August, '08 1 JOUUNAL OF ECONOMIC ENTOMOLOGY 251 



It would seem evident, therefore, that the larvte of the brown-tail 

 moth cannot exist in average size nests where the annual minimum 

 averages —32° to — 35°C. or— 25° to — 31°F. 



The effect of minimum temperature on insect life has been fre- 

 quently recorded, but little study has been given its significance, and 

 only few writers have hazarded the suggestion that the northern dis- 

 tribution of insects might be governed by minimum temperatures. 



Dr. L. 0. Howard is the first writer, known to me, to definitely 

 formulate this principle in America, though he mentions it as excep- 

 tional, and cites only one example. Concerning the American Lo- 

 cust (Srhisioccrea amfvkauay he says, "This species is one of the 

 forms which would seem to indicate that in a few cases, at least, the 

 winter temperature must have some effect in determining distribution. 

 It is exceptional from the fact that it hibernates in the adult condition 

 and we can hardly avoid the conclusion that it is limited in its north- 

 ern range by circumstances M'hich influence successful hibernation. 

 Nothing is better known than that exceptional freezes may kill off 

 thousands of insects; there must therefore be species whose success- 

 ful hibernation is limited to certain degrees of cold." 



Dr. F. H. Chittenden- emphasizes this and states that 

 ". . . . in certain forms of insects the winter temperature must 

 have some effect in determining distribution. While admitting that 

 the past winter was exceptional as regards temperature, the writer 

 feels confident in carrying conclusions still farther in stating that in 

 his opinion, based upon the study of the effect of that winter on injuri- 

 ous northern and southern forms of insects occurring in that portion 

 of the Carolinian or humid life areas of the Austroriparian and Alle- 

 ghanian zones (a climate like that of the District of Columbia), mean 

 winter temperature has more effect upon determining the rarity or 

 abundance of these species than has the mean summer temperature." 



To test this hypothesis, the writer has drawn the average annual- 

 minimum isotherms for the regular stations of the U. S. Weather 

 Bureau (see map 3) together with the maximum annual minimum^ 

 isotherms (see map 4). 



Comparing these with the isotherms of the absolute minima (see 

 map 5), it is seen that the absolute minimum is usually about 10° 

 lower, and the maximum annual-minimum 10° higher than average 



i"Notes on the Geographical Distribution within the U. S. of Certain In- 

 sects Injuring Cultivated Crops," Proceedings Entom. Society, Washington, 

 Vol. 3, p. 225. 



-'Insects and the Weather: OI)servations During the Season of 1899," Bulle- 

 tin 22, n. s., Division of Entomolog.v, U. S. Dept. Agr., p. G2. 



•31. e., the highest annual-niininium rci-orded. 



