MOSQUITOES AND FLIES 



the outer edges and thin on the knifelike inner edges. 

 They appear to be cutting organs, for each is articulated 

 to the lower rim of the head by its expanded base in such 

 a manner that it can swing sidewise a little but can not be 

 protruded and retracted as can the corresponding organ 

 of the cicada. The maxillae (C) are slender stylets, each 

 supported on a basal plate attached to the head; this plate 

 carries also the large, two-segmented palpus {Pip). The 

 maxillae are probably the principal piercing tools of the 

 horsefly's mouth-part equipment. 



The median hypopharynx (Fig. 170 D, Hphv) is a 

 tapering blade somewhat hollowed above, normally ap- 

 pressed, as just observed, against the under surface of the 

 labrum to form the floor of the food canal. The hypo- 

 pharynx itself is traversed by a narrow tube which is a 

 continuation from the salivary duct {SID). The latter, 

 however, just before it enters the base of the hypopharynx, 

 is enlarged to form an injection syringe {Syr). The 

 salivary syringe in structure is a small replica of the mouth 

 pump (A, Pmp), and its muscles arise on the back of the 

 latter. The saliva of the fly is injected into the wound 

 from the tip of the hypopharynx. By reason of this fact, 

 the bite of a fly may be the source of infection to the 

 victim, for it is evident that the injection of saliva affords 

 a means for the transfer of internal disease parasites from 

 one animal to another. 



Behind all the parts thus far described is the median 

 labium (Fig. 170 D, Lb), a much larger organ than any of 

 the others, consisting of a thick basal stalk and two great 

 terminal lobes (La). The soft, membranous under sur- 

 faces of the lobes, which are known as the label/a, are 

 marked by the dark lines of many parallel, thick-walled 

 grooves extending crosswise. These grooves may be 

 channels for collecting the blood that exudes from the 

 wound, or they may also distribute the saliva as it issues 

 from the tip of the hypopharynx between the ends of the 

 labella. The effect of the saliva of the horsefly on the 



