126 THE BIOLOGY OF INSECTS 



equal. It is also evident that in any kind of animal whose 

 members are of one sex or the other, maleness and female- 

 ness may be regarded as alternative characters which might 

 be compared with such characters as dark or pale wings. 

 And it has just been indicated how, in a number of unions 

 between hybrid dominants and pure recessives, we may 

 expect approximately half the offspring to resemble either 

 kind of parent. Further, sex is, in most cases, a truly 

 inborn character. It seems likely, therefore, that the 

 determination of sex follows from what are called '' Mende- 

 lian " factors ; if this be so one sex-factor must be dominant 

 over the other, and members of the one must be hybrid 

 (heterozygous), those of the other pure (homozygous) for 

 the sex-factors. For example, if all males have the zygotic 

 composition M/", there will be two kinds of sperms with 

 factors for one sex or the other, while all females will have 

 the composition ff and all the eggs will be alike carriers of 

 the feminine factor. Such an tgg if fertilised by a male- 

 determining sperm will develop into a male, but if by a 

 sperm of the other kind, into a female. 



Clear evidence that sex is indeed thus determined in 

 various insects has been afforded by studies of the germ- 

 cells and by breeding experiments. From what has been 

 already stated about reducing- divisions (pp. 117-118) it 

 will have been realised that the number of chromosomes in 

 a zygote-nucleus and in the nuclei of body-cells is commonly 

 an even number — half thereof derived from the gamete 

 contributed by either parent. Many years ago, however, 

 H. Henking (1891) observed that in a bug (Pyrrhocoris) two 

 kinds of sperms are produced, distinguished by one kind 

 having a chromosome fewer than the other. Subsequent 

 work by E. B. Wilson and his colleagues (see his text-book, 

 1925) showed that such a condition occurs in many bugs 

 (Hemiptera) and other insects, whose body-cells have one 

 chromosome less in males than in females, a difference of 

 three or four in the chromosome number of the two sexes 

 {e.g. 26 male to 30 female) being sometimes noted. Usually, 

 however, in the body-cells of such differentiated insects the 



