4 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I42 



during embryonic development are added to the blastocephalon. The 

 imaginal head, therefore, is a syncephalon and evidently is a product 

 of evolution. The cephalized body segments include those of the 

 mandibles (fig. i B, Md), the first maxillae (iMx), and the second 

 maxillae (2Mx), so that in the modern adult head the organs of feed- 



^— .-emH 

 ■-4 Ant 

 iz -V-SAnt 



0- 



^ E 



Fig. I. — Examples of arthropod embryos, illustrating particularly the embryonic 

 head (eniH), or blastocephalon. 



A, General structure of a young embryo, ventral, diagrammatic. B, Embryo 

 of a mantid, Paratenodera sinensis (from Hagan, 1917). C, Embryo of a 

 hemipteron Ranatra fusca (from Hussey, 1926) with distinct gnathal tagma (Gn) 

 between head and thorax. D, Embryo of a spider, Agelcna labyrinthica (from 

 Balfour, 1880). E, Embryonic nauplius stage of a crustacean, Leander serratus 

 (from Sollaud, 1923). F, Head region of an amphipod embryo, Gammarus pulex 

 ptilex (from Weygoldt, 1958). G, Later stage of same (from Weygoldt, 1958). 



ing become closely associated with the mouth and the anterior sense 

 organs. 



There is no question that the three segments mentioned above, the 

 so-called gnathal segments, become an intimate part of the definitive 

 cranium in both the insects and the chilopods. It is commonly as- 

 sumed that a premandibular, or first postoral body segment is also 

 included in the adult head. The principal evidence of the existence of 

 this segment, however, is the presence of a pair of premandibular 



