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 colonies on the banks of streams or deep lakes, and construct dwellings which are very well arranged. In their methods we find combined the woven shelter with the house of built earth. Their cabins are established over the highest level of the water and look like little domes. In building them the animals begin by placing reeds in the earth; these they interlace and weave so as to form a sort of vertical mat. They plaster it externally with a layer of mud, which is mixed by means of the paws and smoothed by the tail. At the upper part of the hut the reeds are not pressed together or covered with earth, so that the air may be renewed in the interior. A dwelling of this kind, intended to house six or eight individuals who have combined to build it, may measure up to 65 centimetres in diameter. There is no door directly opening on to the ground. A subterranean gallery starts from the floor and opens out beneath the water. It presents secondary branches, some horizontal, through which the animal goes in search of roots for food, while others descend vertically to pits specially reserved for the disposal of ordure.