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ANNUAL EEPORT SMITHSONIAN INSTITUTION, 1931 



rior arras commonly extend upward to the facial wall of the head 

 and attach to the latter in the neighborhood of the antennal bases. 

 These branches constitute the dorsal tentorial arms (B, DT). 



The posterior tentorial bar always forms a bridge between the 

 lower ends of the postoccipital suture; it never departs from this 

 position. The roots of the anterior arms vary in position, though 

 in pterygote insects they always lie somewhere in the pleurostomal 

 or epistomal sutures. In most of the apterygote insects, however, 

 the anterior tentorial arms arise from the ventral wall of the head 

 near the base of the hypopharynx. In their origin, therefore, the 

 anterior arms are sternal apophyses, on which the ventral muscles 

 of the mouth appendages take their origin. It can not be explained 

 exactly how the primitively ventral arms have acquired lateral or 





Ant 



Poo Cv 



FiGCEB 12. — Diagrams showing hypognatbous (A) and prognathous (B) types 



of head structure 

 A.nt, antenna; at, anterior tentorial pit; Civ, clypeus ; Cv, neck; cvpl, cervical 



sclerites ; B, compound eye ; es, epistomal suture ; Qu, gula ; L6, labium ; Lm, 



labrum ; Mi, mandible ; Mt, mentum ; Mm, maxilla ; ace, occipital condyle ; 



¥mt, prementum ; Poc, postocciput ; pos, postoccipital suture ; ft, posterior 



tentorial pit ; s(js, subgenal suture. 



facial attachments on the walls of the cranium in pterygote insects, 

 but the altered position of their bases has come about probably 

 either by a lateral migration before the mandibles, or by the estab- 

 lishment of secondary connections with the cranial walls accompa- 

 nied by a loss of the primary sternal connections with the floor of 

 the head. 



Modifications in the form of the head. — The relative size or the 

 shape of an insect's head is no index of the brain power of the 

 insect; on the contrary it usually expresses the strength of the jaws, 

 or some other quality connected with feeding. In the biting and 

 chewing insects the parietal areas of the cranium are often enlarged 

 to accommodate the jaw muscles; in sucking insects the facial area 

 may be amplified to provide space for the muscles of the suction 

 pump. 



