NO. 7 THE INSECT CRANIUM SNODGRASS 3 



junction between bones of the vertebrate skull, which to the earlier 

 anatomists suggested seams of stitching. In entomology the term 

 "suture" is carelessly used for any external groove of the integument, 

 regardless of its nature or of how it may have been formed. 



Since the integument of an animal is continuous over all parts of 

 the body in postembryonic stages, the term "suture" can be applied 

 only to lines of union between separate centers of dermal hardening. 

 Thus, Ferris (1942), in discussing the head of Symphyla, says of the 

 cranial lines that they "are sutures in the strictest application of the 

 term, being seams between the various areas of sclerotization." How- 

 ever, by what evidence he arrives at this positive information he does 

 not tell us. It is probable that the postoccipital sulcus of the insect 

 head is a line of union between primitive segments, and on the ventral 

 side of the head of some insects there are lines which clearly indicate 

 the union of primarily separate sclerotic areas. Otherwise there is no 

 proof that any of the lines on the insect's cranium are true sutures ; 

 neither nymphs nor larvae show separate centers of sclerotization in 

 the head integument, and the embryonic head is unsclerotized. Cer- 

 tainly most of the so-called "sutures" of the insect head are grooves 

 incidental to the formation of ridges on the inner surface, and these 

 endoskeletal ridges are structural features serving to strengthen the 

 cranial walls. The external grooves, or sulci, of the endoskeletal 

 ridges, therefore, are not "sutures" in any true sense. Strenger 

 (1942), in a "functional analysis of the orthopteran head," though 

 calling these sulci "sutures" (Nahte), concludes that they are in 

 general strengthenings along lines of stress in the head capsule. 



There is, however, on the head of nearly all immature insects, and 

 on that of some adults, another kind of line which is not marked by a 

 groove externally, nor does it ordinarily form a ridge on the inner 

 surface. This line is that commonly known as the "epicranial suture." 

 Typically it has the form of a Y, inverted if viewed from in front. 

 The stem, called the "coronal suture" or "metopic suture," is median 

 on top of the head; the arms, or "frontal sutures," diverge laterally 

 and downward to different points on the face in different insects. 

 Functionally this "epicranial suture" is the line along which the head 

 cuticle of the immature insect splits at ecdysis. It is in no sense a 

 suture, and is here termed the ecdysial cleavage line of the head. 

 DuPorte (1946) calls it the "ecdysial line or suture." In nymphal 

 and larval stages the cleavage line usually appears as a pale double- 

 edged tract of the head cuticula, the stem of which is continuous from 

 the similar mid-dorsal line on the thorax. According to Duarte (1939) 

 the line along which the cuticle splits on the thorax of Locusta is 



