187] HEAD OF DIPTERA— PETERSON 17 



the tentorium on the cephalic aspect and the arms of the epicranial 

 suture are usually closely associated in insects, there is every reason to 

 believe that the tentorial thickenings (t. th) mark the course of the 

 suture (a. e. s). Furthermore, the location of the thickenings of the 

 tentorium is very similar to the location of the slit-like invaginations 

 of Leptis (Fig. 35). These thickenings (t. th) have been considered as 

 marking the course of the arms of the epicranial suture. The extent 

 of the tentorial thickenings varies considerably, as shown in the figures. 

 In Tetanocera (Fig. 55), Chloropisca (Fig. 51), Heteroneura (Fig. 49), 

 and others, the tentorial thickenings extend to the antennal fossae (a. f). 

 No sutures are present between the dorsal ends of these thickenings. 



Fronto-clypeus. — The front (fr) and clypeus (c) of all insects are 

 unpaired selerites located between the arms of the epicranial suture 

 (a. e. s). The labrum (1) is also an unpaired sclerite attached typically 

 to the ventral margin of the clypeus. These three selerites and their 

 parts are not always distinguishable. This is particularly true of the 

 front and clypeus in Diptera. The dotted, transverse line uniting the 

 invaginations of the anterior arms of the tentorium (i. a) in the hypo- 

 thetical head indicates the position of the fronto-clypeal suture. In a 

 few of the Orthorrhapha, suture-like marks, depressions, or thickenings 

 extend across the chitinized portion of the fronto-clypeus. These marks 

 in Chironomus (Fig. 12), Mycetophila (Fig. 11), and Rhabdophaga 

 (Fig. 6) resemble the fronto-clypeal suture as indicated in the hjT)o- 

 thetical type. It is possible that they are remnants of this suture. 

 Excepting in the forms named, one can not be sure of the presence of 

 a fronto-clypeal suture ; consequently the entire area between the labrum 

 and the arms of the epicranial suture has been designated as the fronto- 

 clypeus (fr. c). The absence of the fronto-clypeal suture in Diptera 

 is not unusual, since it is wanting in many generalized insects. For 

 those who may wish to divide the fronto-clypeus into two areas, the 

 dorsal half would be the front and the ventral half the clypeus. A 

 large portion of the fronto-clypeus is membranous in Rhabdophaga (Fig. 

 6), Rhyphus (Fig. 9), and Sciara (Fig. 17), and the chitinized part 

 is greatly reduced. The variations found in the Nematocera are rep- 

 resented in the figures. 



The Brachycera show two lines of development in the modification 

 of the area enclosed by the arms of the epicranial suture. Both of these 

 started from a form which possessed an epicranial suture similar to that 

 of Leptis (Fig. 35). The line of development seen in Psilocephala, 

 Platypeza, Scenopinus, Lonchoptera, and Aphiochaeta is considered first. 

 The chitinized fronto-clypeus of Leptis resembles the fronto-clypeus of 

 a number of the Nematocera, as Sciara (Fig. 17). From this simple 



