the labrum at its broad upper end. These 

 are stout sinuous rods of heavily pigmented chitin, pointed at the lower 

 end, and with a short barb-like projection just above the point. Their 

 ends are received into a pair of small pits in the labrum. The upper 

 ends of the apodemes are flattened and broadened, for the insertion of a 

 pair of powerful muscles. 



The hypopharynx is a soft tongue-shaped organ, rather shorter than 



