58 JOURNAL OF ECONOMIC ENTOMOLOGY [Vol. 1 



tent caterpillar has but one generation both in the North and South, 

 hibernating over winter in the egg, in which the larva is completely 

 formed. The critical point must be much higher in the South, for the 

 low temperatures which are effective in the North are rarely reached 

 in the South. 



The time at which insects enter hibernation in the fall is undoubt- 

 edly largely influenced by temperature, and in many species it may 

 be the controlling factor, but there are many cases where hibernation 

 commences before there is a material drop in temperature, and that 

 the lowering temperature is the controlling factor is disproved by the 

 fact that insects subjected to high temperatures before the normal 

 time for hibernation persist in hibernating or else remain slightly 

 active, but rest and do not feed or reproduce for a considerable length 

 of time. This has been shown by Tower in his remarkable experi- 

 ments upon the Potato Beetle and in our own experiments. This 

 paper of Dr. Tower*^ will ever remain an entomological classic and 

 should receive the study of everyone interested in economic entomol- 

 ogy. Tower has shown that all of the beetles of this genus have two 

 generations and no more, and that after the second generation there 

 must be a time of rest before reproduction continues. In some cases 

 this is hibernation and in others aestivation, which Tower maintains 

 are practically the same as regards the life history of the insect. In 

 both cases the insect prepares for the resting stage by losing about 30% 

 of its gross weight through the loss of water, which enables it to with- 

 stand a lower freezing point and higher temperatures than if the pro- 

 toplasm were not thus condensed. 



In endeavoring to determine the thermal constant for emergence we 

 have placed the hibernating stage in greenhouses heated at different 

 temperatures and compared the amount of temperature accumulated 

 in each up to emergence with that accumulating in nature. The first 

 year the insects were not placed in the greenhouse until in January, 

 after they had been subjected to considerable cold weather. They 

 emerged in due time in a fairly normal manner. In 1906 the insects 

 were brought into the greenhouse earlier in the season before they had 

 been subjected to much cold out of doors. As a result it required 

 much more heat over a longer period to force their emergence and 

 they emerged irregularly for several weeks. Lots brought in later 

 in the winter emerged much more normally. We were therefore led 

 to suspect that the cold of winter had a positive influence in deter- 

 mining the length of the hibernation of these insects, which subse- 



dW. L. Tower, Evolution in Chrysomelid Beetles of the Genus Leptinotarsa. 

 Carnegie Institution, No. 48. 1906. 



