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 
