NO. 3 



INSECT HEAD SNODGRASS 



19 



creeping of earlier ancestral forms. The question of whether the 

 appendages were first used for propulsion through the water, or for 

 progression on a solid support will not be discussed here, but, in the 

 course of their evolution, the appendages have become specialized to 

 serve a great variety of functions. Moreover, by the functional 

 grouping of the appendages, the corresponding body segments have 

 themselves become differentiated into groups forming often quite 

 distinct body regions (fig. 13 B), of which the head of an insect is one 

 of the most highly evolved. 



Fig. 13. — Young insect embryos at a stage when the thorax is already 

 differentiated, but in which the gnathal segments are not yet added to the 

 protocephalon to form the definitive head. 



A, embryo of Lepisma saccharina (from Heymons, 1897). B, embryo of 

 Ranatra fusca (from Hussey, 1926). 



Ab, abdomen; Ccr, cercus; Gn, gnathal segments; ///, tritocerebral segment: 

 Li, first leg; Lm, labrum ; Md, mandible; iMx, first maxilla; sMx, second 

 maxilla ; Pp, " pleuropodium " ; Pre, protocephalon ; Th, thorax. 



THE PROTOCEPHALON 



The arthropods differ from the annelids in the possession of a com- 

 posite head, or syncephalon, formed by the union of several of the 

 anterior segments with the prostomium. 



In the embryonic development of most Arthropoda the head is 

 first differentiated as a swelling of the anterior end of the body, form- 

 ing the so-called cephalic lobes (fig. 8 A, B, Pre). On this region are 

 developed the labrum (D, Lm), the eyes, the stomodeal invagination 

 (B, Stom), or mouth (D, Mth), the antennae (Ant), and in some 

 cases the postantennal appendages, when the last are present (fig. 

 22 A, 2Ant). The cephalic lobes soon become a very definite em- 

 bryonic head (fig. 13A, B, Pre), which either remains as the entire 



