MOUTH-PARTS. 25 



absent, the mandibles most frequently; when present 

 they are always slender and firm. The hypopharynx is 

 unpaired and slender, grooved on the upper side and 

 sometimes converted into a nearly complete tube. The 

 labrum, also unpaired, is usually elongate and grooved 

 on the under side, forming by apposition with the hypo- 

 pharynx a complete tube. The mandibles are frequently 

 absent ; in fact I do not know of their occurrence in any 

 flies with a simple third antemial joint, and they may be 

 absent in the male when present in the female, as in the 

 Tabanidae. They are always piercing organs, thin, firm, 

 chitinous and usually slender. The two maxillae, like- 

 wise piercing organs, find their highest development in 

 such predaceous flies as the Asilidse. Like the mandi- 

 bles they are chitinous and slender. In some they are 

 more or less flattened, and may have curiously shaped 

 projections at the tip; usually they are bristle-like. 

 They lie with the maxillae within the sheath of the labi- 

 um, at either side of the labrum and hypopharyiix. In 

 some cases the labrum is short, and serves only as a cov- 

 er for the proximal part of the hypopharynx, but usually 

 it is as long as or longer than the hypopharynx and has 

 a simple groove on the under side. The hypopharynx 

 is always present in flies in which the mouth-parts are 

 functional. It is, more often, a slender, firm organ, 

 grooved upon the upper side, which by apposition with 

 the labrum forms a distinct tube. In some, however, 

 it may form an almost complete tube in itself. 



Leaving out of account the degraded, but highly spe- 

 cialized Pupipara, the labium is always a sheath for all 

 the other organs except the palpi, but is separable at the 

 will of the insect. It is not used in piercing; it is either 

 bent backward in the middle, as in the mosquito, or the 

 piercing parts are thrust out at the extremity as in most 

 of the predaceous flies. To facilitate this protrusion of 

 4 



