This process of feeding and moulting is continued for several weeks, the number of moults being usually four. During the later stages the increase in size is more marked each time the skin is shed, until the caterpillar finally reaches its full growth as a larva and is ready for the wonderful change to the quiet chrysalis in which all its caterpillar organs are to be transformed into the very different organs of the butterfly. In the case of butterfly larvae one of the most interesting features of the growth of the caterpillar is that of the remarkable changes in colors and patterns of marking which the caterpillar undergoes. One who had not followed these changes would often be at a loss to recognize caterpillars of slightly differing sizes as belonging to the same species. These changes commonly show a remarkable adaptation to the conditions of life, and generally tend to the concealment of the caterpillar upon its food plant. The stages of growth of the green-clouded swallowtail caterpillar are illustrated on plate opposite page 80. Before each moult the caterpillar is likely to spin a silken web upon the leaf surface. It then entangles its claws in the web to hold itself in place while the skin is cast. (_See plate, page 17._) _The Change to the Chrysalis_ (_See plate, pages 32-33._) A week or ten days after the last moult of its caterpillar growth the larva commonly becomes full fed and ready to change to the chrysalis state. The details of the way in which this is accomplished vary greatly with different butterflies, as will be noted in the stories of many species later in this book. In general, however, the caterpillar provides a web of silk which it spins against some surface where the chrysalis will be secure and in this web it entangles its hind legs. Sometimes there is the additional protection of a loop of silk over the front end of the body. After the legs have become entangled the caterpillar hangs downward until the skin splits open along the median line of the back and gradually shrinks upward until it is almost free, showing as it comes off a curious creature which has some of the characteristic features of a chrysalis. It is seldom at this stage of the same shape as the chrysalis. When the caterpillar's skin is nearly off this chrysalis-like object usually wriggles its body quickly in a manner to entangle a curious set of hooks attached to the upper end in the web of silken thread. This hook-like projection is called the cremaster, and it serves a very important purpose in holding the chrysalis in position. [Illustration: Swallowtail Chrysalis, showing (_b_) the loop of silk over thorax. (After Riley)]