48 



MOSQUITOES OF NORTH AMERICA 



of the third palpar segment, serrate on its inner margin at the tip and better 

 developed than in most JSTematocera ; and a pair of short mandibles, broad, thin, 

 and weakly chitinized. As in other nematocerous flies, there is a well-developed 

 labrum-epipharynx and an elongate flattened hypopharynx. In the males the 

 mandibles are wanting. 



" In the larva the mouth is of the biting type, with short-toothed and heavy 

 mandibles, short, jawlike maxilla with distinct one-segmented palpus, and a 

 small, strongly chitinized labium or labial plate. In addition, labrum, epi- 

 pharynx, and hypopharynx are all well developed. 



"The head of the larva having a thoroughly opaque, strongly chitinized cuticle. 



hnxp. 



mx. 



ihyp. ! tmc/. 

 ^ i.md. 



Fig. 2. — Frontal section through tJie head of old larva of SiinuUum sp., show- 

 ing forming imaginal parts. 



I.e., larval cuticle ; i.d., imaginal derm ; I.md., larval mandible ; i.md., imaginal 

 mandible ; l.mx., larval maxilla ; i.mx., imaginal maxilla ; l.mx.p., larval maxillary 

 palpus ; l.hyp., hypopharynx. 



it was impossible to clear whole heads sufficiently to make visible the develop- 

 ing imaginal head and its parts, so that the method of sections had to be relied 

 on to reveal the internal conditions. These sections of heads of larvae of various 

 ages show plainly that the general method of development of the imaginal parts 

 within the larval head, and the correspondence between forming imaginal parts 

 and the corresponding larval parts already noted in the other orders of holo- 

 metabolous insects, hold good in the Diptera. Fig. 1 shows in sagittal longitud- 

 inal section the forming imaginal head parts within the larval head. This sec- 

 tion shows particularly well the relation of the forming imaginal antenna to the 

 larval antenna. In the larva the antennse are very small compared with their 

 size in the imago, and the imaginal antenna is thus forced, in its development, to 

 occupy a region in the larval head not included in the larval antenna. But the 

 tip of the imaginal organ lies fairly within the larval organ, thus indicating by 

 correspondence in position, what is plainly obvious from anatomical considera- 

 tion, the homology between the larval and imaginal organs. Similarly the 

 forming imaginal mouth parts are to be found in unmistakable correspondence 

 or homologous relation with the larval parts. By tracing the development of 

 the parts, marked in fig. 1 as the forming imaginal mouth parts, through larvae 



