72 EDMUND B. WILSON 



known representative of a type in which a pair of idiochromo- 

 somes can be identified in both sexes, but are of equal size in 

 both, and in which, accordingly, no visible sexual differences ap- 

 pear in the diploid nuclei. These conclusions, as is now appar- 

 ent, were based upon a wrong identification of the idiochromo- 

 some-pair, which is not the smallest pair, as I then believed, 1 but 

 one of the largest. When this fact was recognized, the true con- 

 ditions soon became evident. 



I was led to re-examine Nezara hilaris by the fact (very sur- 

 prising to me) that in Nezara viridula, a southern species closely 

 similar to N. hilaris, the idiochromosomes of the male are ex- 

 tremely unequal in size, and the dimorphism of the spermatid- 

 nuclei is correspondingly marked. Upon returning to the study 

 of N. hilaris it soon became manifest that the dimorphism is 

 present in this species also, though in far less conspicuous form. 

 The size-difference between the X- and Y-chromosomes is here 

 often so slight that I did not at first distinguish it from an incon- 

 stant fluctuation of size, such as is sometimes seen between the 

 members of the other chromosome-pairs. When, however, the 

 identity of the XY-pair was correctly recognized, its constancy 

 of position and of size in the second division enabled me to make 

 an accurate comparison between it and the other bivalents; and 

 this fully established the constant inequality of its members, 

 which is constantly greater than that now and then seen in other 

 pairs. Both species also exhibit some other very interesting 

 features that I overlooked in my former studies. 



Nezara can therefore no longer stand as a representative of 

 the third of the types distinguished in my third 'Study,' but 

 belongs with Euschistus, Lygaeus, etc., in the second type 



1 This was in part because in most of the other forms known at the time the 

 idiochromosomes are in fact the smallest, or one of the smallest, pairs. In part, 

 also, I followed Montgomery ('01) who described in this species two small " chro- 

 matin nucleoli" in the spermatogonial groups, and believed them to be identical 

 with the chromatic nucleolus of the growth-period. In a later paper ('06) Mont- 

 gomery states these "chromatin nucleoli" to be "apparently not quite equal in 

 volume," and asserts that I was in error in describing them as equal. In my 

 material they are certainly equal in the great majority of cases. However, this 

 is not the idiochromosome-pair. 



