Some caterpillars make use of this spinning apparatus only on a few special occasions, but others, more especially some of the smaller species, seem to have it always in use, so that if at any time you suddenly start them into the air by giving a smart tap to the plant or twig on which they rest, they invariably fall slowly on the end of a growing web, the spinning of which they stop as soon as they consider they have fallen far enough. Sometimes, as you are walking through a wood, you will see hundreds, nay, thousands of little caterpillars thus suspended, swinging gently in the breeze. Not long since, after only a few minutes' walk among the trees of Epping Forest, I found I was decorated with several dozens of these swingers with which I had come into collision--in this case consisting chiefly of the larvae of the Green Tortrix Moth (_Tortrix viridana_). Now let us examine the caterpillar's limbs. Attached to each of the second, third, and fourth segments is a pair of true walking legs, corresponding with those of the perfect insect. These are covered with a hard and shining substance, and are also each provided with a hook. The fifth and sixth segments have no limbs at all, nor have the eleventh and twelfth, but some or all of the others (seventh, eighth, ninth, tenth, and thirteenth) are furnished with a pair of claspers which we shall presently describe. First, as regards the number of claspers, it will be seen from what has just been said that this is not always the same. Some caterpillars possess five pairs, thus making up the total number of walking appendages to sixteen. In fact, we must regard this as the _usual_ number. But there are at least a few hundred exceptions to the rule. Many of the _Bombyces_ (page 217), for example, have only four pairs of claspers; and in others of the same group the fifth pair is present, but only partially developed, and quite useless for walking. Look at the peculiar caterpillar of the Lobster Moth (fig. 23)--a creature that differs from most other caterpillars not only in its claspers, but in many other respects too. Observe its long and slender legs, its humped middle segments, and its upturned hindermost segment, of enormous size and mounted with a pair of clubbed 'horns.' This last segment you will observe, has no claspers. [Illustration: FIG. 20.--WALKING LEG OF A CATERPILLAR.] [Illustration: FIG. 21--LARVA OF THE YELLOW UNDERWING MOTH (_Pronuba_).] [Illustration: FIG. 22.--LARVA OF THE CRIMSON SPECKLED MOTH (_Pulchella_).] [Illustration: Fig. 23.--LARVA OF THE LOBSTER MOTH (_Fagi_).] Another allied caterpillar is that of the Iron Prominent Moth (fig. 24). This one also has humped segments, and the claspers of the thirteenth segment are imperfectly developed.