NO. I THE INSECT HEAD SNODGRASS 39 



tions concerning the nature of the labrum, segmentation of the em- 

 bryonic head lobe, and the homology of the antennae with trunk limbs. 



THE LABRUM 



The writer formerly expressed the opinion that the labrum is the 

 anatomical anterior pole of the arthropod and that its ventral posi- 

 tion in some cases is secondary. Dahl (1956), however, has vigor- 

 ously opposed this view as turning "the available evidence upside- 

 down." On the other hand. Young (1959) reasserts that "the labrum 

 is the anterior end of the arthropod." It is true, of course, that the 

 labrum is formed on the underside of the embryonic head lobe, but 

 in a variable position, and the fact remains that the anteriormost 

 nerve endings are on the labrum regardless of its position. 



Since the labrum in so many cases is developed from a pair of 

 lobes that unite, and in the adult insect is often emarginate medi- 

 ally, some writers have expressed the opinion that the labrum repre- 

 sents a pair of appendages. The insect labrum is consistently pro- 

 vided with two pairs of antagonistic extrinsic muscles from the 

 frons, and usually with internal compressor muscles. Judging from 

 the anatomical literature on the arthropods it would appear that mus- 

 culature of the labrum is exceptional. In the shrimp Penaens 

 sctiferiis, however, Young (1959) finds a highly complex labral mus- 

 culature including 12 bilateral pairs of intrinsic muscles running in 

 all directions through the labrum, and two pairs of extrinsic muscles 

 inserted on its base. Because of its inconsistency the labral muscu- 

 lature gives no clue to the nature of the labrum, but the labral in- 

 nervation has been invoked by several writers as evidence that the 

 labrum is not the simple lobe of the head it appears to be. The 

 labrum is said to be innervated from the postoral tritocerebral ganglia 

 of the brain, but this fact has led to two quite different theories as 

 to the morphological status of the labrum. (As will be shown, the 

 nerves in question really go from the labrum to the tritocerebral 

 ganglia.) 



One interpretation of the labrum, the Ferris-Henry theory, is 

 correlated with a comparative study of the annulate nervous system 

 by Miss Henry (1948). She starts with the assumed principle that 

 nerves are always confined to the segment of their ganglionic origin. 

 Then she logically contends that, since the labrum is innervated from 

 the tritocerebral ganglia, it must be the segment of these ganglia, and 

 is therefore the first segment of the arthropod head, equivalent to the 

 prostomium of the earthworm. Though in no modern arthropod, 



