Introduclion. 5 



lo four times as long as the whole body. As belonging to the niouth 

 parts are still two organs concerned, the hypopharynx and the labrum ; 

 the hypopharynx is a prolongation of the lower wall of the pharynx, 

 and in it opens the salivary duet; it lies in the bottom of the canal 

 in the labium and is often pointed triangular or sometimes more 

 bristle-shaped, shorter or longer; it may be developed as a stinging 

 apparatus, as in the Asilidæ. The labrum doses the labial canal 

 above and is generally somewhat semitubular, with the hoUow surface 

 downwards, it may be of different lengths and also vary somewhat in 

 form, but the length is generally in proportion to the length of the 

 labium; in the Culicidæ it is used to sting with. All the parts 

 mentioned form together the proboscis. The mouth parts are inserted 

 at the mouth-aperture, but the case may here be somewhat different 

 in an interesting way; in some flies as in many Orthorrhapha, f. inst. 

 Tahanid(f>, and in at all events most of the Nematocera the connecting 

 membranes of the mouth parts are small and narrow, and the lower 

 part of the head is well chitinised : ihe proboscis is then generally 

 only slightly pro- and retractile. In other Diptera as in Stratiomyiidæ 

 and, I tliink, in all Gyclorrhaphæ the lower part of the head is 

 membraneous and not chitinised. and at the same time more or less 

 prolonged; probably also the connecting membranes of the mouth 

 parts enter into the formation of this part. I term this part, from 

 the firm and chitinised mouth-edge to the base of the proboscis, the 

 oral cone, and it is of importance to distinguish it from the proboscis 

 itself with which it in earlier times has often been confounded, (stipes 

 Latreille, Riissel (Proboscis) Meigen); it is not a part of the mouth 

 but the lower part of the head ; it may sometimes be of considerable 

 length. In the flies in which the lower part of the head is firm and 

 chitinised as in the Tabanidæ, the clypeus is not, or not distinctly, 

 separated from the epistoma; on the contrary in flies where an oral 

 cone is present, the clypeus is separated and lies on the front side of 

 the cone; it is generally elongated and reaches from the epistoma to 

 the labrum, often, as in the Si/rphidæ, it is deeply excised at the apex, 

 and this excision may be so large that the clypeus becomes horseshoe- 

 shaped as in most Muscidæ; also in flies without an oral cone, the 

 clypeus may be excised at the apex, f. inst. in the males of the 

 Tabanidæ. Often the membraneous part between the clypeus and the 

 lower end of the epistoma is narrow, but sometimes broader. The 

 clypeus may be divided in two parts as in some Syrphidæ, a small 

 basal part being found, connected with the epistoma. In the interior 

 of the head lies the pharynx with its lower end reaching down to the 

 insertion of the mouth parts; its hinder wall is chitinised, and in flies 



