24 ILLINOIS BIOLOGICAL MONOGRAPHS [194 



the occipital foramen. The structures pertaining to this modification 

 have been designated as the parocciput (pocc). Each postgena (po) 

 is also secondarily differentiated along its mesal margin by a chitinized 

 thickening which extends between the occipital foramen and the invagi- 

 nations of the posterior arms of the tentorium. This thickening has 

 been designated as the parapostgenal thickening, while the area mesad 

 of it is the parapostgena (ppo). The two mesal projections of the 

 parocciput on the lateral margin of the occipital foramen serve as points 

 for the articulation of neck sclerites and mark the ventral boundary of 

 the occiput. 



The occipital foramen (o. f ) is centrally situated in all but a few 

 genera, such as Tipula (Fig. 95), Limnobia (Fig. 93), Psorophora (Fig. 

 96), and Bibio (Fig. 92), in which it is near the dorsal margin. The 

 size of the occipital foramen is more or less constant thruout the order, 

 but in Psychoda (Fig. 82) and Promachus (Fig. 84) it is comparatively 

 much larger than in Pipunculus (Fig. 106) and Exoprosopa (Fig. 98). 

 The shape of the occipital foramen varies somewhat, but usually it is 

 in the form of a figure eight. The constrictions in the lateral margins 

 are generally due to the mesal projections of the parocciput, which 

 vary to some extent in their situation. The projections in Exoprosopa 

 (Fig. 98), Pipunculus (Fig. 106), and Mydas (Fig. 99) meet on the 

 meson and completely divide the occipital foramen into two openings. 

 The neck sclerites (n. s) always articulate with these mesal projections 

 and are represented in a number of the figures. 



The occiput (occ) of all genera figured resembles in general the 

 occiput of the hypothetical head, since no sutures separate the vertex, 

 the occiput, and the postgenae. The position of the occipital foramen 

 and the contour of the caudal surface determine the amount of variation 

 in the occiput as well as in the postgenae. In some genera, Empis (Fig. 

 164) and Bibiocephala (Fig. 156), the caudal aspect is convex; while 

 in others, Exoprosopa (Fig. 98) and Pipunculus (Fig. 106), it is de- 

 cidedly concave. Suture-like markings or depressions are present near 

 the dorsal margin of the caudal aspect in the heads of Tabanus (Fig. 

 74), Stratiomyia (Fig. 104), Bibio (Fig. 91), Bibiocephala (Fig. 83), 

 Leptis (Fig. 103), Psilocephala (Fig. 100), and others. These depres- 

 sions mark the place of contact of the mesal portions of the ocular 

 sclerites with the head-capsule, and are in no way homologous with the 

 sutures about the occiput in generalized insects. 



The area about the dorsal and lateral margin of the occipital fora- 

 men, the parocciput (pocc), is more or less differentiated from the re- 

 mainder of the occiput in all the species studied. In the more generalized 

 forms, Bibiocephala (Fig. 83), Trichocera (Fig. 78), Tipula (Fig. 95),. 



