94 INSECT PHYSIOLOGY 



For the most part, bodily characters are determined genetically. 

 When multinucleate eggs are fertilized by several spermatozoa carry- 

 ing different genes, mosaics result in the adult insect, exactly com- 

 parable with the sexual mosaics or gynandromorphs. But characters, 

 ordinarily determined by the genetic constitution, can sometimes be 

 elicited by changes in external conditions during development. Ac- 

 cording to the theory of Goldschmidt, these effects can be explained, 

 as the occurrence of intersexes can be explained, by supposing that 

 many morphological variations are induced by changes in the veloci- 

 ties of particular processes of development. Such changes in time 

 relations may be brought about, on the one hand, by changes in the 

 genes, or on the other, by alterations in the temperature or other 

 factors, which influence one process more than another. 



Nowadays it is more usual to picture the different morphological 

 characters in different individuals of a polymorphic species, or at 

 different stages in the life cycle of a given individual (as in meta- 

 morphosis) or in the different parts of the body, as being the result 

 of activation of different components of the gene system. In some 

 way that is not yet understood the juvenile hormone, the various 

 inductors responsible for differentiation, and the factors responsible 

 for the appearance of the different forms in insects subject to 'en- 

 vironmentally controlled polymorphism', all have the effect of bring- 

 ing into action particular elements of the gene system : gene function 

 is said to be 'switched'. Other types of polymorphism are due to 

 genetic differences between individuals; the balance between the 

 numbers of each type being determined by natural selection. 



Arrested growth or diapause 



Growth and development in insects may be temporarily arrested at 

 any stage in the life cycle by adverse conditions of many kinds. Low 

 temperature, drought, starvation, lack of vitamins, or other essential 

 substances may lead to a state of 'quiescence'. But many insects, 

 particularly in temperate latitudes, periodically enter a state of arres- 

 ted growth which is not readily eliminated when conditions again 

 become favourable. For this more deep-seated arrest the term 'dia- 

 pause' is commonly used. 

 The state of diapause may supervene at different stages in different 



