14 



SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL, I42 



prognathous. Since this condition is of common occurrence among 

 insects of various orders, some writers have contended that the 

 primitive insects were prognathous. This, however, seems improba- 



Poo Cvx 



Fig. 5. — Different positions of the head or mouth parts relative to the body. 



A, Hypognathotis position of head, diagrammatic, head vertical, mouth parts 

 hang downward. B, Prognaihous i>osition of head, diagrammatic, head hori- 

 zontal, mouth parts anterior. C, Auchenorhynchons position of mouth parts, 

 cicada, beak projects from below the neck. D, Sternorhynchous position of mouth 

 parts, aphid, beak held against undersurface of thorax when not in use. 



Aclp, anteclypeus ; cvpl, cervical plates; es, epistomal sulcus; Gu, gula; Mt, 

 mentum ; occ, occipital condyle ; Poc, postocciput ; pos, postoccipital sulcus ; 

 Prmt, prementum; sgs, subgenal sulcus; Smt, submentum. 



Other lettering as on figure 4. 



ble, because prognathism involves extensive readjustments in the 

 structure of the head, particularly of the undersurface, which are 

 specializations, and are not the same in different prognathous insects. 

 Furthermore, some insects are prognathous in the larval stage, and 

 revert to the hypognathous condition in the adult. The structural 

 changes correlated with prognathism will be fully discussed in a 

 following section on the back of the head (p. 21). Prognathism, 



