24 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL, I42 



An unusual condition is seen in the hymenopterons Proctotrnpes 

 and Pelecinus (fig. loH). The hypostomal bridge here appears to 

 be limited laterally by the ventrally elongate posterior tentorial pits, 

 from which are given off separately {pf, pt", pt'") three parts of the 

 tentorium. 



By another line of modifications the posterior wall of the head be- 

 comes closed between the occipital foramen and the base of the labium 

 by a bridge that unites the postgenae. This postgenal bridge has its 

 inception in a pair of median lobes of the postgenae (fig. ii A, F, 

 PgL) below the tentorial pits, or distal to them in prognathous species. 

 If a hypostomal bridge is already present (A, HB) it may be com- 

 pressed between the postgenal lobes, as in the honey bee (B). A 

 union of the lobes, as in the wasp (C), then establishes a postgenal 

 bridge (PgB) between the foramen and the labium that has sup- 

 planted the hypostomal bridge. The bridge itself may then be length- 

 ened downward (D) until it forms a large area on the back of the 

 head, as seen in the hymenopteron Pristocera (E), The tentorial pits 

 here retain their primary relation to the occipital foramen, but the 

 labium becomes far removed from the pits by intervention of the 

 lengthened postgenae. The postgenal bridge differs from the hy- 

 postomal bridge in that it has no connection with the postocciput 

 (C, Poc), and is continuous only with the postgenae. 



In other cases, particularly in nematocerous fly larvae in which 

 there is no hypostomal bridge, a postgenal bridge may be formed by 

 direct confluence of a pair of hypostomal lobes (fig. ii F, PgL). In 

 the same way the bridge becomes lengthened (G) between the 

 foramen and the mouth parts. The median postgenal suture (D, ms) 

 is usually retained as a groove, but it may be partly suppressed (G), 

 and in others (H, I) it becomes entirely obliterated by complete 

 union of the postgenae. The broad enclosure of the back of the head 

 in Notonecta (H) and Naucoris (I) is evidently a postgenal bridge, 

 since it is continuous with the postgenae and not with the post- 

 occiput. 



In many insects, especially prognathous larval forms, the tentorial 

 pits lie near the center of the posterior or under surface of the head 

 (fig. 12, pt). Inasmuch as the pits retain their primitive association 

 with the base of the labium (A, B) the condition here must there- 

 fore be interpreted as brought about by a lengthening and approxi- 

 mation of the postgenae proximal to the pits, since the lower ends of 

 the postoccipital sulcus when present (A, pos) are continued to the 

 pits. In some beetle larvae, as in the carabids (C, D), the postgenae 

 are farther lengthened distal to the pits, and come together medially, 



