34 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8l 



of the head, but it must be admitted that muscle bases can undergo 

 rather extensive migrations. That the gnathal segments contribute a 

 consideral)le part to the cranial walls of the definitive insect head is 

 clearly suggested by Heymons' figures of the development of Forficula 

 (fig. i6), and, as already shown, there can be no doubt that these 

 segments enter bodily into the head composition of Crustacea that have 

 a well-defined composite head. With insects, it is a question of the 

 degree of reduction that the gnathal segments have suffered after their 

 union with the protocephalon. 



By whatever phylogenetic course the cephalic region of the insect 

 body has arrived at its definitive state, it acquired long ago a cranium- 

 like form, and a definite structure that has since been modified only in 

 superficial characters, adaptive to diflferent modes of living and to 

 dififerent ways of feeding in the various groups of modern insects. 



THE HEAD CAPSULE 



The chitinous walls of the definitive head capsule constitute the 

 epicranitiiii. In an adult insect head preserving the typical embryonic 

 position, with the facial aspect directed forward (fig. i8B), the 

 mouth parts are suspended from the ventro-lateral edges of the epi- 

 cranium. A pair of compound eyes (E) typically have a lateral or 

 dorso-lateral position, and three ocelli (O) occur between them on 

 the dorsal or facial area of the head (A). The antennae (Ant) vary 

 in their location from positions just alcove the bases of the mandibles 

 (fig. 50 A, Aut) to a more median site on the dorsal part of the face 

 (fig. 18 A. B). The top of the head, or vertex (fig. 18 A, B, J\\-). 

 is marked by a median coronal suture (A, cs) that turns downward 

 on the face and divides into the frontal sutures (fs). which diverge 

 ventrally to the anterior articulations of the mandibles (c). The 

 coronal suture and the frontal sutures together constitute the epi- 

 cranial suture. The lines of these sutures are marked internally by 

 ridges, and the coronal ridge is sometimes developed into a plate 

 supporting muscle attachments. The median facial region between 

 and below the frontal sutures is the frons (Fr). ventral to which is 

 the clvpeus (Clp). with the iabruni (Liu) suspended from the lower 

 margin of the latter. 



The posterior surface of the epicranium (fig. 18 C) is occupied 

 by the opening (For) from the head cavity into the neck, usually a 

 large aperture, properly termed the foramen magnum by analogy with 

 vertebrate anatomy, but commonly called the " occipital foramen " 

 by entomologists. The surface of the head surrounding the foramen 



