_Fortifications of Bees._--As these facts take place always they may be called instinctive; but that is not the case with regard to defences elevated with a view to a particular circumstance, and which disappear when the danger to which they correspond disappears. Such are the labours of the bees to repel the invasions of the large nocturnal Death's-head Moth. (Fig. 42.) He is very greedy of honey, and furtively introduces himself into the hives. Protected by the long and fluffy hairs which cover him, he has little to fear from stings, and gorges himself with the greatest freedom on the stores of the swarm. Huber, in his admirable investigations,[113] narrates that one year in Switzerland numbers of hives were emptied, and contained no more honey in summer than in the spring. During that year Death's-head Moths were very numerous. The illustrious naturalist soon became certain that this moth was guilty of the thefts in question. While he was reflecting as to what should be done, the bees, who were more directly interested, had invented several different methods of procedure. Some closed the entrance with wax, leaving only a narrow opening through which the great robber could not penetrate. Others built up before the opening a series of parallel walls, leaving between them a zigzag corridor through which the Hymenoptera themselves were able to enter. But the intruder was much too long to perform this exercise successfully. Man utilises defences of this kind; it is thus at the entrance of a field, for example, he places a turnstile, or parallel bars that do not face each other; the passage is not closed for him, but a cow is too long to overcome the obstacle. In years when the Death's-head Moth is rare the bees do not set up these barricades, which, indeed, they themselves find troublesome. For two or three consecutive years they leave their doors wide open. Then another invasion occurs, and they immediately close the openings. It cannot be denied that in these cases their acts agree with circumstances that are not habitual.[114] [113] Huber, _Nouvelles observations sur les Abeilles_. [114] These facts have recently been observed and recorded afresh by Mr. Clifford in _Nature Notes_, January 1893. _Precautions against inquisitiveness._--I will finally quote a fact of defence which took place under circumstances that were absolutely exceptional, and which therefore exhibits genuine reflection in these insects. During the first exhibition of 1855 an artificial hive was set up, one face of which was closed by a glass pane. A wooden shutter concealed this pane, but passers-by opened it every moment to contemplate the work of the small insects. Annoyed by this inquisitiveness, the bees resolved to put an end to it, and cemented the shutter with _propolis_. When this substance dried it was no longer possible to open the shutter. The bees were visible to nobody. [Illustration: FIG. 43.] [Illustration: FIG. 44.]