225] HEAD OF DIPTERA— PETERSON 55 



One of the most important conclusions concerning the generalized 

 head-capsule relates to the position of the epicranial suture. The stem 

 of this suture along the dorso-meson represents the line of fusion of 

 the paired sclerites of the head, while the arms of the suture ventrad 

 of the antennal fossae enclose the unpaired sclerites of the head. This 

 suture resembles the epicranial suture in the immature stages and the 

 adult forms of all the generalized members of the more common orders. 



Two unpaired sclerites, front and clypeus, are enclosed by the fork 

 of the epicranial suture, and in all but one or two genera form a con- 

 tinuous area called the fronto-clypeus. 



The labrum is an unpaired, distinct, tongue-like structure situated 

 ventrad of the fronto-clypeus. It is joined to the epipharynx and the 

 resulting structure is known as the labrum-epipharynx. 



The tormae are chitinized lateral pieces of the epipharynx which 

 project cephalad and unite with the fronto-clypeus in generalized Dip- 

 tera. They are also present in such generalized insects as the Orthop- 

 tera. In the more specialized Diptera the tormae are interpolated be- 

 tween the fronto-clypeus and the labrum, and in all but a few genera 

 lose all connection with the chitinized portions of the fronto-clypeus. 

 Their exposed surface is best seen from a cephalic view. 



The crescent-shaped frontal suture dorsad of the antennal fossae 

 marks the line of invagination of the ptilinum. The origin of the 

 ptilinum has not been determined. 



The vertex is the paired continuous area on the cephalic aspect of 

 the head, and the region of the vertex ventrad and mesad of each com- 

 pound eye is a gena. 



The compound eyes are usually large and located on the cephalo- 

 lateral aspects of the head. They show secondary sexual characters in 

 a greater number of species than do any other of the fixed and movable 

 parts. The three ocelli are arranged in the form of a triangle and 

 located on the vertex dorsad of the bifurcation of the arms of the epi- 

 cranial suture. 



The occiput and postgenae are continuous areas of the caudal sur- 

 face. The former occupies the dorsal portion and is secondarily modified 

 about the occipital foramen to form the parocciput. The postgenae are 

 the two areas of the ventral half, separated by a membrane in gener- 

 alized forms and united ventrad of the occipital foramen in all the 

 Brachycera and the Cyclorrhapha. They are also secondarily divided 

 into parapostgenae along the mesal membrane. 



The tentorium of generalized Diptera is represented by the usual 

 three pairs of arms and a rudimentary body. It undergoes striking 

 modifications, and influences to a considerable extent the detailed struc- 



