NO. 3 INSECT HEAD ^SNODGRASS 33 



and that their signiticance has not been fully taken into account by 

 those who have formulated theories of arthropod relationships and 

 descent. 



II. GENERAL STRUCTURE OF THE INSECT HEAD 



The almost complete suppression of the primitive intersegmental 

 lines in the insect head makes a study of the head segmentation in 

 insects a difficult matter, and investigators differ widely in their 

 views as to the parts of the adult head that have been derived from 

 the several head segments. Since the prostomial region and the three 

 segments of the protocephalon are never distinct, even in the earliest 

 embryonic stages, it seems fruitless to speculate as to what areas of 

 the adult cranium are to be attributed to them individually, but the 

 general protocephalic region must be at least the region of the clypeus 

 and frons, the compound eyes, and the antennae. In as much as the 

 muscles of the three pairs of gnathal appendages have their origins in 

 the posterior parts of the head, it is reasonable to assume that the 

 areas upon which these muscles arise represent the walls of the gnathal 

 segments that have been added to the protocephalon. 



According to Heymons (1895), who bases his conclusions on a 

 study of the embryonic development of the head in Periplaneta and 

 Anisolahis, the entire cranium except the frons and the region of 

 the compound eyes and the antennae is formed from the walls of the 

 mandibular, maxillary, and labial segments. Janet ( 1899) , taking the 

 attachments of the muscles of the appendages on the head walls as 

 criteria of the respective segmental limits, maps the cranium into 

 areas that closely correspond with the segmental regions claimed by 

 Heymons. From Riley (1904), on the other hand, we get a quite 

 different conception of the definitive head structure. According to 

 Riley's account of the development of the head of Blafta, the great 

 cephalic lobes of the embryo form most of the adult head capsule. 

 The dorsal and lateral walls of the gnathal segments, Riley says, are 

 so reduced by the posterior growth of the cephalic lobes that little 

 remains of them in the adult head — only the extreme posterior and 

 postero-lateral parts of the cranial walls, and the postoral ventral region 

 being refera1)le to them. This view must assume that the muscles of 

 the gnathal segments have moved forward to the protocephalic region 

 as their own segments became reduced, and it would nullify the evi- 

 dence of head segmentation based on muscle attachments. The writer 

 is inclined to agree with Heymons and Janet that the muscle attach- 

 ments on the lateral and dorsal walls of the cranium should be pretty 

 closelv indicative of the limits of the gnathal terga in the composition 



