The Cycle of the Year 87 



decrease the amount of brood when the honey-flow stops 

 while Carniolan and Caucasian bees rear more brood "out 

 of season," or after the honey-flow. But all races rear 

 some brood "out of season," so that the final discontinuance 

 of brood-rearing cannot be considered as due to lack of 

 incoming nectar or pollen. Even among colonies of the 

 same race there is considerable variation in a single apiary. 

 Of course, no colony can rear brood without food for the 

 young larvae. The stoppage of brood-rearing is sometimes 

 attributed to low outside temperature but, as stated earlier, 

 brood is sometimes reared in the coldest months, in fact 

 it is usually begun then in colonies wintered out of doors. 

 As will appear later, the cold outside during January is 

 the cause of a higher temperature within the cluster than 

 is usual in the autumn and, combined with the effects of 

 the accumulation of feces, is the cause of higher cluster 

 temperature than occurs with the same intensity of cold 

 in December. Furthermore, when a colony begins brood- 

 rearing in the winter, the presence of brood seems to induce 

 the production of sufficient heat to care for it, the resulting 

 temperature being sufficient to induce more egg-laying, so 

 that brood-rearing once begun continues through the re- 

 mainder of the winter. Since a moderately low outer tem- 

 perature may cause more active heat production in a small 

 colony than in a strong one, this may explain some of the 

 variation observed in the time that brood-rearing ceases. 

 A small colony may have a higher cluster temperature than 

 a strong colony, the greater activity in the center of the 

 cluster being necessary to produce the required temperature 

 in the shell of the cluster, which is a less efficient insulator 

 in a weak colony. The structure of the cluster is described 

 in the following section. If an explanation for the cessation 

 of brood-rearing is sought, the paradoxical conclusion is 

 reached that in the fall (1) the outside temperature is not 

 high enough for brood-rearing without artificial heat pro- 

 duction and (2) it is not low enough to cause the bees to 

 produce sufficient cluster heat for brood-rearing. Egg- 



