12 SOME ASPECTS OF MORPHOLOGY 



conclusions, showed that the pair of small plates (superlinguae of 

 Folsom) overlying the hypopharynx in Collembola and Thysanura 

 Entognatha are probably homologous with paragnaths. On the 

 other hand, he contended that true maxillul^e co-exist with them 

 in the groups just named and therefore cannot be their homologues 

 as Crampton thought. In summing up the position it may be 

 said that non-appendicular plates or superlinguas (paragnaths) 

 undoubtedly exist among the Apterygota, but evidence of the 

 occurrence of maxillulae is very unconvincing and points to the 

 contrary. 



The available evidence relative to the segmentation of the insect 

 head leads to the conclusion that it has resulted from the 

 amalgamation of at least six embryonic somites. The present 

 status of knowledge of the problem may be summarised as follows : 



1. The pre-antennal or ocular segment is usually regarded as 

 being the first true cephalic segment, its neuromere being the 

 protocerebrum or first division of the brain. The protocerebrum 

 is probably composite since it is believed to comprise also the 

 primitive archicerebrum. The existence of rudimentary coelom 

 sacs and very small evanescent appendages pertaining to the pre- 

 antennal segment in the Phasmid Carausius (Fig. 2B) has not so 

 far been confirmed in any other insect. It is now tolerably well 

 established that coelom sacs, in close association with labrum, 

 occur in Carausius, Locusta and Rhodnius. It appears unlikely 

 that two pre-antennal segments are exhibited in the Insecta and no 

 other Arthropoda and the possiblity that the labral coelom is 

 part of that pertaining to the ocular segment cannot be overlooked. 

 Evidence that the labrum itself represents a pair of true appendages 

 is very hypothetical, especially in view of the fact that this organ 

 arises from an unpaired fundament in the Apterygota and other 

 insects. 



2. The antennary segment is innervated by the deutocerebrum 

 or second neuromere of the brain : its appendages are the antennae 

 and coelom sacs are present in the embryo. 



3. The intercalary segment is vestigial, but evidence of its 

 existence is found in the embryos of most insects. Rudimentary 

 coelom sacs are retained in some cases, and vestigial evanescent 



