24 



SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8l 



of the antennal ganglia, and which later fuse with the brain ganglia, 

 are probably to be explained as the true segmental ganglia of the 

 preantennal appendages." 



It may be questioned if there is ever a true segmental separation 

 between the ocular and antennal region of the insect head, since what- 

 ever division does occur between the two parts appears relatively late 

 in development, and is, therefore, probably of a secondary nature. 

 Holmgren (1916), from a comparative study of the histology of the 

 brain of annelids and arthropods, concluded that the protocerebral 

 and deutocerebral parts of the definitive arthropod brain are secondary 

 subdivisions of one primitive nerve mass, which, moreover, Holmgren 

 would identify with the archicerebrum of the annelids. This con- 

 clusion is scarcely tenable, because, interpreted literally in terms of 

 annelid structure, it would assign the antennae to the prostomium. 

 and because it disregards the evidence of the postoral position of both 

 the preantennal and antennal rudiments in the embryo. 



It is usually assumed that the compound eyes of crustaceans and 

 insects belong to the preantennal segment, which, on this assumption, 

 is designated the "ocular" segment. Heymons (1921), however, 

 claims that in Scolopendva the eyes and the optic lobes are derived 

 from the ectoderm of the prostomial region. It is perhaps not neces- 

 sary to believe that the grouped ocelli of the Chilopoda, even the 

 composite " pseudo-compound " eyes of Scutigera, are related to the 

 true compound eyes of crustaceans and insects, since the details of 

 structure in the two cases are quite dififerent ; but it would seem less 

 probable that the optic lobes of the brain should have a separate 

 origin in the different arthropod groups. In many of the Crustacea, 

 the compound eyes are pedunculate, being situated on segmented 

 stalks having an ample musculature innervated from the protocere- 

 brum, and this fact gives strong support to the idea that the eye- 

 stalks are appendages of the preantennal segment. Experiments have 

 shown that if an eye-stalk is amputated, an antenna-like organ is 

 often regenerated from the stump, on which an eye is not developed. 

 These results recall the experiments of Schmitt-Jensen (1913, 1915) 

 who cut ofif the antennae of a phasmid (Caraiisius inorosiis) and found 

 that the appendages were regenerated in a form closely resembling 

 the tarsi of the thoracic legs, each, in some cases, with a pair of ter- 

 minal claws and a pulvillus. 



It is difficult to evaluate these regeneration phenomena, for it seems 

 highly improbable that the insect antenna ever had the specialized 

 structure of the thoracic appendages of modern adult insects. Many 



