On each ring, except the second (including now the three thoracic with the nine abdominal; and so making twelve rings), the third, and the last, there is an oval or roundish mark which indicates the position of the breathing hole (_spiracle_). Through these minute openings air enters to the breathing tubes (_tracheæ_), which are spread throughout the interior of the caterpillar in a seemingly complicated kind of network of main branches and finer twigs; air is thus conveyed to every part of the body. In the event of one or two air-holes becoming in any way obstructed, the caterpillar would possibly be none the worse; but if all the openings were closed up effectually, it would almost certainly die. Total immersion in water, even for some hours, is not always fatal. Turning again to the "feet" of the caterpillar, it will be seen from the figure that the true legs (_a_) differ from the false legs (_b_) in structure. The former are horny, jointed, and have terminal claws; the latter are fleshy, with sliding joints, and the foot is furnished with a series of minute hooks which enable the caterpillar to obtain a secure hold when feeding, etc. The false legs are also the chief means of locomotion, as the true legs are of little service for this purpose. The true legs, however, appear to be of use when the caterpillar is feeding, as the leaf is held between them so as to keep it steady whilst the jaws are doing their work. [Illustration: FIG. 2. (_a_) True and (_b_) false legs.] [Illustration: FIG. 3. _a_, labrum; _b_, mandible; _c_, antenna; _d_, ocelli; _e_, maxilla; _f_, labium; _g_, spinneret; _h_, labial palp.] In the accompanying figure of the head of a caterpillar the mouth parts are clearly shown. The biting jaws (_mandibles_) are slightly apart, above them is seen the upper lip (_labrum_), and below them is the under lip (_labium_ or _lingua_). The _maxillæ_ are very tiny affairs, but they should be noted because in the butterfly they become the basal portions of the two tubes which, when united together, form the sucking organs (_proboscis_). The eyes, or ocelli as they are termed, are minute, and are said to be of slight use to the caterpillar as organs of sight, so that it probably has to depend on its little feelers (_antennæ_) for guidance to the right plants for its nourishment. Attention should also be given to the spinneret, as it is by means of this that the silken threads, etc., for its various requirements are provided; the substance itself being secreted in glands placed in the body of the caterpillar. The palpi are organs of touch, and seem to be of use to the caterpillar when moving about.