205] HEAD OF DIPTERA— PETERSON 35 



arms of the tentorium. Structurally the hypothetical mandibles do not 

 resemble the biting mandibles of the Orthoptera, but their situation 

 and their association with the invaginations of the anterior arms of the 

 tentorium are the same, which is far more important in determining 

 their homology' than any particular form thej' may assume. 



The mandibles vary in their structure. In Psorophora (Pig. 251) 

 they are long and needle-like, while in Tabanus, Culicoides, and the 

 male of Simulium (Fig. 252) they are sword-shaped, and in Dixa (Fig. 

 254) spindle-like. The mandibles in the females of all species of Simu- 

 lium (Fig. 250) examined are a trifle longer than those in the males 

 (Fig. 252) and much broader at their distal ends. The greatest spe- 

 cialization in structure and point of attachment with the head occurs 

 in the long, thin, saw-like mandibles of Bibiocephala (Fig. 256) and 

 Blepharocera. In these forms they are longer than the labium, blunt 

 at the end, and toothed along the mesal margin, fitting against a similar 

 edge on the lateral margin of the hypopharynx. 



All mandibles (md) of the Diptera are connected with the head- 

 capsule cephalad of the maxillae (mx) and caudad of the labrum- 

 epipharynx (1. ep) and the fronto-cljijeus (fr. c). In this respect they 

 resemble the hypothetical type. In Psorophora, Dixa, Simulium, and 

 Tabanus they are associated with the invaginations of the anterior arms 

 of the tentorium. The proximal ends of the mandibles of Psorophora 

 (Fig. 159) are bent cephalad, and articulate with the head-capsule at 

 the distal ends of the crescent-shaped tentorial thickenings (t. th) which 

 arise from the margins of the invaginations of the anterior arms of the 

 tentorium. In Dixa (Fig. 254) the mandibles connect with the head- 

 capsule at the ventro-caudal angles of the elypeus. An indistinct thick- 

 ening extends dorsad from the point of articulation of each of the man- 

 dibles toward the invaginations of the anterior arms of the tentorium. 

 The mandibles of Simulium (Fig. 250 and 252) and Tabanus (Fig. 

 317) connect with the head-capsule directly ventrad of the invagina- 

 tions of the anterior arms of the tentorium, but no direct connection 

 occurs between them. In the female of Simulium the mandibles artic- 

 ulate -n-ith a hook-shaped projection of the vertex. The mandibles of 

 Tabanus (Fig. 255) are bifurcate at their proximal end and the lateral 

 bifurcation articulates with the liead. The location of the mandibles of 

 Bibiocephala (Fig. 155) and Blepharocera is generalized with respect 

 to their position between the maxillae and the fronto-clj-peus, but their 

 point of attachment with the head-capsule is decidedly specialized. 

 They unite with chitinized pillars (Fig. 83) on the caudal aspect 

 ventro-laterad of the invaginations of the posterior arms of the tento- 

 rium. The proximal portion of eacb mandible is a long chitinized strip 



