NO. I 



THE INSECT HEAD SNODGRASS 



37 



The entognathous hexapods — Protura, Collembola, and Diplura — 

 have no head structure corresponding anatomically with the tentorium 

 of the ectognathous insects. When the cleared head of one of these 

 forms is examined by transmitted light there is seen a pair of long 

 skeletal arms (fig. 19 B, S) extending posteriorly from the hypo- 



\i i — — ^ 

 A 



Fig. 19. — Noncuticular endosternal structures in the head of Diplura and 



Collembola. 



A, Hetcroiapyx gallardi, Diplura, sternal arms of head {S) connected by inter- 

 nal arched ligament {Lg), giving attachment to maxillary muscles, dorsal. B, 

 Canipodca sp., Diplura, hypopharynx and sternal arms with internal ligamentous 

 bridge, dorsal. C, Onychinrus fimentarius, Collembola, sternal arms of head and 

 endosternal superstructure with supports (a, p) on sternal arms (adapted from 

 Denis, 1928). D, Aniirida maritima, Collembola, same parts as at C (from Denis, 

 1928). 



pharynx. These arms have been mistaken for tentorial apodemes, 

 but actually they are sternal arms contained in the walls of the 

 gnathal pouches, as shown by the writer (1951) and by Tuxen 

 (1952). Folsom (1900) described their superficial origin in the em- 

 bryo of Collembola. The sclerites clearly pertain to the maxillary 

 segment since the cardines are articulated on their posterior ends 

 (D, Cd). 



In the Diplura (fig. 19 A, B) an internal membranous bridge {Lg) 

 is arched upward between the sternal arms and gives attachment to 

 ventral muscles of the maxillae (A). In the Collembola an elaborate 

 superstructure is built upon the sternal arms (C, D), consisting of 



