1917] Hibernation: A Periodical Phenomenon 183 



insects to hibernate. Table IV gives further support to this 

 conclusion and also indicates that various successive treatments 

 with different humidities are also of no avail. Death was not 

 due to poisoning of the larvas by fumes given off from H2SO4, 

 for Noctua unipuncta moths were reared from first stage larvae 

 at vapor tentions 3.4, 9.0, 11., 13.5, 15.0. 



Since larvae before experiencing winter go into a hibernating 

 condition from which various combinations of the three stimulii 

 high temperature humidity and food cannot "arouse" them, 

 we must conclude that this quiescence is predetermined. The 

 practice of collectors and the experiments of Weissmann,^ 

 have shown that a period of low temperature makes it possible 

 to activate hibernating insects by high temperature. Kirby 

 and Spence^ have suggested that this predetermination is 

 instinctive as they observed that before winter insects suddenly 

 at a definite date, independent of weather conditions, start an 

 excited search for winter shelters. This "instinct " has probably 

 been noticed by every collector of insects. However, this 

 "instinct," if not directed by any external stimulus is rather 

 hard to explain in cases where a summer and a winter generation 

 occur. 



Pictet*' studied the Lasiocampa quercus and Dendrolimus 

 larvas which hibernate before the temperature has lowered. 

 In the case of Lasiocampa quercus, the adult emerges in July, 

 but larvae appear in August and hibernate, beginning again 

 to develop in Spring and pupating in June-July. By keeping 

 the larvae on ice it was possible to cause them to pupate in 

 May. Continued selection of precocious larvae for six gen- 

 erations decreased the length of larval life from 245 to 112 days. 

 The pupal period was lengthened sufficiently to make up for 

 the difference. Similar experiments with Dendrolimus pini 

 gave a second generation and no persistence of the normal 

 cycle. Pictet believes that this difference is due to the fact 

 that Lasciocampa quercus feeds on the leaves of deciduous trees, 

 while Dendrolimus pini feeds on the leaves of evergreen trees. 



We have seen that certain insects have a definite periodical 

 hibernation which is hereditary. This quiescence can only be 

 overcome by a certain period of low temperature and the 

 organisms then by a compensatory lengthening of the next 

 stage regains its normal rhythm. Other insects are more 

 plastic and instead of showing a definite period of hibernation, 

 merely remain quiescent during periods of low temperature and 



