NO. 7 THE INSECT CRANIUM SNODGRASS 5 



when present, separates the clypeal area from the frontal area, but it 

 is clearly a secondary device for strengthening the head wall between 

 the bases of the jaws, and, considering the irregularity of its occur- 

 rence, it may be suspected of having been independently developed in 

 some cases. However, this epistomal sulcus lies posterior to the 

 muscles that pertain to the clypeus, and separates these muscles from 

 those which arise above it on the f rons. 



The frons is usually defined as the facial area above or behind the 

 clypeus embraced by the arms of the "epicranial suture" ; but, thus 

 defined, it has no definite anatomical status because of the different 

 courses followed by the said arms of the "suture" (fig. I C-G). 

 According as the arms go behind or between the antennae, the 

 "frons" in some cases carries the antennae, in others it does not, and 

 this inconsistency is clearly seen on the head exuviae after ecdysis. 

 DuPorte (1946), therefore, would bound the frons laterally by 

 grooves present in some insects extending dorsally from the anterior 

 mandibular articulations. These lateral grooves, however, in some 

 insects run laterad of the antennae and in others mesad of them, and in 

 many cases the "frontogenal sulcus" of DuPorte is part of the groove 

 commonly known as the frontoclypeal, or epistomal, sulcus. In short, 

 the "frons" is a name rather than an anatomical reality, but as a name 

 without a definition it is perhaps the more useful inasmuch as any 

 writer may apply it as he chooses. In the present paper the term 

 frons is used in an indefinite sense for the facial area of the head 

 above the clypeus, or the clypeal area, just as vertex applies to the top 

 of the head, and genae to the sides. The areas designated by these 

 names are topographical but not anatomical. When the arms of the 

 cleavage line go posterior to the antennae (fig. i C, D) they approxi- 

 mately define the frons ; when the arms go between the antennae the 

 area they embrace is frontal (F), or frontoclypeal (G), but is not 

 the frons or the frontoclypeus. The part cut out at ecdysis may be 

 termed the apotome, which will be either a frontal apotome, or a 

 frontoclypeal apotome, according to the position and extent of the 

 cleavage lines. 



Riley (1904), in his account of the embryonic development of the 

 head of Blatta, contended that the labrum, the clypeus, and the frons 

 are all derived from the "procephalon" (meaning the preoral lobe 

 of the embryonic head terminating with the labrum). "The vertex, 

 the compound eyes, and with them the ocular sclerites and the genae," 

 he says, "are formed from the fused cephalic lobes and thus, with the 

 front, clypeus and labrum, belong to the ocular or protocerebral 

 segment." The Y-shaped "epicranial suture" of the head, Riley 



