HEAT PRODUCTION OF HONEYBEES IN WINTER. 3 



possible to carry out but one experiment a year with a given colony. 

 Circumstances incident to the war prevented continuation of this 

 work, but the results obtained in this experiment are of such economic 

 importance, as well as scientific interest, that it seems desirable to 

 publish them without further delay. 



SOURCE OF HEAT IN THE WINTER CLUSTER. 



The effect of external temperature on the activity of a colony of 

 bees is conspicuous. The bee is similar to other cold-blooded animals 

 in that it lacks the means for internal regulation of body temperature 

 that are found in birds and mammals, and hence the temperature of 

 its body is affected by that of the surrounding air. As the tempera- 

 ture of the air in the hive falls in winter the bees become less active 

 until a certain critical temperature (14° C.) is reached, at which 

 they undertake by muscular activity, not unlike that of shivering, to 

 produce heat in order to keep warm. Between the combs and some- 

 times extending above or below them they form an approximately 

 spherical and fairly compact cluster, with the bees on the outside 

 comprising a sort of shell with their heads turned toward the center. 

 This shell may be several layers thick, the number of layers and the 

 compactness of the cluster depending upon the size and condition of 

 the colony and the temperature of the air in the hive. The bees in 

 this shell remain quiet, except for an occasional shifting of position, 

 but those in the space inside the shell become very active, moving 

 about, shaking their bodies, and fanning vigorously with their wings, 

 thus producing heat to warm the cluster. 



By means of many thermocouples fastened in different parts of 

 the hive Phillips and Demuth (loc. cit.) were able to measure the 

 temperatures at various points within and around the winter cluster. 

 They found that when the temperature of the air within the hive 

 and surrounding the bees was between 14° and 20° C. the bees remain 

 quietly on the combs but not clustered, their body temperatures 

 being, of course, approximately that of the surrounding air. While 

 the upper temperature limit of this quiescent condition is not defi- 

 nitely fixed, varying with the condition of the bees and the weather 

 outside the hive, the lower limit is quite accurately determined by 

 the needs of the bees. When the air temperature falls to 14° C. the 

 bees come together to form the winter cluster. If the temperature 

 falls still lower, they begin to generate heat within the cluster, and 

 frequently the inner temperature rises considerably above those tem- 

 peratures at which the bees were able to exist without activity. 

 Temperatures as high as 30° to 35° C. are not uncommon, and, indeed, 

 were observed even when the air outside the cluster was as low as 



