84 EDMUND B. WILSON 



GENERAL 



5. The idiochromosomes 



The case of Nezara shows how readily a morphological dimorph- 

 ism of the spermatid-nuclei may be overlooked when the X- and 

 Y-chromosomes do not differ markedly in size ; and it emphasizes 

 the necessity for the closest scrutiny of these chromosomes in the 

 study of this general question. In my fourth 'Study' I placed 

 with Nezara hilaris, as a second example of my original 'third 

 type/ the lygaeid species Oncopeltus fasciatus (Dall.), on the 

 strength of Montgomery's account of the conditions in the male 

 ('01, '06) and my own unpublished observations on both sexes. 

 While I have carefully re-examined this case also, I am not yet 

 prepared to express an unqualified opinion in regard to it. Cer- 

 tainly, in very many of the cells of this species, at every period of 

 the spermatogenesis, the idiochromosomes (which are always sep- 

 arate up to the second division) seem to be perfectly equal. A 

 slight inequality may indeed be seen in some cases ; but as far as I 

 can yet determine this seems to fall within the range of the size- 

 variation in other chromosomes and gives no positive ground for 

 the recognition of a morphological dimorphism in the spermatozoa. 

 A similar condition has been described in several other insects, not- 

 ably in some of the Lepidoptera (Stevens, '06 ; Dederer, '08 ; Cook, 

 ' 10) , in the earwig Anisolaba (Randolph, '08) and apparently also 

 in the beetle Hydrophilus according to Arnold ('08). I see no rea- 

 son to question these observations; but the interpretation to be 

 placed on them is by no means clear at present. The experimental 

 evidence on the Lepidoptera seems to demonstrate that in at least 

 one case that of Abraxas according to Doncaster and Raynor, 

 it is the eggs and not the spermatozoa that are sexually dimorphic ; 

 that is, in the terms that I have recently suggested ('10a), in 

 this case it is the female that is sexually 'digametic' while the 

 male is 'homogametic.' Baltzer's careful work on the sea- 

 urchins ('09) clearly demonstrates a cytological sexual dimorphism 

 in the mature eggs of these animals, and shows that the sperm- 

 nuclei are all alike. In cases, therefore, where no visible dimorph- 

 ism of the spermatid-nuclei is demonstrable, two possibilities 



