199] HEAD OF DIPTERA— PETERSON 29 



position is characteristic of these invaginations in the Brachycera, and 

 the figures show the details of the variations in the position of the 

 invaginations oU the posterior arms of the tentorium. 



Two lines of specialization appear in the tentorium of the Diptera, 

 one in the reduction of the dorsal arms and the other in the union of the 

 dorsal arms with the anterior arms. The two types of invaginations 

 described for the cephalic aspect of the head bear directly upon this 

 problem. The most important evidence in proof of these two types of 

 development is found in the structure of the arms. 



In Sciara (Fig. 150), Bibio (Fig. 153 and 154), Psorophora (Fig. 

 159), Trichocera (Fig. 158), Bibiocephala (Fig. 155), Dixa (Fig. 163), 

 and others, two long narrow rods extend on each side between the 

 invaginations on the caudal aspect and the invaginations on the cephalic 

 aspect. These rods are composed of the posterior arms (p. a) and the 

 anterior arms (a. a) of the tentorium. The dorsal arms are completely 

 reduced in these forms. Other genera show completely developed dorsal 

 arms or rudiments of the same. The dorsal arms (d. a) are distinct 

 and free in Pipunculus (Fig. 151). They arise from the anterior arms 

 and project cephalad to the cephalic aspect of the head, where they 

 connect with small but distinct ental projections adjacent to the anten- 

 nae. The cephalic ends of the dorsal arms are very delicate and easily 

 broken in dissecting. There are no invaginations on the ectal surface. 

 In Chiron omus (Pig. 152) the tentorial arms are swollen near the mid- 

 dle of their length, and the distinct humps on the dorsal side are 

 interpreted as rudiments of the dorsal arms. Promachus (Fig. 147) 

 has two long, free, finger-like projections, arising from the ocular 

 sclerite near the antennae, which project toward the tentorium proper. 

 These projections are apparently dorsal arms of the tentorium, or 

 derivatives of the same that have retained their connection with the 

 ocular sclerite near the mesal margin of the compound eye but have lost 

 their connection with the tentorium proper. A similar relationship 

 exists between the dorsal arms and the ocular sclerite in Tabanus (Pig. 

 22). If the above structures in Promachus are dorsal arms, then the 

 anterior arms are large (Fig. 148) and the slit-like invaginations on the 

 cephalic aspect are only the invaginations of the anterior arms of the 

 tentorium. 



The tentoria of the Nematocera above described are in the ventral 

 half of the head-cavity and their situation is dependent upon the posi- 

 tion of the invaginations. Usually the invaginations of the anterior 

 arms are ventrad of the invaginations of the posterior arms ; but Bibio- 

 cephala (Fig. 155) is an exception to this rule if the tentorium in this 

 genus is composed of only the anterior and posterior arms — and there 



