ed every
Brazilian species of _Melipona_ at Bordeaux) to the Jardin
d'Acclimatation. It was even seen that the door might be put up under
certain circumstances in open day, as for example, when a storm or
sudden cold delays the appearance of the workers. If one of them
happened to be late it had to perforate the partition, and the hole
was then stopped up again.

[Illustration: FIG. 42.]

_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]