Another of the varied duties performed by the younger worker bees is that of ventilating the hive by fanning with their wings. On a warm day a number of them may be seen located outside on the alighting-board working these appendages at the utmost velocity so as to drive a current of pure air within; while inside, but not exactly opposite to their comrades, are another troop, who by the same process are engaged in driving the foul air out. Other detachments are in the hottest weather posted in different parts of the interior, and the whole relieve each other in pickets. Huber ascertained that the inside air of a hive is thus preserved nearly as pure as that without. In older works on the subject we are told of the sentinels of bees, but this idea is now abandoned as a fiction. It arose naturally enough out of the above office of fanners, as well as from the fact that if a rap be given upon the alighting-board a bee will immediately appear without. So too if danger appears, and if any bees are outside either as fanners or for their own relief from the heat, these will promptly perform the _duties_ of sentinels. But as to any of them being posted specially for that purpose, it is sufficient to say that at the season when enemies are most to be feared there are no guards at the gates to be found. It has been much queried whether bees ever go to sleep during the working season, as it is known that at night, when not gathering abroad, they are engaged in ceaseless activity at home. Huber, however, observed frequent instances of bees placing their heads' in empty cells and remaining perfectly motionless in that position for from fifteen to twenty-five minutes, in his opinion evidently asleep. Von Berlepsch has repeatedly observed similar occurrences both with workers and queens--not with drones, but then, says he, what is the whole life of these but sleep?--and he considers that there can exist upon the point no doubt whatever. "The more active the bees are," he remarks, "the more will they sleep, like every creature."