30 ILLINOIS BIOLOGICAL MONOGRAPHS [200 



is no evidence to the contrary. In some genera, as in Lonchoptera 

 (Fig. 177), Rhabdopliaga (Pig. 170), and Empis (Fig. 164), the tento- 

 ria are not free rods extending thru the head cavity, but are completely 

 united with the ventral margin of the head, or nearly so. The tentorium 

 of Aphiochaeta (Fig. 174) is reduced to two small ental projections 

 adjacent to the occipital foramen, while in Tipula (Fig. 178) the ten- 

 torium is apparently wanting. 



In a majority of the Brachycera the tentorial arms are specialized 

 by fusion, and Tabanus (Fig. 143) illustrates an early stage in this 

 development. The principal difference between the tentorium of Taba- 

 nus and the hypothetical type is the presence of a thin chitinized plate 

 in the V-shaped opening between the anterior and dorsal arms. Simu- 

 lium (Fig. 144), of the Nematocera, has a similar plate, and these two 

 genera clearly demonstrate the first stage in the fusion of these two 

 arms. The cephalic end of the tentorium in My das (Fig. 146), Leptis 

 (Fig. 145), Scenopinus (Fig. 149), and Exoprosopa (Fig. 162) is a 

 broad uniformly chitinized triangular area. This condition is accounted 

 for on the basis of the union of the anterior and dorsal arms. The 

 invaginations on the cephalic aspect of these forms agree in all respects 

 with this interpretation. In Tabanus (Fig. 20) the invaginations on 

 each side are joined together by the epicranial siiture, while in the 

 above forms the invaginations are slit-like and occupy the greater part 

 of the arms of the epicranial suture. The slit-like invaginations are 

 easily explained if the anterior and dorsal arms are considered as united. 



The posterior arms of the tentoria of the Nematocera and the 

 Brachycera vary in shape, size, and location. The anterior and posterior 

 arms are united within the head and no sharp line can be drawn be- 

 tween them. The body of the tentorium (b. t) is represented by small 

 projections on the mesal surface of the posterior arms of most genera. 



Many interesting features occur in the modifications of the tentoria 

 of this group. In Dolichopus (Fig. 43 and 168) it appears to be fused 

 with the dorsal margin of the slit-like openings on each side between 

 the mesal margin of the compound eye and the fronto-clypeus. The 

 tentorium of Mydas (Fig. 146) is large and tubular, and it is possible 

 to push a good-sized needle thru the opening on the cephalic aspect to 

 the opening of the posterior arms on the caudal aspect. 



The tentoria of the genera possessing a ptilinum differ principally 

 from the foregoing in the degree of fusion with the head-capsule. In 

 most genera of this group the tentorium is completely united with the 

 head, but in a number of the Aealyptratae the tentorial arms arise as 

 free rods from the invaginations on the caudal aspect and project to 

 the latero-ventral margins of the head-capsule, with which they unite 



