191] HEAD OF DIPTERA — PETERSON 21 



chitinized and separated from the fronto-clypeus by a suture. In all 

 other genera there is a distinct membranous area present between the 

 fronto-clypeus and the labrum. This area is very extensive in the 

 Cyclorrhapha and includes the ectal exposure of the tormae. The la- 

 brum of a few scattered genera, such as Rhabdophaga (Fig. 6), Myceto- 

 bia (Fig. 7), Chironomus (Fig. 12), Scenopinus (Fig. 41), and others, 

 is completely membranous, while in still others it is nearly so, as in 

 Mydas (Fig. 30). The figures of the cephalic aspect of the head and 

 the lateral views of the epipharynx and the hypopharynx show the 

 shape and extent of the chitinization of the labrum. 



The labrum of Dixa (Fig. 501), Trichocera (Fig. 499), Sciara 

 (Fig. 513), Bibio (Fig. 523), Simulium (Fig. 497), Culicoides (Fig. 

 521), Tabanus (Fig. 20), and Dolichopus (Fig. 528) is distinctly sepa- 

 rated from the epipharynx (ep) by a membrane. This condition is 

 best seen in a lateral view. A majority of the forms studied have little 

 or no membrane between the labrum and epipharynx. This is particu- 

 larly true of the Cyclorrhapha. The surface of the labrum of all Dip- 

 tera is more or less convex. In a large number of the genera the con- 

 vexity is very decided and of such a nature as to surround the cephalic 

 and lateral aspects of the epipharynx. The epipharynx in these forms 

 can only be seen in a caudal view. In the Calyptratae, the labrum and 

 epipharynx are firmly united in one piece. 



The labrum of Simulium (Fig. 2 and 489) is unique in that the 

 chitinized part consists of a narrow mesal piece which bifurcates at its 

 distal end. These bifurcations give rise to special small hook-like struc- 

 tures (h) which have been incorrectly interpreted as mandibles (Smith, 

 1890). The labrum and epipharynx of Psorophora (Fig. 504) fit to- 

 gether very closely. By careful dissection they may be separated, as 

 seen in the drawing. So far as observed, no membrane is present be- 

 tween them. The proximal end of the labrum is crook-like in form, 

 and muscles connect with this portion. 



Vertex. — The vertex (v) of a hypothetical head (Fig. 1) consists 

 of the paired continuous areas on the cephalic aspect of the epicranium. 

 It is interpreted as including all the cephalic and dorsal aspects of the 

 epicranium except the front. In a number of the Diptera, as heretofore 

 described, the stem of the epicranial suture (s. e. s) is present and marks 

 the line of fusion of the two halves of the vertex, upon w T hich the ocelli 

 and the antennae are located. The shape and size of the chitinized 

 portion of the vertex is largely determined by the size of the compound 

 eyes, the location and extent of the membranous area about the base 

 of the antennae, and the location of the arms of the epicranial suture. 

 The variations in the size and shape of the vertex are shown in the figures 

 of the cephalic aspect of the head. 



