onger habitable; but they have all contributed
to diminish the resistance of the wood, and this continues as long as
the race which they produced makes its way towards the centre of the
stake.

[Illustration: FIG. 22.]

An insect, the _Xylocopa violacea_ (Fig. 22), related to our
Humble-bee, from which it differs in several anatomical characters,
and by the dark violet tint of its wings, brings an improvement to the
formation of the shelter which it makes in wood for its larvae. Instead
of hollowing a mere retreat to place there all its eggs
indiscriminately, it divides them into compartments, separated by
horizontal partitions. It is the female alone who accomplishes this
task, connected with the function of perpetuating the race. She
chooses an old tree-trunk, a pole, or the post of a fence, exposed to
the sun and already worm-eaten, so that her labour may be lightened.
She first attacks the wood perpendicularly to the surface, then
suddenly turns and directs downwards the passage, the diameter of
which is about equal to the size of the insect's body. The _Xylocopa_
thus forms a tube about thirty centimetres in length. Quite at the
bottom she places the first egg, leaving beside it a provision of
honey necessary to nourish the larva during its evolution; she then
closes it with a partition. This partition is made with fragments of
the powder of wood glued together with saliva. A first horizontal ring
is applied round the circumference of the tube; then in the interior
of this first ring a second is formed, and so on continuously, until
the central opening, more and more reduced, is at last entirely closed
up. This ceiling forms the floor for the next chamber, in which the
female deposits a new egg, provided, like the other, with abundant
provisions. The same acts are repeated until the retreat becomes
transformed into a series of isolated cells in which the larvae can
effect their development, and from which they will emerge either by
themselves perforating a thin wall which separates them from daylight,
or by an opening which the careful mother has left to allow them to
attain liberty without trouble.[87]

[87] Reaumur, _Memoires pour servir a l'histoire des
Insectes_, pp. 97 _et seq._

_Woven dwellings._--The second class of habitation, which I have
called the woven dwelling, proceeds at first from the parcelling up of
substances, then of objects capable of being entangled like wisps of
wood or stra