122 ETHEL NICHOLSON BROWNE 



species common in the western United States, N. shooterii and 

 N. indica* The other American species are N. variabilis, N. 

 uhleri, N. howardii, N. raleighi, N. mexicana, N. montezmna, 

 N. bifasciata and N. nigra, the two latter ones being peculiar 

 to South America. The genus Notonecta, together with Buenoa 

 and the Pleinae belong to the family Notonectidae. 



III. BRIEF SUMMARY OF THE CHROMOSOMES OF N. UNDULATA, 

 N. IRRORATA, AND N. INSULATA 



For a full and detailed account of the chromosomes of these 

 species, the reader is referred to my former paper (Browne '13). 

 I here merely outline the chief features of the chromosome groups 

 in order to have a basis for reference and comparison with those 

 of the three other species described in detail in this paper. 



In N. undulata the number of chromosomes in the first sper- 

 matocyte division is 14, consisting of a ring of 12 chromosomes 

 surrounding two small central ones (fig. 1). In side views, these 

 central pairs often appear linearly arranged, owing to their 

 proximity and to the precocious division of one pair (fig. 2) . In 

 the second division the number is reduced by one because of the 

 late conjugation of X and Y, which are not associated in the 

 first division. In this division the XY dyad lies in the center 

 of a ring of 12 chromosomes (figs. 3 and 4). The components of 

 this pair are considerably unequal in size, and, owing to their 

 presence, the sister groups which go into the spermatids and 

 spermatozoa are of two kinds (fig. 5 A, B). The spermatogonia! 

 group contains 26 chromosomes (fig. 6). There are five large 

 chromosomes in the diploid group, representing two large ones 

 of the haploid and X which is next in size to these. There are 

 five small chromosomes, representing the two small central ones 

 of the first division and Y which is the next smallest after these. 



In N. irrorata, the number of chromosomes in the first division 

 is 13, and there is only one small one within the ring (figs. 7 and 

 8). In the second division there are 12 chromosomes and the 

 XY dyad lies in the center of the ring (figs. 9 to 11). The sper- 

 matogonia! number is 24 (fig. 12). There are seven large chro- 

 mosomes, corresponding to three large ones of the haploid group 



