NO. I 



THE INSECT HEAD — SNODGRASS 



modern heads. In some insect embryos, however, the thorax with its 

 six legs (fig. I C) is already differentiated as the locomotor section 

 of the body while the segments of the gnathal appendages (Gn) are 

 still a small body section between the blastocephalon and the thorax. 

 If we can trust the embryo, therefore, the insects may have been 

 hexapods before the gnathal segments became a part of the head. In 

 this respect the primitive insects must have resembled an anostracan 

 crustacean (fig. 2C). 



emH 



Gr^c 



Md Mx 



Fig. 3. — Structure and composition of the adult insect head. 



A, Diagram of the head showing probable approximate regions derived from 

 the embryonic blastocephalon (emH) and four postoral segments (I-IV). B, 

 An adult head of generalized structure, intersegmental lines obliterated except 

 for the persisting groove (pos) between third and fourth segments. 



In conclusion, it appears that we may safely infer from embryonic 

 evidence that the modern insect head has been evolved by the addition 

 of four postoral segments (fig. 3 A) to the primary head {emH), 

 represented in the embryo by the blastocephalon. This concept of the 

 composition of the adult insect head is certainly suggested by the nor- 

 mal development of the embryo. It is somewhat disconcerting, there- 

 fore, when we read the results of experiments by Haget (1955) on 

 the embryo of Leptinotarsa. Haget reports that when the gnathal 

 segments of the embryo are destroyed, a complete cranium is re- 

 formed by the cephalic lobe alone. Very probably, however, this is 

 a curious case of regeneration, and has no phylogenetic significance. 



The present discussion and the diagram (fig. 3 A) allot no space to 

 a theoretical "superlingual" segment between the premandibular and 

 mandibular segments, the existence of which is generally discredited 



