REPRODUCTION AND HEREDITY 131 



When two x chromosomes are present the creature must be 

 female. 



Sex-chromosomes of the same essential type as those of 

 Drosophila have now been demonstrated in other insects 

 of the same order (Diptera) as well as in grasshoppers 

 (Orthoptera) and beetles (Coleoptera). In several types of 

 moths (Lepidoptera), the males are pure and the females 

 hybrid for sex- characters : this was worked out by 

 L. Doncaster (1908, 1914) in his famous studies on the 

 breeding of the Magpie Moth {Abraxas grossulariata) with 

 its pale variety lacticolor, known only from female specimens, 

 and affording an example of sex-linked inheritance con- 

 trasting with that of Drosophila. In bees, wasps, and most 

 Hymenoptera that have been studied there is a remarkable 

 difference between the two sexes in the chromosome- 

 number, females having twice as many as males ; the 

 meaning and results of this condition will be discussed later 

 in this chapter. 



The facts set forth in the preceding pages might be 

 thought to suggest strongly that an insect's sex is a definite 

 and irrevocable character determined by the constitution of 

 the germ-cells of the creature's parents. Yet, there are 

 other facts well known to students of insects which warn us 

 that by resting in such a conclusion we miss part of the 

 truth of the matter. Insects are hardly evenjl truly herma- 

 phrodite, but abnormal individual specimens in which the 

 characters of the two sexes are more or less combined are 

 well known ; such creatures are called gynandromorphs. 

 In certain moths in which the male differs from the female 

 in wing colour or pattern and has more elaborately developed 

 feelers than she, such gynandromorphs can be very easily 

 recognised. In the simplest case a moth may, for example, 

 be male on the left and female on the right side, Hke the 

 *' Emperor " Moth (Saturma pavonia) depicted on Plate V, 

 the difference affecting not only the visible (" secondary ") 

 characters of wings and feelers, but frequently also the 

 essential organs of reproduction, so that there is on the 



