254 SOME ASPECTS OF ECOLOGY 



that a " nun moth temperature index " may be obtained for a 

 given locahty by dividing the sum of monthly temperature 

 indices for the months in which development has gone on by the 

 thermal constant for the total development. The monthly 

 indices are obtained by subtraction of the threshold of 

 development from the mean monthly temperature and by the 

 multiplication of the remainder by the mean number of days in 

 that month during which any particular stage of the insect occurs. 

 If the temperature index for the month is equal to or in excess 

 of 1, then the existence of the insect is, at any rate, theoretically 

 possible. If it be less than 1, then the total temperatures in a 

 given locality are below the limit required to allow of the develop- 

 ment of a complete generation. By means of this index Zwolfer 

 has provided maps showing the world range of L. monacha, and 

 these are in general accord with what is known relative to its 

 actual distribution. These maps further indicate where the 

 insect may be expected to occur in Asia, where its range is very 

 little known. Typical nun moth areas or regions, it may be 

 added, have a temperature index ranging from 1-1 to 1-4. Zwolfer's 

 ideas have so far been very little tested, and there are not many 

 species of insects for which the necessary data are available. 

 The codling moth, Mediterranean fruit-fly and the Dytiscus 

 beetle, however, may be indicated as being species with reference 

 to which his method might be applied. 



Davidson (1934) has studied the distribution of the Collembolan 

 Sminthurus viridis in Australia in relation to climatic factors. 

 The regions favourable to the insect and where it may become 

 established are determined primarily by rainfall and temperature. 

 The range of mean monthly temperatures most favourable 

 to the insect lies between about 11° and 16°. This author 

 has shown that these temperature conditions are intimately 

 connected with those of moisture. The ratio of mean monthly 

 rainfall to evaporation as determined by the saturation deficiency 

 of the air is a guide to the months in which the moisture will be 

 favourable for population increase. Where the rainfall evaporation 

 ratio falls below a value of 1, fluctuating moisture conditions will 

 inhibit multiplication. When this ratio is much in excess of 1, 



