pplying
itself to one small corner of the work.

The vaulting is executed by the method already described; horizontal
ledges, slanting from the summit of pillar or wall, are formed to meet
one another. The insects are intelligent enough to begin their labour
at the spots best fitted to give strong support to the overhanging
materials, as for instance, at the angle of two walls. There is so
much activity among the workers, and they are so anxious to take
advantage of the damp, that the storey is sometimes completely
finished in seven or eight hours. If the rain suddenly stops in the
course of the work, they abandon operations, to complete them as soon
as another shower falls.

I have already had occasion to speak of the covered passages and
Aphis-pens built by Ants outside their dwellings. Besides these
constructions, they also make roads in the fields, tearing up the
grass and hollowing out the earth so as to form a beaten path free
from the lilliputian bushes in which there would be danger of becoming
entangled, on returning to the nest laden with various and often
embarrassing burdens.

Nor are Ants by any means alone in exhibiting the results of
individual skill and reflection. It will, however, be sufficient to
mention only one other example, that furnished by Spiders. McCook, in
his great work, after elaborately describing and carefully
illustrating the skill exhibited in individual cases by Spiders in
their aerial labours, considers himself justified in concluding as
follows:--"The manner in which the ends of the radii which terminate
upon the herb are wrapped roundabout and braced by the notched zone;
the manner in which the wide non-viscid scaffold lines are woven in
order to give vantage ground from which to place the close-lying and
permanent viscid spirals, upon which the usefulness of the orb
depends--all these, to mention no other points, seem to indicate a
very delicate perception of those modes (shall I also say principles?)
of construction which are continually recognised in the art of the
builder, the architect, and the engineer."[107]

[107] _American Spiders_, vol. i. p. 228.

_Dwellings built of hard materials united by mortar._--Among mammals
few animals have become so skilful in the art of building houses as
the insects we have just been considering. There are, however, two who
equal if they do not surpass them--the Musk-rat and its relative, the
Beaver.

The Musk-rats of Canada live in