18 ILLINOIS BIOLOGICAL MONOGRAPHS [188 



condition it is possible to develop the type of fronto-clypeus found in 

 Psilocephala (Fig. 33 and 36). This came about by a membranous 

 development on the meson and on the lateral margins of the fronto- 

 clypeus and the loss of the arms of the epicranial suture directly ven- 

 trad of the antennae. The membranous development of the fronto- 

 clypeus of Platypeza (Fig. 32) resembles that of Psilocephala. Sceno- 

 pinus (Fig. 41 and 42) belongs to this same line, but in this genus the 

 antennae are adjacent to the fronto-clypeus and no portion of the 

 chitinized vertex exists between them. The form of the chitinized 

 portion of the fronto-clypeus resembles closely that of Platypeza (Fig. 

 32). Aphiochaeta (Fig. 31) and Lonchoptera (Fig. 37) apparently 

 belong to this same series. If such is the case, the arms of the epicranial 

 suture do not project dorsad but are represented by the nearly straight 

 ventral margin of the cephalic aspect. This condition must have come 

 about by the straightening out of the usual u-shaped depression, and 

 the chitinized part of the fronto-clypeus is located ventrad of the mar- 

 gin of the head. The tentorial thickenings along the ventral margin 

 of the head in Lonchoptera afford evidence favorable to the above inter- 

 pretation. A similar type of development occurs in Bibio (Fig. 14), 

 in which the invaginations for the anterior arms of the tentorium are 

 located on the ventral margin of the head-capsule latero- ventrad of 

 the antennal fossae. All the other Brachycera and Cyclorrhapha figured, 

 show the presence of sclerites designated as the tormae and located 

 ventrad of the fronto-clypeus, and this fact places them in the line of 

 specialization which leads toward a muscid type. 



The fronto-clypeus (fr. c) is present in all Diptera and constitutes 

 a prominent portion of the head-capsule. In Tabanus (Fig. 20 and 

 21) the fronto-clypeus is the entire area ventrad of the epicranial suture 

 and outside of the tormae and the labrum. The sutures separating the 

 fronto-clypeus from the genae (ge) are very indistinct. No arms of 

 the epicranial suture are present in Promachus (Fig. 22), Empis (Fig. 

 40), and Pipunculus (Fig. 38) ; consequently the dorsal extent of the 

 fronto-clypeus can not be determined, and the area ventrad of the 

 antennae is considered as the fronto-clypeus. The fronto-clypeus of 

 Mydas (Fig. 30) resembles that of Leptis, and from a type similar to 

 Mydas it is possible to develop the fronto-clypeus of Exoprosopa (Fig. 

 29), Eristalis (Fig. 25), and probably Stratiomyia (Fig. 27). The 

 fronto-clypeus of Mydas closely resembles that of the Acalyptratae and 

 the Calyptratae, as will be seen by comparing Mydas with Tetanocera 

 (Fig. 55), Chloropisca (Fig. 51), Chyromya (Fig. 50), and Musca (Fig. 

 72). It is not a completely chitinized area in all of the genera studied, 

 and the significance of this mesal membranous area in Sepsis, Oecothea, 

 and Calobata has been suggested in the discussion on the ptilinum. 



