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SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I42 



retractor muscle (E, rprmt). Incidentally it may be noted that the 

 word nientum, meaning "chin," is incongruously applied to any part 

 of the labium, or "lip," but we cannot stop here to reform this ac- 

 cepted terminology. 



POG 



Fig. 9. — Examples of simple modifications of the relatively generalized structure 

 (A) of the back of the head. 



A, Posterior surface of a head having a relatively generalized structure asso- 

 ciated with a centrally placed occipital foramen. B, Xyela minor, adult, Hy- 

 menoptera. C, Melandrya, larva, Coleoptera. D, Popillia japonica, larva, Coleop- 

 tera. E, A myrmelionid adult, Neuroptera. 



A series of modifications in the posterior surface of the head, de- 

 parting from the relatively generalized structure shown at A of 

 figure 9, begins with the formation of a pair of opposing lobes of 

 the hypostomata (fig. 10 B, HL) that intrude between the occipital 

 foramen and the base of the labium. A union of these lobes then 

 produces a hypostomal bridge (D, HB) ventral to the tentorial pits 

 (pt). A suggestion of the lobes is seen on the head of Pteronidea 

 ribesii (A), but they are fully developed on the head of a caterpil- 

 lar (C). The bridge is a narrow bar in Tabanus (E) ; in other 

 Diptera it becomes a wide plate, as in the asilid (F), and in a muscoid 



