350 EDMUND B. WILSON 



I. THE MATURATION-DIVISIONS IN ONCOPELTUS AND LYGAEUS 

 WITH REFERENCE TO THE SEX-CHROMOSOMES 



In Oncopeltus the diploid number of chromosomes is sixteen 

 in both sexes (figs. 1 to 5, photos, 1, 2) in Lygaeus fourteen (fig. 6), 

 and the second spermatocyte-division shows half these numbers 

 that is, eight in the former case, seven in the latter. The first 

 division shows in each case one more than the haploid number, 

 owing to the fact, repeatedly described heretofore in other Hemip- 

 tera, that in the first division the X- and F-chromosomes divide 

 as separate univalents, while in the second they are united to 

 form a bivalent. In both Oncopeltus and Lygaeus these chro- 

 mosomes conjugate in the final anaphase of the first division, 

 just as the cell is about to divide. 



1. The diploid chromosome-groups 



The spermatogonia! and oogonial divisions require but brief 

 description since they present no striking features and the size- 

 differences are but shghtly marked. In Lygaeus bicrucis (fig. 6) 

 the fourteen chromosomes are in the main similar to those of L. 

 turcicus as described in my first and third 'Studies' ('05, '06) 

 though the F-chromosome is relatively smaller in the latter spe- 

 cies. The X-chromosome can not be identified by the eye, but 

 must be at least twice the size of the F-chromosome, as indicated 

 by the maturation-divisions and by the spermatogonia! groups 

 themselves. Unfortunately my material of this species does not 

 show a single good equatorial plate in the female; but the relations 

 are here no doubt the same as in L. turcicus. 



In Oncopeltus, of which I have abundant material of both sexes, 

 the size-differences of the chromosomes are even less marked than 

 in Lygaeus, and it is impossible to identify pairs of different 

 sizes. Careful study fails to reveal any differences between the 

 diploid groups of the two sexes that are sufficiently marked or 

 constant to give any certain result (figs. 1 to 5). In the male one 

 chromosome not infrequently is somewhat smaller than the others 

 (fig. 2), and this may be the F-chromosome; but very often this 

 is not evident, even in other spermatogonia from the same cyst 

 (figs. 1,3). As will be shown beyond, the X- and F-chromosomes 



