INSECT DIAPAUSE AND ENVIRONMENT 125 



significance in the natural environment can be utilized by 

 natural selection as an appropriate signal. Thus the hypopus 

 of the mite Histiostoma (this is the extra non- feeding nymphal 

 instar which appears when poor nutritive conditions supervene) 

 can be induced to moult and transform into a feeding nymph by 

 the smell of yeast^^. 



However, temperature is undoubtedly of very general signi- 

 ficance. The silkworm Qgg — in this respect a classical object of 

 study — has a temperature optimum for the completion of 

 diapause of about 8°, which is well below the threshold for 

 normal development. The temperatures required by different 

 species show a general correlation with their environments, 

 those from temperate or cold climates usually needing a tempera- 

 ture in the range 0-10°, while 10-15' is often more suitable for 

 species from warmer regions. Higher temperatures are also 

 effective in insects from temperate climates which have an 

 aestivating rather than a winter dormancy {e.g. the winter moth 

 Operophtera^'^). 



The precise temperature requirements for terminating dia- 

 pause are complex and variable. No two species are exactly alike 

 in this respect. The silkworm egg needs 40 or so days of chilling. 

 But longer or shorter periods may be required by other species 

 with a diapause of greater or lesser intensity. It is also very 

 common to find that the temperature optimum varies during the 

 course of diapause completion. I shall consider some of the 

 physiological implications of this phenomenon in my second 

 lecture. 



DORMANCY AND THE LIFE CYCLE 



Although it is not always possible to estim.ate the precise 

 significance of the m.ore complex temperature requirem.ents, it 

 is safe to infer that they represent a successful adjustment to the 

 average temperature conditions of the local environment. A 

 dormant period of optimal length ensures that the active growth 

 of the population is delayed until weather and food supplies 



References p. 128 



