“The chromosomes in relation to the determination of sex in insects”

Material procured during the past summer (1905) demonstrates with great clearness that the sexes of Hemiptera show constant and characteristic differences in the chromosome groups, which are of such a nature as to leave no doubt that a definite connection of some kind between the chromosomes and the determination of sex exists in these animals. These differences are of two types. In one of these, the cells of the female possess one more chromosome than those of the male; in the other, both sexes possess the same number of chromosomes, but one of the chromosomes in the male is much smaller than the corresponding one in the female (which is in agreement with the observations of Stevens on the beetle Tenebrio). These types may conveniently be designated as A and B, respectively. The essential facts have been determined in three genera of each type, namely (type A), Protenor belfragei, Anasa tristis, and Alydus pilosulus, and (type B), Lygœus turcicus, Euschistus fissilis, and Cœnus delius. The chromosome groups have been examined in the dividing oögonia and ovarian follicle cells of the female and in the dividing spermatogonia and investing cells of the testis in case of the male.

Type A includes those forms in which (as has been known since Henking's paper of 1890 on Pyrrochoris) the spermatozoa are of two classes, one of which contains one more chromosome (the so-called “accessory” or heterotropic chromosome) than the other. In this type the somatic number of chromosomes in the female is an even one, while the somatic number in the male is one less (hence an odd number), the actual numbers being in Protenor and Alydus ♂ 14, ♀ 13, and in Anasa ♂ 22, ♀ 21.