48 MOSQUITOES OF NORTH AMERICA 
of the third palpar segment, serrate on its inner margin at the tip and better 
developed than in most Nematocera; 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. 
“Tn 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, 
A 
e \ § ae 7 il : % 
{ id 
Lama. 
Fic, 2.—Frontal section through the head of old larva of Simulium sp., show- 
ing forming imaginal parts. 
lc., larval cuticle; ¢.d., {maginal derm; I.md., larval mandible; ¢.md., imaginal 
mandible ; Lme., larval maxilla; t.ma., imaginal maxilla; l.ma.p., larval maxillary 
palpus; Lhyp., 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 larve 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 antenne 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 larve 
