283 



which are represented in figures 7 and 8, Plate XXIII. The labrum 

 hangs over in front of the head, and can be drawn backward so as to 

 close over the mouth orifice. The function of the hooks present on the 

 cpipliarynx, or under surface of the labrum, is probably that of retain- 

 ing food in the mouth, but they are also used in assisting the larva in 

 its movements within its burrow, and also over any surface, as in pro- 

 gressing it generally grasps the sides of the burrow or some other ob- 

 ject with the mouth parts, drawing the body forward at the same time. 

 To the lateral plates are attached the antenncu and mandibles; on each 

 lateral plate there are generally two black pigment spots, which are ru- 

 dimentary eyes; the lateral plates curve down over the side of the head 

 and meet in the center of the under surface, which junction is marked 

 by a faint suture. The antenna: are in many larvae very small, in others 

 of considerable size, and in Tanypus and its allies are retractile within 

 the head for almost their entire length. The usual form of antenna 

 consists of a large and stout basal joint, on which there is generally a 

 sensory spot, or a hair, and on the apex of this joint one simple, gen- 

 erally hairlike, process of varying length, and a process with three to 

 five distinct jc^nts, which presumal)ly represents the true continuity of 

 the antenna. The mandibles in all the species which I have examined 

 are large and heavily chitinized, generally toothed on their inner sur- 

 face, and move on an articulated base so as to close inward ; when com- 

 pletely closed their apices are visible behind the anterior transverse 

 margin of the labial plate. The brushlike hairs which are present on 

 the mandibles of the larvae of Sinmlidcc and Culicidcc are much less 

 prominent in the larvce of Chironomidce, though still distinguishable. 

 The maxilUr are much retracted and rather rudimentary in many spe- 

 cies, and but little use has been made of them in descriptions. The 

 character which has been used more than any other for the separation 

 of larvae of this family lies in the structure of the labial plate, or sub- 

 mentuni. In Chironomus and several other genera, Orthocladins and 

 Crieotopns in particular, this plate is exposed and is therefore easily 

 accessible ; but in all the species of Tanypincc which are represented in 

 the material before me, the labial plate is very small and generally re- 

 tracted within the mouth, or occupies a vertical position so that its 

 form is indistinguishable ; the labial papill(U of a species of CJiirononnis 

 are as in Figure 10, Plate XXIII. A further discussion of this matter 

 will be found under Tanypin(e. The larval head of Dixa differs very 

 considerably from that of any chironomid, and shows the dorsal 

 sclerites clearly (PI. XXIII, Figs. 9 and 12). 



Abdomen. — The thorax and abdomen of the larva combined con- 

 sist of twelve segments, which are almost devoid of hairs in most of 

 the aquatic species. Several of the terrestrial species lire figured here- 



