24 MEDICAL ENTOMOLOGY 



each retraction the barb-like teeth are drawn through the tissues. In 

 each case the armature of the blade and the musculature are admirably 

 adapted to one another. One might compare the action of the mandible 

 to that of a circular saw cutting through a short arc, and that of the 

 maxilla to a file, thrust in and out of the wound. As the wound is deep- 

 ened the cutting blades are gradually lowered into it. (Plate IV, fig. 4.) 



The labrum-epipharynx has its two constituent parts fused together to 

 form a flattened spatulate slip, convex on its dorsal surface, and resembling 

 a two-handed sword when seen from the front. The labral lamina is 

 thinner than the epipharynx, and does not reach quite to the distal end. 

 The two are only loosely united at the proximal end, where the labrum is 

 attached to the clypeus by a short tongue-shaped piece, in which is 

 inserted a short muscle. The epipharynx is separated from the labrum 

 by a small amount of cellular tissue, and at the upper end it fuses with 

 the dorsal plate of the buccal cavity, there being no suture to break 

 the continuity of the chitin. Its distal end is blunt, and has on it three 

 sets of minute tubercles, two lateral and one median. These are not 

 cutting teeth, but have another function, which will be referred to later- 

 The ventral surface is strongly concave, and forms the dorsal wall of 

 the food channel. 



The hypopharynx resembles the labrum-epipharynx in shape, but is 

 much thinner and more slender, and is produced to a narrower point ; the 

 extreme tip, in fact, is very soft and thin. It is a little shorter than the 

 labrum-epipharynx, and flatter on section. When the parts are in sitti 

 it closes in the gap between the two lateral edges of the groove in 

 the epipharynx. At its base the hypopharynx is directly continuous with 

 the ventral plate of the buccal cavity. It is pierced throughout its length 

 by the salivary duct. Where it merges with the buccal cavity there is a 

 small chitinous pouch, the salivary pump, on the ventral surface. (Plate 

 V, fig. 1.) This is interposed between the salivary duct in the head and 

 that in the hypopharynx, and is surrounded by a small fan-shaped 

 muscle, by which the supply of saliva to the wound is believed to be 

 regulated. (Plate V, fig. 3.) 



The labium, unlike the other mouth parts, is a soft organ, and has a 

 wall composed of thin plates of chitin and membrane, enclosing a 

 series of muscles and other structures. It and the palps are the only 

 parts which can be seen on external examination, the other organs 

 being entirely concealed in a groove on its anterior surface when the 

 proboscis is in the position of rest. It is divided in its distal third 

 into two labella. The wall of the labium . proper is formed, on the 



