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 (Plp). The maxillae are probably the principal piercing tools of the horsefly's mouth-part equipment.
The median hypopharynx (Fig. 170 D, Hphy) is a tapering blade somewhat hollowed above, normally appressed, as just observed, against the under surface of the labrum to form the floor of the food canal. The hypopharynx itself is traversed by a narrow tube which is a continuation from the salivary duct (SlD). 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 sort, membranous under surfaces of the lobes, which are known as the labella, 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
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