Not less significant is the case of the silk-cocoons. The cocoons spun by the silkworm are egg-shaped, and consist of a single thread many thousand yards in length, which is wound round the spinning caterpillar so that not a space is left uncovered. The web is firm, tough, and very difficult to tear; therefore we must grant that the pupa resting within will enjoy a very considerable degree of security against injury. But the moth must be able to get out, and that this may be possible the caterpillar is impelled by instinct to make its spinning movements such that the cocoon is eventually looser at the anterior end, so that the insect, when it is ready to emerge, can tear it asunder with its feet and make a way out for itself. For this very reason, because the silk must be torn and spoilt by the emerging insect, silk-breeders kill the pupating insect before it begins to make its way out. But there are species whose cocoons are provided from the very start with an outlet, for the caterpillar spins the silk round itself in such a way that a round opening is left. But this opening would be not only a convenient door for the butterfly to emerge by, but an equally convenient entrance for all its enemies. It is, therefore, closed up. In the case of the 'emperor moth' (_Saturnia carpini_) this is effected by means of a circle of stiff bristles of silk on the inside (Fig. 33), the points of which bend outwards like those of a weir-basket (_r_); from the inside the emerging moth can easily push aside the bristles, while the threatening enemy from without is scared off by the stiff points of the bristles. [Illustration: FIG. 33. Cocoon of the Emperor Moth (_Saturnia carpini_), after Rösel. _A_, enclosed pupa. _B_, emerging moth. _r_, hedge of bristles. _fl_, wings.]