22 MEDICAL ENTOMOLOGY 



the epipharynx and the hypopharynx separate from the stomodaeum 

 may be termed the mouth. The distal aperture between the epi- 

 pharynx and the hypopharynx is the prestomum. 



In addition to a channel by which the fluid food can be conveyed to 

 the mouth, some apparatus is necessary to provide a sucking force. 

 This is formed by a modification of the stomodaeum within the head. 

 The walls of this are strongly chitinized, and are provided with muscles 

 which pass between them and the walls of the head capsule ; on contrac- 

 tion these muscles draw the walls of the tube apart and so produce a 

 negative pressure. This causes the fluid to flow up the food channel. 



The appendages are modified for their specialized function in the 

 blood-sucking forms by elongation, by the reduction of superfluous parts, 

 and by the development of a suitable armature. (Plate III, fig. 4.) In the 

 simpler forms, such as the mosquitoes and the horse flies, the wound is 

 made by the mandibles and the first maxillae, the labium serving 

 mainly as a sheath for the other appendages and the food channel. 

 They are specialized into piercing stylets, elongated on account of the 

 depth of skin which has to be pierced in order to reach to the level 

 of blood ; the number of joints, in the case of the maxilla, is reduced to 

 ensure rigidity, and the armature and musculature are so modified as to 

 be suitable for making a deep wound of narrow bore. In the higher 

 Diptera the mandibles and first maxillae are not present, and the whole 

 function of making the wound is transferred to the second maxillae, at 

 the distal end of which there is developed a highly specialized arrange- 

 ment of teeth. 



Considered with regard to their mouth parts, the blood-sucking 

 Diptera fall into two well-defined classes, those in which the mandibles 

 and first maxillae are present and functional, and those in which they are 

 absent, and their function fulfilled by the second maxillae. These two 

 classes correspond with the two sub-orders into which the Diptera are 

 divided, the Orthorrapha and the Cyclorrapha. Some examples of each 

 will now be described in detail. 



The proboscis of Tabanus and of the allied genera (excluding Pan- 

 gonia) is relatively short, and hangs downwards from the lower surface 

 of the head. All the parts enumerated at the commencement of this 

 section are present and functional. 



The mandibles (Plate IV, fig. 4) are broad flat blades of yellow 

 chitin, slightly recurved on their inner edges. The cutting armature 



_ ._ is limited to the distal portion of the inner edge, and 



Tabanus 



consists or a row of extremely fine serrations like those 



