INTRODUCTION. XV 



bles 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 com- 

 plete tube. The labrum, also unpaired, is usually elongate 

 and grooved on the under side, forming by apposition with 

 the 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 antennal 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, likewise piercing organs, 

 find their highest development in such predaceous flies as the 

 Asilidae. Like the mandibles they are chitinous and slender. 

 In some they are more or less flattened, and may have curi- 

 ously shaped projections at the tip; usually they are bristle- 

 like. They lie with the maxillae within the sheath of the 

 labium, at either side of the labrum and hypopharynx. In some 

 cases the labrum is short, and serves only as a cover 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 almost a complete tube in itself. 



Leaving out of account the degraded, but highly specialized 

 Pupipara, the labium is always a sheath for all the other or- 

 gans 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 the piercing parts, the proximal 

 portion is more or less membranous and retractile; or, the 

 inner organs may be capable of elongation, being coiled 

 up in some cases, as in Pangnnia, within the pharyngeal 



