160 SEX-DETERMINATION [CH. 



X-chromosome, but only one T element. In a local race 

 of the Pentatomid species Thyanta custator, for example, 

 WILSON found that two chromosomes both behave as mates 

 for the single 2^-chromosome ; they pair with it in the pro- 

 phase and both separate from it in the second spermatocyte 

 mitosis, the T going to one pole and the two X elements to 

 the other. The female has four X elements, so that two are 

 present in every mature egg. In another Hemipteran, 

 Acholla, F. PAYNE has described an X-group consisting of 

 five chromosomes, two fairly large and three very small, all 

 of which pair with a single 1^-chromosome and pass together 

 to one pole while the T goes to the other (PL XVII, 10, 1 1 ). 

 Another complication, with which the origin of these mul- 

 tiple idio-chromosomes is possibly connected, arises from 

 the occasional presence of supernumerary chromosomes, 

 such as have been described by WILSON in three species 

 of the Hemipteran genus Metapodius (PL XVIII). In 

 these species the typical condition is that both sexes 

 have 22 chromosomes, the male an unequal X-T pair, 

 and the female two similar X elements. In some races, 

 however, the 1^-chromosome is absent in the male, giving 

 21 in the male and 22 in the female. In other races 

 there may be a variable number of small "supernumerary" 

 chromosomes, which behave exactly like idio-chromosomes 

 in the spireme stage and in the mitotic figure. They may 

 attach themselves to either the X- or the l"-chromosome, 

 and pass with it to its pole of the spindle, and the varia- 

 bility of their behaviour in this respect leads to the produc- 

 tion of individuals with any number up to 28 (i.e. twenty v 

 autosomes, one pair of idio-chromosomes, and up to six 

 supernumeraries). WILSON suggests that they arose origin- 

 ally by abnormal division in which both X and T went 

 together to one pole ("non-disjunction," see below, p. 232), 



