197] HEAD OF DIPTERA—PETERSOX 27 



capsule. The free euds of the posterior arms are fused aud form the 

 body of the tentorium. 



The tentorium undergoes a considerable amount of variation in 

 the different orders, but so far as observed the above associations be- 

 tween the invaginations and the fixed and movable parts of the head 

 are always retained by the more generalized members of each order. 

 This is also true for a generalized hpothetieal dipterous head. The 

 tentorium (t) of such a head (Fig. 140 and 141) is considerably modi- 

 fied when compared with the tentorium of a generalized insect. Two 

 pairs of invaginations are present on the cephalic aspect of the head 

 (Fig. 1). The dorsal, indistinct pair (i. d), just ventrad of the anten- 

 nae, are homologous with the invaginations of the dorsal arms of the 

 tentorium, while the prominent pair (i. a) of invaginations ventrad of 

 these and located in the arms of the epicranial suture (a. e. s) and 

 adjacent to the lateral ends of the fronto-clypeal suture are the invagi- 

 nations of the anterior arms of the tentorium. One pair of invagina- 

 tions (i. p) is present on the caudal aspect of the head-capsule (Fig. 

 73) somewhat ventrad of the ventro-lateral margins of the occipital 

 foramen. These are the invaginations of the posterior arms of the 

 tentorium. Each lateral half of the tentorium is Y-shaped (Fig. 141), 

 the stem of the Y arising from the invaginations on the caudal aspect, 

 its caudal portion being a part of the posterior arms (p. a) of the tento- 

 rium. The large ventral arm of the Y and the cephalic portion of its 

 stem, constitute the anterior arm (a. a), and the small dorsal arm of 

 the Y is the dorsal arm (d. a) of the tentorium. These two arms con- 

 nect with their respective invaginations on the cephalic aspect. The 

 body of the tentorium (b. t) is apparently represented by a small, rudi- 

 mentary, mesal projection arising from the posterior arms near the 

 caudal portion of the stem of the Y. 



The association between the movable appendages and the invagi- 

 nations of the tentorium is discussed under the respective appendages. 

 From this point, the tentorial structures as they occur in the various 

 genera are compared with the hypothetical type and the line of speciali- 

 zation noted. The forms without a ptilinum are considered first. The 

 parts of the free tentorium, not completely fiised with the head-capsule, 

 are indicated in the figures by dotted lines. 



The tentorium of Tabanus (Fig. 142 and 143) is generalized and 

 closely resembles the hj-pothetical type ; consequently it furiiislies a 

 good starting point for a discussion. Two pairs of invaginations are 

 present on the cephalic aspect (Fig. 20) ; of these the invaginations for 

 the anterior arms (i. a) are the more prominent. The dorsal arms 

 (i. d) arise from the head-capsule just ventro-laterad of the antennae 



