Studies on Chromosomes 3 



role in sex-production, but could at that time produce no evidence 

 in support of the suggestion. I have now the evidence to show 

 that this suggestion was in accordance with the facts; for in at 

 least four genera, Lygaeus, Euschistus, Coenus and Podisus, both 

 sexes show the same number of chromosomes, but the small 

 idiochromosome is present only in the male. Somewhat earlier, 

 and independently, Stevens ('05) determined a precisely similar 

 fact in the case of a beetle, Tenebrio, which indicates that the 

 phenomenon is of wide occurrence in the insects. These results 

 confirm the correctness of my conclusion that the heterotropic 

 or "accessory" chromosome has become unpaired in the male 

 sex through the disappearance in that sex of its mate, and give a 

 complete explanation of the fact that in forms possessing the 

 heterotropic chromosome the male number is odd and one less 

 than the female number. I believe that these facts may give 

 the basis for a general theory of sex-production. 



I. DESCRIPTIVE 

 A. General Character of the Chromosome- groups 



In two preceding papers (Wilson, '05, i; 05, 3,) (where due 

 acknowledgment is made to previous observers in this field) 

 I have described in some detail the general nature of the chro- 

 mosomes in these insects. For such an investigation as the present 

 one, the Hemiptera present peculiar advantages, owing above all 

 to the short and regular form of the chromosomes, and the relative 

 lack of crowding in the equatorial plate. I have employed almost 

 exclusively Flemming's strong fluid as a fixative, staining the 

 sections with iron-haematoxylin and extracting until the cytoplasm 

 is nearly or quite colorless. The best preparations thus obtained 

 leave nothing to be desired in point of brilliancy and clearness, 

 and show the chromosomes with a distinctness that is hardly 

 exaggerated by the black and white figures here reproduced. 

 The very large number of sections now at my disposal (including 

 all those of Paulmier and a still greater number of new prepara- 

 tions of my own) has enabled me in the case of nearly every 

 species to examine numerous division-figures (of which only the 



