SEGMENTATION OF THE HEAD 13 



appendages (homologous with the second antennae of Crustacea) 

 occur in certain Apterygota, as well as in the cockroach, silkworm 

 and honey bee among Pterygota. In Campodea (Uzel, 1897) and 

 the grasshopper Dissosteira (Snodgrass, 1928) rudiments, which 

 appear to represent these appendages, are discernible in the 

 adult insect. Its nerve centre, or tritocerebrum, forms the third 

 neuromere of the brain, but it subsequently becomes largely 

 incorporated with the deutocerebrum. 



4, 5, 6. The mandibular, first maxillary and second maxillary 

 segments are all well developed, and bear corresponding 

 appendages. Coelom sacs are present in the embryo, and the 

 neuromeres coalesce to form the infra-oesophageal ganglion. 



The completed head-capsule is so highly modified and con- 

 solidated that it no longer betrays obvious evidence of its 

 segmental composition. It becomes, therefore, a difficult and 

 largely speculative problem to assign the regions, or parts of the 

 head, to their original segments. The whole subject is admirably 

 discussed from a comparative standpoint by Snodgrass (1928), who 

 points out that since the segments of the protocephalic region are 

 never distinct, even in the earliest embryonic stages, it seems 

 fruitless to hazard what areas of the adult head are to be attributed 

 to them individually. The original protocephalic region must at 

 least be the area occupied by the labrum, clypeus and frons, 

 together with the compound eyes and antennae. Also, since the 

 muscles of the three pairs of gnathal appendages have their 

 origins in the posterior part of the head, Snodgrass concludes that 

 the areas upon which these muscles arise are parts of the walls of 

 those segments that have been added to the protocephalic region. 

 It seems probable that the frontal suture marks off the original 

 prostomial area from that derived from the segmental elements, 

 and, if this be correct, the labrum,. clypeus and frons represent 

 differentiations of the original prostomium. In the primitive 

 Machilidse the posterior part of the epicranium is crossed by a 

 prominent suture just behind the eyes. This suture extends 

 downwards to a point between the bases of the mandible and 

 maxilla. Snodgrass advances reasons which suggest that this 

 suture separates the mandibular segment from that of the maxilla 



