2 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,! 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 
‘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. 
