70 ETHEL NICHOLSON BROWNE 



If this pair is absent, N. giauca differs radically from the other 

 three species, and it may be that this discrepancy is analogous to 

 that found in Metapodius (Wilson '09 a) where in different indi- 

 viduals of the same species a F-chromosome may be present or 

 absent; or in the mosquitoes, where there is a typical unequal 

 XF-pair in Anopheles punctipennis, while in two other genera, 

 Culex and Theobaldia, the differential chromosomes are absent 

 (Stevens '11). It is possible that the X and Y chromosomes are 

 present in N. giauca, but are of practically equal size as in Onco- 

 peltus and Nezara hilaris (Wilson '11), and have been overlooked. 

 Pantel and Sinety call attention to the presence of an extra large 

 chromosome, which they call the 'chromosome exceptionelle,' 

 suggesting that it may be an accessory chromosome, but they are 

 convinced that it participates in both divisions. This body seems 

 quite similar in appearance and behavior to the large chromosome 

 described in N. insulata. It is unfortunate that N. giauca cannot 

 be brought into line with the three American species in regard to 

 chromosome number, but the account of Pantel and Sinety is 

 inadequate to permit the attempt. 



B. Discussio7i 



1 . The relation of chromosome number to species. It is doubtful 

 whether every chromosome in the three species can be homolo- 

 gized individually, for the size relations are different in some 

 respects. By comparing the spermatogonial groups of N. undu- 

 lata and N. irrorata (figs. 13, 14, 24) it is evident that there are 

 5 large chromosomes in the former, and 7 large ones in the latter; 

 and the XF-chromosomes are of different relative size in the three 

 species. On the other hand, one largest chromosome can be 

 traced throughout the history of all three species; likewise the 

 smallest chromosome, not only by its size but especially by its 

 position in the first spermatocyte division. We may also homolo- 

 gize the second small chromosome which is present in the first 

 division of N. undulata, in the center of the group, with the one 

 of similar size which is sometimes present in N. insulata in the 

 same position. And since the steps in the process of fusion 



