195] HEAD OF DIPTERA — PETERSOX 25 



Sciara (Fig. 81), and Bittacomorpha (Fig. 85), it is only a thickened 

 edge ; but in a large nximber of species thruout the order it is a clearly 

 defined piece, set off from the occiput proper bj- a secondary suture. 

 The indefiniteness of this piece in a large number of the generalized 

 Diptera and the want of an homologous part in generalized insects 

 support the view that it is only a secondary modification of the occiput. 



The parocciput (poce), in most genera, occurs as a narrow piece 

 about the dorsal and lateral margin of the occipital foramen, and its 

 ventral ends project mesad. In the heads of the Cyclorrhapha thi-ee 

 secondarily developed, chitinized thickenings (th) arise from the ental 

 surface of the parocciput; two of these project dorso-laterad from the 

 lateral portions of the parocciput, and the third is on the meson. These 

 thickenings are also present in some of the Brachj-cera, such as Dolieho- 

 pus (Fig. 112). Their greatest development is found in Eristalis (Fig. 

 113), where two dorso-lateral thickenings (th) extend to the caudal 

 margins of the compound eyes and a third thickening, on the meson, 

 bifurcates a short distance dorsad of the occipital foramen, the two arms 

 connecting with the dorso-mesal angles of the compound eyes. In the 

 genera figured, the dorso-lateral thickenings are, on the whole, better 

 developed than the thickening on the meson. In Thelaira (Fig. 128) 

 and ilusca (Fig. 133) the dorso-lateral thickenings project dorsad to 

 the margin of the head. The area included between them is called by 

 several writers the epicephalon, or the occiput; and tho it is entirely 

 different in origin from similarly situated areas in Tabanus (Fig. 74) 

 and other genera, the same name is applied in the different cases. These 

 names and others used by systematists have no morphological signifi- 

 cance for they can not be homologized with the primary selerites of a 

 generalized insect. 



The postgenae (po) of the hypothetical dipterous head have been 

 carefully compared with those of the heads of such generalized insects 

 as the Orthoptera. The mesal membranous area between the postgenae 

 is homologous with the membrane of the neck and with the membrane 

 surrounding the proximal ends of the maxillae and the labium. There 

 are no sutures or selerites along the mesal portions of the postgenae ia 

 such generalized insects as the Orthoptera ; consequently the parapost- 

 genae (ppo) described above can not be homologous with any primary 

 sclerite. In Diptera the parapostgenae are undoubtedly special modi- 

 fications of the postgenae. 



The postgenae and the parapostgenae of a majority of the Nematoc- 

 era resemble those of the hypothetical head. In Chironomus (Fig. 

 88) and Trichocera (Fig. 78) the parapostgenal thickenings are want- 

 ing. The invaginations for the posterior arms of the tentorium in 



