GROWTH 95 



insects: in the recently laid egg, or the egg in which the germ band is 

 newly formed (e.g. in the silkworm and many other Lcpidoptcra), in 

 the egg containing a fully develped larva (in the mosquito Aides 

 aegypti, &c), in one or other of the larval stages, or in the pupa. This 

 ^tate is most commonly an adaptation for survival without feeding 

 during the winter. It often appears to arise without any external 

 stimulus, but that is because the stimulus that induces diapause may 

 occur weeks or months before the effects become visible. The stimuli 

 concerned are closely related to the season: they may be a fall in 

 temperature, or the poor nutritive quality of foliage in the late 

 summer; but the most important of all is generally the length of day. 

 Many Lepidoptera, for example the cabbage butterfly Picn's, which 

 normally pass the winter in a pupal diapause, can be induced to 

 breed continuously if the larvae are exposed to a length of day which 

 exceeds fourteen hours or so. In the silkworm, which lays its eggs 

 early in the summer, the effect of day length is reversed : if the female 

 moth is exposed to a day length of fourteen hours or more she lays 

 hibernating eggs. 



In certain species, for example the moth Acronycta rumicis in 

 Russia, the critical daylength required to induce diapause varies 

 widely in different geographical races in accordance with the latitude 

 to which they are adapted: a given race transplanted to a different 

 latitude may be unable to survive because it fails to go into diapause 

 at the proper time. In some insects, the Nymphalid butterfly Ara- 

 schnia levana is the most familiar example, the diapause pupa gives 

 lise to a 'spring form' which is strikingly different from the 'summer 

 form' that emerges from non-diapause pupae. Here 'seasonal poly- 

 morphism' is genetically linked with diapause. 



Once diapause is established it is not quickly terminated. Most 

 insects with a winter diapause (such as the silkworm egg) require a 

 period of exposure to low temperature before growth is renewed. It 

 seems that the metabolic processes which have to be gone through 

 before diapause is brought to an end have a curiously low optimum 

 temperature. Certain tropical species, on the other hand, enter dia- 

 pause in the hot dry season; here the termination of diapause re- 

 quires a period of exposure to a high temperature. 



The immediate cause for diapause is the lack of the hormones 



