Studies on Chromosomes 17 



show the equal pairing of all the chromosomes with equal clear- 

 ness, the absence of the small idiochromosome being evident 

 (Fig. 4, /). The small pair (/) evidently correspond to the small 

 pair in the male (4, />) and the large idiochromosome-pair must 

 therefore be represented by one of the larger pairs. Fig. 4, n, o, 

 show the spermatogonial and oogonial groups of E. fissilis, show- 

 ing the same relations as in E. servus, save that the small pair are 

 relatively larger. 



The above-described species of Euschistus, while agreeing pre- 

 cisely in the general relations, present individual differences so 

 marked as to show that even the species of a single genus may be 

 distinguishable by the chromosome-groups. In this case the 

 most interesting feature is the series shown in the inequality 

 of the idiochromosomes, which becomes progressively greater 

 in the series (i) E. servus, (2) tristigmus, fissilis, (3) ictericus, 

 (4) variolarius, the inequality in the last case being fully as great 

 as in Lygaeus. I may again mention the fact that in the opposite 

 direction the genus Brochymena often shows the idiochromosomes 

 less unequal than in E. servus; in Mineus they are sometimes 

 of nearly equal size (Fig. 4, />, </), while in Nezara no inequality 

 exists. Practically all intermediate conditions are therefore shown 

 within the limits of a single family between the extreme inequality 

 shown in E. variolarius and no inequality at all. It is quite clear 

 from the observations here brought forward that this progressive 

 differentiation has occurred only in the male sex, as I conjectured 

 in my first paper. 



/. Coenus delius 



The relations in this form are so closely similar to those seen 

 in Euschistus servus or fissilis, as described above, as hardly to 

 require separate description. Fig. 5, b, shows the spermatogonial 

 metaphase-group; 5, i, the corresponding oogonial group. Both 

 these preparations show very clearly the small pair (j) of ordinary 

 chromosomes (not so well shown in the figure of the spermat- 

 ogonial group in my first paper). Here, as in Euschistus, it is 

 evident that the large idiochromosome is much larger than the 

 members of the small pair. 



