STUDY OF CHROMOSOMES IN NOTONECTA 141 



absolute determination), N. shooterii and N. undulata, and is 

 the largest in N. indica. Y is the third smallest in N. undulata, 

 about the fifth in N.. irrorata, N. indica, N. insulata and N, 

 glauca, and about the eighth in N. shooterii. A similar pro- 

 gressive difference in the relative size of these chromosomes 

 has been shown to occur in the different species of Euschistus 

 (Wilson '06). 



VIII. CORRELATION OF SOMATIC CHARACTERS AND CHROMOSOMES 



The diagnostic specific characters of Notonecta used by ento- 

 mologists are the body size, distance between the eyes, width of 

 the pronotum and size of the scutellum. The coloration and 

 markings are to some extent characteristic of the different spe- 

 cies, but there is a great variation in color in certain species from 

 a pure white to an almost black, so that this character is regarded 

 as of little value in the determination of species. In the accom- 

 panying table, I have listed the size relations of the different 

 parts in the six species, most of the figures being taken from 

 Bueno ('05). An outline of the number and relations of the chro- 

 mosomes is also given in the table. 



In general a definite number and arrangement of the chromo- 

 somes is characteristic of each species, and is therefore probably 

 correlated with the characters of that species. A reference to 

 the table will show, however, that the species cannot be lined 

 up with regard to their chromatin content and with regard to 

 each of the somatic characters with any great agreement. It 

 would seem that the distance between the eyes, which is perhaps 

 the most reliable characteristic for the entomologist, is not cor- 

 related with the most striking chromosome difference, that of 

 number. There are two characters, however, in which the 14- 

 chromosome species differ from the 13-group. The species with 

 14 chromosomes are the small forms — N. undulata, N. indica 

 and N. shooterii — whereas the species with 13 chromosomes are 

 the large forms, N. insulata, N. glauca and N. irrorata. Sec- 

 ondly, there is a physiological difference between the 14-species 

 and the 13; the former have a rapid evolution of the germ cells, 



