201] HEAD OF DIPTERA — PETERSON 31 



and continue cephalad as thickenings adjacent to the ventral margin 

 of the head, as in Sapromj'za (Fig. 171), Loxoeera (Fig. 169), Euaresta 

 (Fig. 175), Calobata (Fig. 183), Chrysomyza (Fig. 181), Drosophila 

 (Fig. 172), Chyromya (Fig. 179), Heteroncura (Pig. 176), and Teta- 

 nocera (Fig. 180). In those forms where the tentorium is completely 

 fused with the head, as in Sepsis (Fig. 184), Chloropisca (Fig. 189), 

 Coelopa (Fig. 182), and Borborus (Fig. 188), it is a continuous thick- 

 ening from the latero-ventral angle of the occipital foramen to the 

 cephalo-ventral aspect of the head-capsule. The tentorium between the 

 invaginations for the posterior arms and the ventro-lateral margins of 

 the head-eapsule is apparently wanting in Musca (Fig. 194), Thelaira 

 (Fig. 196), Arehytas (Fig. 197), and some other genera; in one or 

 two cases it is possible to trace a faint mark which would indicate the 

 line of connection. The tentoria of some of the genera of the Acalyp- 

 tratae and the Calyptratae show an iinusual development of the tento- 

 rial thickenings (t. th) in that they extend about the entire caudal part 

 of the ventral margin of the head. In some cases these tentorial thick- 

 enings reach the occipital foramen, as in Calobata (Fig. 114), Scatoph- 

 aga (Fig. 135), Heteroneura (Fig. 126), Lispa (Fig. 116), and Mj-ios- 

 pila (Fig. 120), while in Musca (Fig. 133), Coelopa (Fig. 121), 

 Hydrotaea (Fig. 127), and other genera, there is no such connection. 



The invaginations of the posterior arms of the tentorium of the 

 Acalj-ptratae and the Calyptratae are situated laterad or latero-ventrad 

 of, and adjacent to, the occipital foramen. In many of the species 

 figured the invaginations are merely long, heavily chitinized furrows 

 extending latero-ventrad from the occipital foramen, and very often 

 it is difficult to locate them definitely. 



Two mesal projections arise from the proximal portions of the 

 posterior arms in a majority of the Cyelorrhapha. In some species 

 these structures are well developed, and their mesal ends apparently 

 join on the meson, cephalad of the occipital foramen. These structures 

 are similar to those described for the Brachycera and are rudiments of 

 the body of the tentorium. 



No invaginations of the tentorium occur on the cephalic aspect in 

 any of the forms which possess a ptilinum. On account of the decided 

 specialization of this aspect, it is very difficult to know just what has 

 happened. The tentorium is represented by thickenings which extend 

 from the ventral to the cephalic aspect of the head. The extent of 

 these thickenings varies; in some genera they continue to the antennal 

 fossae, while in others they are practically wanting. 



