189] HEAD OF DIPTERA— PETERSON 19 



Tonnac. — The tormae (to) in generalized insects are cliitioized 

 pieces which belong to the lateral portions of the epipharynx in the 

 region of the clypeo-labral suture and connect with the cljT)eus or la- 

 brum at the lateral ends of the suture. These are well illustrated in such 

 Orthoptera as Periplaneta (Pig. 514), Melanoplus (Fig. 515), and 

 GryUus (Fig. 516). 



The tormae of generalized Diptera also connect with the inner sur- 

 face of the ventral portion of the frouto-clypeus. They are not well- 

 developed structures or readily distinguishable from the fronto-cl.vpeus 

 in a number of species of the Nematocera. This seems to be due to the 

 decidedly convex nature of the fronto-clypeus and the close proximity 

 of its lateral portions to the lateral margins of the epipharynx. The 

 tormae of Leptis (Fig. 520), Psilocephala (Fig. 36 and 533), Scenopi- 

 nus (Fig. 41 and 538), Aphiochaeta (Fig. 31 and 544) Louchoptera 

 (Fig. 37 and 539), and Platypeza (Fig. 32 and 543) connect with the 

 fronto-clypeus and thus resemble the Nematocera and the hypothetical 

 type. In Tabanus, the tormae (Fig. 494) resemble the above genera in 

 their connection with the fronto-clypeus, but they have been enlarged 

 ventrad until they are exposed between the clypeus and the labrum 

 (Fig. 20 and 494). The exposed portions of the tormae resemble two 

 small, triangular sclerites with their pointed ends meeting on the meson. 

 This condition is not unusual since they resemble closely the exposed 

 portions of the tormae located at the lateral ends of the clypeo-labral 

 suture in Gryllus (Fig. 516). Siraulium (Fig. 2 and 489) also shows 

 exposed portions of the tormae at the ventro-lateral angles of the fronto- 

 clypeus (fr. c). 



The inverted chitinized V-shaped piece ventrad of the fronto- 

 clypeus in Mydas (Fig. 30) has undoubtedly been derived from the fusion 

 of the tormae of some form resembling Tabanus (Fig. 20). The tor- 

 mae are adjacent to the fronto-clypeus in Mydas, but they are not con- 

 nected with the same as in Tabanus. From the type of tormae found 

 in Mydas it is possible to develop the tormae of all other genera. The 

 tormae vary in shape and position as seen in the cephalic views of the 

 head. In Exoprosopa (Fig. 29), Eristalis (Fig. 25), and Stratiomyia 

 (Fig. 27) they show a striking development in that they are located 

 within deep emarginations of the ventral nuirgin of the fronto-clypeus. 

 The tormae of Empis (Fig. 40) closely resemble those of IMydas and 

 belong to the same line of development. In Pipunculus (Fig. 38) the 

 tormae resemble the fronto-clypeus of Sciara (Fig. 17), but as a matter 

 of fact the fronto-clypeus is the area ventrad of the antennae, as shown 

 by the location (Fig. 151) of the dorsal arms of the tentorium (d. a). 

 The tormae of the Acal.vptratae are usually crescent-shape, while in the 

 Calyptratae they resemble the type found in Mydas. 



