NO. 7 THE INSECT CRANIUM SNODGRASS 45 



inflection forming internal ridges ; the ridges are the important 

 structural features, the external grooves, or sulci, are merely incidental 

 to the production of ridges. 



3. The postoccipital sulcus is probably a line of union between two 

 segments, apparently the first and second maxillary segments, and 

 lines of union between sclerotized areas may occur on the ventral side 

 of the head, such as a median "gular suture." 



4. The so-called "epicranial suture," with its coronal stem and 

 frontal arms, is, in the immature insect, a preformed line of weakness 

 in the head cuticle along which the cuticle will split at ecdysis. The 

 cleavage line may be carried over into the adult, though usually it is 

 more or less suppressed or entirely absent in the imago ; other grooves 

 of the adult cranium have frequently been mistaken for it, and 

 designated the "epicranial suture." In some insects the exuvial 

 cleavage line goes through the middle of the head without forking. 



5. The common Y-shaped cleavage line of the head is variable in 

 different insects both as to the length of the coronal stem and the 

 extent and course of the frontal arms. The arms may take five 

 different facial positions (fig. i C-G) ; at one extreme they go to the 

 compound eyes, at the other they go mesad of the antennae to the 

 distal margin of the clypeus. The coronal stem may be very short, and 

 is sometimes absent, in which case the arms proceed separately from 

 the occipital margin of the cranium. 



6. The arms of the Y-shaped cleavage line, therefore, do not define 

 any fixed part of the cranium, and in this sense they have no structural 

 value, nor do their variations in position affect the fundamental 

 structure of the head ; the ecdysial clefts merely cut the head cuticle 

 in different ways in different insects. 



7. On the other hand, whatever may be the course of the arms of 

 the cleavage line, the latter run always through the cranial areas that 

 intervene between the attachments of the mandibular muscles and 

 those of the facial muscles. 



8. The facial muscles attached on the cranium between the arms 

 of the cleavage line are consistently separated into an upper group, 

 including the labral muscles, the precerebral pharyngeal muscles, and 

 the dorsal muscles of the hypopharynx, and a lower group composed 

 of the cibarial muscles. The two groups are always separated by the 

 frontal ganglion and its brain connectives. 



9. The frontal ganglion, being developed from the anterior or 

 dorsal wall of the stomodaeum just within the mouth, and connected 

 with the primarily postoral premandibular ganglia, must have been 

 originally a preoral nerve center of the ventral nerve cord. The 



