16 ILLINOIS BIOLOGICAL MONOGRAPHS [186 



tentorium usually helps to determine the location of the epicranial su- 

 ture. In Ptychoptera (Fig. 15) the invaginations of the anterior arms 

 ■of the tentorium are located in the distinct V-shaped depression on the 

 -chitinized area ventfad of the antennae. Undoubtedly this depression 

 marks the position of the epicranial suture. Tipula (Fig. 18) has a 

 very specialized head and shows no epicranial suture or tentorium. 



Only the arms of the epicranial sutures are present in the Brachye- 

 era. On the whole these sutures are not as well developed in the 

 Brachycera as in the Nematocera. When present (a. e. s) they are long 

 and slit-like in all the genera except Tabanus. This condition is due 

 to the fusion of the invaginations of the dorsal arms and the anterior 

 arms of the tentorium along each suture. The arms of this suture in 

 Tabanus (Fig. 20 and 21) unite the invaginations on each lateral half 

 of the head, but they are not decidedly slit-like. 



The arms of the epicranial suture (a. e. s) in Tabanus (Fig. 20) 

 have the usual inverted-u shape and their ventral ends terminate at the 

 ventral margin of the head. The arms are indistinct ventrad of the 

 invaginations of the anterior arms of the tentorium. The invaginations 

 (i. a) in Promachus (Fig. 22) are slit-like and situated near the ventro- 

 lateral angles of the compound eyes. The epicranial suture is wanting 

 dorsad and ventrad of the invaginations of the anterior arms, and in 

 this respect Promachus differs from Leptis and Tabanus. From Leptis 

 (Fig. 35) it is possible to homologize the arms of the epicranial suture 

 of all the Brachycera and those of the Cyclorrhapha. The arms of the 

 suture in Leptis are long and slit-like and coincide with the invagina- 

 tions of the tentorium on the cephalic aspect of the head. They extend 

 dorsad from the ventral margin of the head to a point ventrad of the 

 antennae, where they unite and enclose a convex mesal area called the 

 fronto-clypeus (fr. c). This suture (a. e. s) in Platypeza (Fig. 32) 

 closely resembles that of Leptis. The dorsal ends of the arms of the 

 epicranial suture are wanting in Psilocephala (Fig. 36), Mydas (Fig. 

 30), Exoprosopa (Fig. 29), Eristalis (Fig. 23 and 25), and Scenopinus 

 (Fig. 41 and 42), and in other forms. Scenopinus shows a striking 

 variation in that the vertex is membranous between the antennae and 

 the fronto-clypeus, and no epicranial suture can be traced thru the 

 membrane. Stratiomyia (Fig. 27) shows a unique development of the 

 slits in that they extend mesad rather than dorsad. This condition is 

 undoubtedly a secondary development. The epicranial suture of Lon- 

 choptera, Aphiochaeta, Pipunculus, and Empis is discussed under fronto- 

 clypeus. 



No epicranial suture or slit-like invaginations are present in any 

 dipteron that has a frontal suture (fr. s) or a ptilinum (pt). Since 



