Most people know the caterpillar of this moth a good deal better than they know the moth itself. I dare say that you have often seen it crawling about in August and September, always walking very fast, as though it were in a great hurry. It is a big, rather hairy creature of a dull yellow colour, with a black head, and with nine black stripes running along its body; and you may find it in numbers, feeding on the leaves of elm, lime, and willow trees. Very often, indeed, it is so plentiful that it strips whole branches of their leaves. When it reaches its full size it comes down from the tree, wanders off to some little distance, hides away under dead leaves or at the roots of a tuft of grass, and turns into a dark brown chrysalis, out of which the moth hatches in the following May or June.
The reason why one sees this handsome moth so very much seldomer than the caterpillar is that it always rests with its wings folded closely against its body, in which position it looks just like a piece of broken stick. But you may often find it clinging to the trunk of an elm or a lime tree, or to a long grass-stem growing underneath it.
You have only to look at this moth to see why its name was given to it, for on each of its front wings it has two large white spots with two small dark spots inside them, one above the other; so that they look very much like the figure 8. But the inner 8 is always a much neater one than the outer, which has a kind of blurred appearance, just as if a drop of water had fallen upon it and made the colours run.
This moth is quite a common one in most parts of the country, and appears on the wing in September. It only flies by night, so that one does not often see it; but it will sometimes fly into a well-lighted room on a dark, warm evening if the window is left open. You can find the caterpillar, however, without any difficulty at all. All that you have to do is to hunt for it on hawthorn or blackthorn bushes during May or the early part of June, and there you are almost sure to see it—a smoky green creature thinly covered with black hairs, and with a yellow stripe running down its back, and another along each side of its body. A little later on it spins a neat little cocoon, made partly of silk and partly of bits of bark and leaf, which it fastens underneath a twig of its food-plant. And in this it changes into a chrysalis.