August, '081 JOURNAL OF ECONOMIC ENTOMOLOGY 249 



129 to 359 larvje. The minimum length of time at — 35° C. neces- 

 sary to kill them does not seem to have been determined. It should 

 also be noted that most of the experiments were made with nests 

 containing but few larva compared with those of this country, rarely 

 containing over 100, while here the average is over 300 larvae, and 

 as has been shown the larger nests better withstand the cold. Gre- 

 villius notes that the larvae in the outer parts of the nests are killed 

 first and that those in the center survive. The size of the nest, there- 

 fore, greatly complicates the determination of the minimum temper- 

 ature for this species. 



Comparing the northern limit of the brown-tail moth in Europe 

 with the annual minimum temperature occurring there (see map 2), 

 Grevillius remarks ( 1. c. pg. 314) that Kasan at the northern limit 

 of the brown-tail moth in Russia has a mean annual minimum of 

 — 32° or — 33°C., and that it is noteworthy that this temperature 

 corresponds with the minimum temperature at which larva? could exist 

 in his experiments, but that there is a possibility that the larva may 

 have adapted themselves to a lower temperature to which they may 

 be exposed for a longer time than in his experiments. The relations 

 of the isotherms of the mean annual minimum temperatures with the 

 northern limits of the brown-tail moth, oak and apple in Europe are 

 certainly suggestive. In northwestern Europe it seems well estab- 

 lished that the pest is kept in check by the greater humidity encour- 

 aging the growth of fungous disease. The southward curve of the 

 boundary of the brown-tail moth to Podolia follows that of the mean 

 annual minimum isotherms, but its extension northeastward to Kasan 

 cannot be accounted for thus. 



From the elaborate researches of Bachmetjew^ concerning the "crit- 

 ical point,"- it would seem that the maintenance of the temperature 

 of the "critical point" for from a few minutes to not over half an 

 hour would result fatally to the insect, and that the time required to 

 produce death at any temperature above the critical point will vary 

 conversely with the difference between it (the body temperature 

 reached) and the critical point. Unfortunately the "critical point" 

 of the body temperature of the brown-tail larvse in their nests is 



iBachmetjew, P., Experimeutalle entomologiscbe Studieii vom Physikalisch- 

 chemischen Standpunkt. {Leipzig, 1901) p. 80-90, 132-135. 



2lt is unfortunate that Bachmetjew has used the term "critical point" to 

 define the temperature at which the protoplasm commences to freeze, in an 

 entirely different sense from that previously employetl by phenologists who 

 desitrnate the temperature above which positive or effective temperatures must 

 be summed as the "critical temperature," as mentioned on pg. 25.5 of this paper. 



