MALE GERM CELLS IN NOTONECTA 73 



in iiiiinber. For example, three species of Podisus have the 

 diploid number of 10, two species 14 (]\Iontgomery, Wilson), 

 In this category belong the three species of Notonecta. But 

 much wider differences in related species may occur, as for exam- 

 ple, in two closely similar species of Banasa, of which B. dimidiata 

 has 16 chromosomes, B. calva has 26 (Wilson '09 b). So in 

 Thyanta, in which a distinction between two species has just 

 recentl}'^ been rediscovered by Barber, two types of chromosome 

 groups occur, 27-28 in T. calceata, and 16 in T. custator. Among 

 the phylloxerans, there is considerable variation; four species 

 have 6 chromosomes in the diploid groups, one species has 8, one 

 12, and one 22 (Morgan '09). Similarly in the aphids, the hap- 

 loid number ranges fi-om 3 in the willow aphid to 16 in the maple 

 aphid (Stevens '06). Likewise Braun ('09) found in fifteen spe- 

 cies of Cyclops a wide range of number, from 6 to 22, although 

 several species have the same number. In the Oenotheras, 

 mutants have been found with 14, 15, 21 and 28 chromosomes 

 (Lutz '12). 



When we come to groups less closely related than species, 

 marked differences in the chromosome number frequently occur. 

 In the family Jassidae, the diploid number varies from 15 to 23 

 in different genera; in the Cercopidae, from 15 to 27; in the Mem- 

 bracidae, from 17 to 21 (Boring '07). In ten genera of the Chryso- 

 melidae, there is a range from 16 to 36 (Stevens) ; in the Coreidae, 

 from 13 to 27 (Montgomery, Wilson). These are a few of the 

 many cases of divergence within a family. There are on the other 

 hand, a few cases where a constancy obtains throughout as wide 

 a group as a family, as for example, in ten genera of the orthop- 

 teran family, Acrididae (McClung '05, et al.), and in four genera 

 of the opjsthobranch mollusks. The constancy in number goes 

 still further in some of the Amphibia, where all the urodeles, so 

 far as examined, apparently have 24 chromosomes in the diploid 

 groups. 



It is therefore evident that while in some cases the chromosome 

 number is the same for members of rather a large group, it is not 

 necessarily the same for even very closely related forms. It is 

 true in general, however, that closely related forms have the same 



