NO. I THE INSECT HEAD SNODGRASS 51 



probably have to wait a long time for this proof to materialize, we 

 may as well in the meantime be content with the facts as they are 

 known. If we must have a theory, that of the prostomial nature of 

 the embryonic blastocephalon is the simplest and the easiest to visual- 

 ize. However, even if we do not know the facts concerning the seg- 

 mentation of the head, and perhaps never shall know them, ignorance 

 in this respect will have no practical effect on an understanding of 

 the head structure in modern arthropods. And really, it would be 

 too bad if the question of head segmentation ever should be finally 

 settled ; it has been for so long such fertile ground for theorizing that 

 arthropodists would miss it as a field for mental exercise. 



THE ANTENNAE 



The principal theoretical question pertaining to the antennae (an- 

 tennules of Crustacea) concerns their possible homology with other 

 appendages. The nature of the antennae then has an important bear- 

 ing on the question of segmentation in the embryonic blastocephalon. 



That the antennae are not organs equivalent to the postoral ap- 

 pendages would seem obvious from the fact that normally they 

 never have a leg structure in any arthropod, and are filamentous 

 even in the trilobites The antennules of Crustacea may be branched, 

 but not in the manner of the second antennae or other truly biram- 

 ous appendages. Furthermore, the antennal nerve centers are always 

 closely associated in the brain with the ocular centers, and are con- 

 nected by a preoral, intracerebral commissure. The only brain ganglia 

 that are known to have a postoral origin are those that become the 

 tritocerebral lobes of the brain. From the likeness of the first an- 

 tennal nerve centers in the arthropod brain to the nerve centers of 

 the palps in the polychaete brain it has been contended that the 

 antennae are homologues of the annelid palps. The antennae are 

 palplike in their embryonic origin, but since the derivation of arthro- 

 pods from polychaetes is an overworked theory, the palps and an- 

 tennae may be quite separate organs in their origin. 



In opposition to the idea that the first antennae are primary pre- 

 oral appendages of the embryonic head lobe, there is often cited the 

 well-known fact that the insect antennae when amputated at or near 

 the base are frequently regenerated in a form having a striking re- 

 semblance to a segmented leg with a pair of apical claws (fig. 21 D). 

 The same results have been obtained by other methods, and leglike 

 antennae are sometimes found in nature. 



Bodenstein and Abdel-Malek (1949) submerged larvae of Dro- 



