TEMPEKATURE OP THE HONEYBEE CLUSTER IN WINTER. 15 



to be expected, as the heat is generated here. The outer shell consti- 

 tutes an ideal insulator for the conservation of the heat, since the 

 bees arranged so close together form small dead air spaces in their 

 interlacing hairs, especially those of the thorax, and afford still more 

 insulation with their bodies. The abdomens of the bees in the outer 

 row are practically separate one from another, and must often be 

 exposed to severe cold. That this method of conserving heat is 

 effective is shown by observations on undisturbed colonies out of 

 doors. For example, on January 14, 1914, there was at 9 a. m. a 

 difference of 68° F. between thermometers 14 (center of the sphere) 

 and 16 (outside the cluster) of Colony D, which were less than 4^- 

 inches apart on the same level in the same space between combs, and 

 a difference of 75° F. between this couple and the bottom board 4^ 

 inches below it. What this difference might sometimes be in colder 

 climates may be imagined. Examples of this kind might be multi- 

 plied indefinitely from the records of these experiments. 



The source of the heat of the cluster must, of course, be the oxi- 

 dation of the food consumed by the bees. The bee is classed as a 

 cold-blooded animal in that the temperature of the individual bee 

 is practically that of the surrounding medium. There is obviously, 

 from the records just given, no internal regulation of the tempera- 

 ture of the body such as is found in birds and mammals, for the 

 temperature of a broodless cluster varies greatly. From the obser- 

 vations made on the various colonies, especially Colony C, it is clear 

 that heat for the warming of the cluster is produced by muscular 

 activity. While, of course, some heat is doubtless liberated by other 

 life processes, this is practically negligible when bees are quiet, as 

 in Colony A when above 57° F. That higher temperatures may be 

 produced, greatly increased muscular activity is required, and in 

 Colony C in cold weather bees in the center of the shell of insulating 

 bees were seen fanning vigorously and executing other movements, 

 such as shaking and rapid respiration. We thus have the para- 

 doxical condition that bees fan to heat the cluster in winter as well 

 as to cool the hive in summer. Observations of this kind were 

 repeated beyond number, and this theory of the method of heat pro- 

 duction is entirely supported by the repeated observation of a hum- 

 ming noise from the cluster during cold weather. 



A few details of the observations on Colony C may be of interest. 

 For example, one bee was observed fanning vigorously for 7-J minutes 

 (9.53 to 10.00-J a. m., Jan. 23) while the other bees kept a space cleared 

 for it. The temperature of the nearest thermometer rose -J° F. during 

 this time. At 9.52 this thermometer was almost a degree cooler than 

 at the time of greatest heat during the fanning. The rapidity of fan- 

 ning of the wings varied, and toward the end of the time it became 

 so slow that the outline of the wings was distinguishable. After the 



