NO. 7 



THE INSECT CRANIUM — SNODGRASS 



Fig. I. — Diagrams showing the cleavage line of ecdysis, its principal variations 

 on the head, and the relation of the frontal arms to the head muscles. 



A, stonefly nymph showing entire cleavage line, with frontal forks on head. 

 B, distribution of head muscles, with cleavage lines omitted. _ C, frontal splits 

 at ecdysis extended into compound eyes. D, frontal splits going between eyes 

 and antennae. E, frontal splits ending at antennal fossae. F, frontal splits going 

 mesad of antennae. G, frontal splits extended to free margin of clypeus. H, 

 coronal line absent, frontal splits proceeding separately from posterior margin 

 of cranium (Pentatomidae). I, frontal arms of cleavage line obsolete, coronal 

 split extended to labrum (Vespidae). 



at, anterior tentorial pit ; cbmcls, cibarial muscles ; Clp, clypeus ; dlch, dilators 

 of cibarium (cbmcls) ; FrGng, frontal ganglion; hphmcls, frontal muscle of 

 hypopharynx (retractor of mouth angle) ; Ibrmcls, labral muscles; Lm, labrum; 

 rndmcl, mandibular muscle; phmcls, dilator muscles of pharynx. 



