Then there is the ordinary Ice-house, a sketch of which is given in the illustration. A pit is first dug in the ground, and thickly lined with dry branches, straw, &c. The roof is constructed in the same manner, only the non-conducting power is increased by a thick coating of earth over the sticks and straw. The door, which is approached by a shelving cutting, is similarly protected, the covering only being removed when the door is opened.
I once made a very effective refrigerator out of two hampers, putting a small hamper inside a large one, and packing the space between them with straw.
In Nature we find many examples of this principle, which enables the inhabitants to bid defiance to frost.
A familiar example may be found in the cocoon of the common Silk-worm (Bombyx mori), and indeed in that of almost any silk-producing insect. When the caterpillar is about to make its cocoon, it begins by a number of rather strong threads attached to different points, and making a sort of scaffolding, so to speak, for the cocoon itself. Upon these is spun a slight outer cocoon of very loose and vague texture—the “floss silk” of commerce, and within that is the cocoon proper, in which the insect lies enclosed. It will be seen, therefore, that there are really three cocoons, one within the other, namely, the scaffold cocoon, the floss cocoon, and the silk cocoon itself, so that the inmate is protected from variations of temperature.
The cocoon of the emperor-moth, which has already been described, is made on the same principle.
There are several caterpillars which are social in their early stages, and which construct a common habitation. The Little Ermine-moth. (Hyponomeuta padella) affords a familiar example of this structure. The caterpillars are great roamers in search of food by day, and travel from branch to branch on their strong silken threads. At night, however, they return to a large white silken habitation which they have spun, and which they divide into many compartments, as may easily be seen by cutting the nest open with very sharp scissors. Within this habitation the caterpillars spin their separate cocoons, so that the system of double walls is thoroughly carried out.
There is another insect, very common on the Continent, but, happily for us, not introduced into England. It is called the Processionary Moth, from its curious habit of marching in exact lines, the head of the second caterpillar touching the tail of the first, and so on. These insects have likewise a common home, and spin their own separate cocoons within it.
There are two other sociable British Moths which make nests on a similar principle. These are the Gold-tailed Moth (Porthesia chrysorrhœa) and the Brown-tailed Moth (Porthesia auriflua). They are both beautifully white insects, but may easily be distinguished from each other, the Gold-tailed Moth having some brown-black spots on the upper wings, and a tuft of golden-yellow hairs at the end of the body; while the Brown-tailed Moth is without spots, and the tail-tuft is brown.
In habits they are very similar, and the description of the nest made by one will answer for that made by the other. I believe that broods of these two species have been known to construct a common nest. The nest is extremely variable in form, because it depends much on the number of twigs which it includes. Interiorly, it is divided into a considerable number of chambers, each containing one or several individuals.