185] HEAD OF DIPTERA— PETERSON 15 



the adults of Gryllus and Periplaneta it is complete and similar to that 

 of Corydalis except that a small portion of each arm is wanting about 

 the antennae and the lateral ocelli. The ventral ends of the arms are 

 commonly called the fronto-genal sutures, and they join with the clypeus 

 as in Corydalis. All insects that have a sucking type of mouth, such 

 as the Hemiptera and Hymenoptera, usually show no signs of the stem 

 of the epicranial suture. The arms, however, are distinct and form the 

 lateral and dorsal boundaries of the large mesal piece commonly called 

 the clypeus. A large number of the Diptera possess an epicranial suture 

 which closely resembles that of the Hemiptera and the Hymenoptera. 

 On the basis of the above interpretation of the epicranial suture it has 

 been possible to homologize the sutures and sclerites, and the invagina- 

 tions of the tentorium on the cephalic aspect. No other interpretation 

 gave satisfactory results. 



The epicranial suture (e. s) in Mycetophila (Fig. 11) is complete 

 and closely resembles the hypothetical type. In Leia it closely resem- 

 bles that of Mycetophila except for the stem of the suture, which is 

 wanting dorsad of the median ocellus. The stem of the epicranial suture 

 in Psorophora (Fig. 10 and 26) and Chironomus (Fig. 12) is repre- 

 sented by a distinct suture in a deep fold on the meson. Other forms, 

 such as Rhabdophaga (Fig. 6), Mycetobia (Fig. 7), and Tabanus (Fig. 

 20), show depressions or thickenings along the meson. These marks 

 may have no significance. Outside of the above-mentioned forms, the 

 stem of the epicranial suture is wanting. 



The arms of the epicranial suture (a. e. s) are present in many 

 Diptera. This is the case in all but a few of the Nematocera, in a ma- 

 jority of the Brachycera, and in many of the families of the Cyclorrha- 

 pha. These resemble, therefore, the adults of the Hemiptera and Hy- 

 menoptera. The arms are present as definite sutures between two 

 chitinized areas in Tabanus (Fig. 20 and 21) and Leptis (Fig. 35), 

 and in the female of Simulium (Fig. 2). The epicranial suture is ap- 

 parently wanting in the male of Simulium (Fig. 3) unless the lateral 

 margins of the convex area represent it. In many genera the epicranial 

 suture is represented by the edge of a chitinized sclerite. This is the 

 case in Chironomus (Fig. 12), Trichocera (Fig. 16), Psorophora (Fig. 

 10), Mycetobia (Fig. 7), and Dixa (Fig. 19). The vertex in the genera 

 just named is membranous between the antennal fossae and the epicra- 

 nial suture. Sciara (Fig. 17), Rhabdophaga (Fig. 6), Bibiocephala (Fig. 

 4 and 5), and possibly Rhyphus (Fig. 9) and Bibio (Fig. 14), have 

 the arms of the epicranial suture represented by the chitinized margin 

 of the vertex, which is adjacent to the membranous portion of the 

 fronto-clypeus. The location of the invaginations of the arms of the 



