GEOGRAPHICAL DISTRIBUTION 253 



likewise preclude the fruiting of suitable hosts for long periods. It 

 would appear, therefore, from these remarks that the insect 

 exhibits a greater tolerance of heat than of cold, and that it is a 

 species incapable of hibernation in the usual sense of the term. 

 In the Hawaiian Islands, for example, the development of the 

 insect progresses most rapidly when the mean temperature 

 exceeds 23-8° to 26-1°. Back and Pemberton state that complete 

 mortality follows continued exposures to temperatures below 10°, 

 and that no immature stages can withstand subjection for two 

 weeks at freezing point. On the basis of the foregoing remarks, 

 inferences may be drawn with respect to the possibility of the 

 insect extending its range into lands free from its presence. The 

 recent incidence of the pest in Florida points to the liability to 

 infestation of all the southern states of North America where 

 suitable climatic conditions prevail, and also of Mexico and 

 Central America. The West Indies, East Indies and many of 

 the Pacific Islands, to mention but a few other areas, afford ideal 

 climatic conditions, and their greatest danger lies, perhaps, in the 

 unintentional transfer of infested fruits carried by travellers 

 enjoying the facilities now available for world tours. 



Zwolfer (1934), in Germany, has made a special study of the 

 temperature relations of the different instars of the nun moth 

 (Lymantria monacha), together with the regional distribution of 

 this insect over Europe. He has determined the thermal constants 

 in day-degrees and the threshold of development for the egg, 

 the six larval instars, pupa and imago. Since the complete 

 development within the egg occurs in the autumn, no growth 

 occurs during hibernation in this stage. Beginning with the first 

 larval instar and ending with the egg stage, the thermal constants 

 and thresholds of development (in brackets) are stated to be as 

 follows : 65(4-9) : 217(3-2) : 84(5-7) ; 84(7-2) : 90(7-6) : 132(7-8) : 

 197(6) : 130(8-4) : 240(6-8). The thermal constant for the total 

 development of this insect is given as 1,240 ^ 40 day-degrees. 

 Zwolfer further mentions that the stages up to the fourth larval 

 instar lend themselves best for control measures, and that their 

 appearance can be calculated by means of experimental data 

 coupled with the use of mean monthly temperatures. He states 



