AQUATIC MIDGES AND SOME RELATED INSECTS. 39 



the Tanypinae, is not confined to this subfamily, but is a common reaction in all free- 

 living species of the Chironominae. 



The author's chief justification for considering the feeding habits of this entire 

 subfamily together is the similarity in the structures of the head and the mouth parts. 

 The figures and the discussion of these parts found chiefly in systematic works have 

 misinterpreted the homology of these parts. On this account, as well as the fact that 

 the mouth parts in this group are of prime importance in any discussion of their feeding 

 habits, they will be discussed in considerable detail. 



MOUTH PARTS. 



The hypopharynx of the larva of the subfamily Tanypinae has commonly been 

 called the labium. But it seems probable from figures 42 and 43, tk, that what has 

 hitherto been called the labial plate is really an especially well chitinized anterior border of 

 the hypopharynx. Its strong development is here associated with its very much greater 

 functional importance in this species. The strong muscles attached to this part of the 

 head swing the plate upward and backward, the entire chitinized framework of the 

 hypopharynx taking part in this movement. The toothed border of the upper chiti- 

 nized bar also serves a similar scraping and cutting function (figs. 42 and 43, hy). 

 The labium proper is double and has been labeled hypopharynx. From its position it 

 seems more properly called the labium, and its double nature finds a partial counter- 

 part in the labium of Chironomus digitatus Malloch (Malloch, 1915, pi. 30, fig. 13). 



In the latter species the central part of the labium is a large rounded light-colored 

 process, while the two sides are black and toothed in a manner very similar to the two- 

 toothed areas in the Tanypinae. The central area appears to be homologous to the 

 "labial papillae" of Malloch, 1915 (pi. 25, figs. 4 and 9; see also fig. 41 in this work). 

 This centrally arranged flap is soft and muscular and has a band of roughened scales 

 on its dorsal surface. The mandibles are opposable and very pointed. They are also 

 able to be used in opposition to the labium. In Tanypus dyari they are furnished 

 with a row of seven lateral teeth (fig. 41, md.), the first of which is especially well 

 developed in practically all species, as is the case in T. dyari. It seems possible that 

 this long-pointed tip, together with the first well-developed tooth, are structures 

 homologous to the double tip so common in the Chironominae, which is frequently 

 mentioned as of specific value. 



The maxilla in the Tanypinae (fig. 40) is very different from the homologous struc- 

 ture in the Chironominae. Here instead of being attached to a flattened plate (figs 

 11 and 12) it is capable of a considerable movement. It consists of a flattened append- 

 age with a roughly circular ehitinous supporting structure made up of several partially 

 fused sclerites (fig. 40). This freedom of movement and increased functional impor- 

 tance of the maxilla have an important bearing on the freedom of the movement of 

 the mandible, as one can readily appreciate who is familiar with the restricted move- 

 ment of the latter in the genus Chironomus due to the fusion of the maxilla to the sides 

 of the head. Correlated with this freedom of movement of the maxilla one is able to 

 note that the mandibles may be employed either above or below the maxilla. The 

 anterior portion of the maxilla (fig. 40) is furnished with a large number of thin plate- 

 like processes which doubtless have a tactile function. 



