366 Relation' of Watee to the Behavioe of 



ROLE OF WATER IN HIBERNATION. 



It is an established fact (Tower, 1906) that in the second or winter genera- 

 tion Leptinotarsa decemlineata in its homozygous state always hibernates under 

 normal conditions, but that desiccation, or cold, or both, might produce hiber- 

 nation at any time. At Tucson it was also found that whenever the conditions 

 became adverse enough to produce desiccation, hibernation was produced. Tower 

 (1906) showed that preparation for hibernation in the winter generation con- 

 sists largely in the reduction of the watery contents of the body and in an 

 elimination of all food and other substances from the alimentary canal. 



These facts indicated that the loss of water appeared to be produced by two 

 different mechanisms. One was controlled by an external medium, while the 

 other was determined by heredity. In the former, water was extracted from 

 the tissues through desiccation due to conditions in the medium, while in the 

 latter water was eliminated from the tissues through internal processes under 

 normal conditions. Tower states : 



" Preparation for hibernation consists in a physiological change in the con- 

 stitution of the body for the time being and a consequent lowering of the 

 freezing-point of its tissues in exactly the same way that spores of many plants 

 and the over-wintering eggs of rotifers prepare for the coming of the unfavor- 

 able conditions in their environment." 



The following experiments were performed to show how the above results can 

 be brought about through desiccation. At Tucson this type of hibernation was 

 quite common, but did not occur in nature at Chicago. 



ENTRANCE INTO HIBERNATION. 



It is evident that in the potato beetle entrance into hibernation may be 

 " induced hibernation," which occurred whenever the evaporating power of the 

 air surrounding the insect removed more water by weight in a given time than 

 was introduced into the organism by food and other agencies. Such desiccation 

 produced in the course of one or two days depended upon the adversity of the 

 conditions, thus effecting change in the beetle's behavior, so that its reactions 

 were reversed and it burrowed in the soil. 



Extensive observations were made at Tucson Station A, where it was dis- 

 covered that this type of hibernation took place whenever sufficiently desiccating 

 conditions existed. The evidence of such a reaction in a large population was 

 determined by comparing the daily environmental readings with counts of the 

 non-hibernating population, which showed that during the rainy season and as 

 long as water was added to the soil no entrance in this type was found, but when 

 water was discontinued desiccation occurred and hibernation resulted. On the 

 other hand, at Tucson Station B, the " induced hibernation " was always 

 observed as the prevailing type of behavior, since in this habitat the conditions 

 were more adverse. Moreover, at this locality, the growth of the food plants was 

 retarded, since the leaves were tougher and showed less water-content ; desicca- 

 tion was also much greater, so that the response of the organisms to such 

 rigorous conditions was sharper than in any other locality under observation. 

 This clearly demonstrated when a comparison of the daily environmental 

 records were made with the daily count of beetles, which were found out of the 



