26 ILLINOIS BIOLOGICAL MONOGRAPHS [196 



Simulium (Fig. 77) are adjacent to the occipital foramen, consequently 

 the parapostgenae are confined to the lateral margins of the occipital 

 foramen. In Tabanus also the invaginations are adjacent to the occipi- 

 tal foramen, and the postgenae are connected ventrad of the occipital 

 foramen in the male and by a narrow strip in the female. 



The area ventrad of the occipital foramen is a continuous chitinized 

 piece in all of the Cyclorrhapha and the Orthorrhapha. There is only 

 one probable explanation of the origin of this area. It has been derived 

 from the fusion of the mesal margins of the postgenae. The evidence 

 for this interpretation is found in a number of the Nematocera. The 

 mesal margins of the postgenae in Trichocera (Fig. 78) and Sciara 

 (Fig. 81) are curved mesad and in some cases actually join, as in the 

 female of Bibiocephala (Fig. 83). The peculiar elongated heads of 

 Limnobia (Fig. 93), Tipula (Fig. 95), and Psorophora (Fig. 96) show 

 a distinct depressed line on the meson along which the postgenae have 

 joined. In a number of the genera of the Orthorrhapha and the Cy- 

 clorrhapha the ventral margin of the caudal aspect is decidedly concave. 

 This condition may be due to a former stage in the development of the 

 fused postgenae. In all cases where the area ventrad of the occipital 

 foramen is chitinized, the invaginations of the posterior arms of the 

 tentorium are somewhat adjacent to the occipital foramen and the 

 attachments of the maxillae are removed to or beyond the ventral mar- 

 gin of the head. Sciara (Fig. 81) is a good example of an early stage 

 in the development of the above relationship. The variations in the 

 shape and extent of the postgenae and the parapostgenae are well illus- 

 trated by the figures. 



Tentorium. — There is present within the head of generalized insects 

 a definite arrangement of chitinized rods and plate-like structures which 

 go to support the internal organs and furnish places for the attachment 

 of muscles. These rods or plates arise from three pairs of openings on 

 the head known as the invaginations of the anterior arms, dorsal arms, 

 and posterior arms of the tentorium. The invaginations of the anterior 

 arms are usually associated with the lateral margins of the clypeus, 

 with one of the points of articulation of the mandibles, and frequently 

 with the ventral ends of the arms of the epicranial suture. The invagi- 

 nations of the dorsal arms are associated with the points of attachment 

 of the antennae and near the dorsal portions of the arms of the epi- 

 cranial suture. The invaginations of the posterior arms are associated 

 with the occipital foramen and the points of attachment of the maxillae. 

 The three pairs of arms unite within the head ; the small dorsal arms 

 unite with the larger anterior arms, and these, in turn, join with the 

 posterior arms, which are confined to the caudal portion of the head- 



