128 THE BIOLOGY OF INSECTS 



the tvvo kinds of sperms are distinguished not by the total 

 absence of a certain chromosome, but by a marked difference 

 in size or shape between the two of a pair. Thus, in a bug 

 {Lygaens) investigated by Wilson, the full (zygote) number 

 is fourteen ; in a female the two " sex-chromosomes " are 

 equal in size, while in a male (Fig. 33) one of them {x the 

 " female-producing ") is much larger than the other (y). 

 The two members of this unequal pair go into different 

 daughter-cells at any reducing division, and fertilisation by 

 an .Y-bearing sperm will result in a female-producing zygote, 

 or by a jy-bearing sperm in a male-producing one. 



These facts establish the conclusion that sex- determina- 

 tion is closely dependent on certain factors situated in 

 certain definite chromosomes of the germ-cells. That the 

 same is true for other inherited characters is shown by 

 what is called " sex- linked " inheritance — a term applied to 

 cases in which some readily observed feature in body- 

 structure or appearance is inherited by means of a factor 

 carried in the same chromosome that bears the factor for 

 sex (either male or female). The insects in which this 

 condition has been most thoroughly studied are small 

 Fruit-flies {Drosophila) which breed very quickly and 

 prolifically under observation in the laboratory, and are 

 thus particularly suitable for studies in heredity, as a 

 number of generations can be observed in a comparatively 

 short time. Investigations on these flies have been carried 

 on through several years by T. H. Morgan and his colleagues 

 (191 6, etc.) ; only a brief summary of some of the results 

 can be given here. The normal colour of the eyes of some 

 of these flies is red, but a male variety appeared in which 

 the eyes were white. Breeding experiments show that the 

 normal red eye-colour is dominant to white, and that males 

 are hybrid and females pure as regards the sex-factors. 

 The crucial test of sex-linkage is furnished by pairing 

 w^hite-eyed females with hybrid red-eyed males ; the off- 

 spring of such a cross are half white-eyed males and half 

 red-eyed females. If we indicate graphically the con- 

 stitution of the parents. 



