Fig. 22.—Head of larva of Goat Moth, seen from behind. Copied from Lyonnet.
Paired appendages of perfectly simple form are therefore the first stage through which all Insect-jaws must pass. Our second stage is a little more complex, and not nearly so universal as the first. A caterpillar (fig. 22) has its own special wants, and these are met by the unequal development of its jaws. The mandibles are already as complete as those of the Cockroach, which they closely resemble, but the maxillæ are stunted cylinders formed mainly of simple rings, and very like the antennæ. They show, however, the beginnings of three processes (palp, galea, and lacinia), which are usually conspicuous in well-developed maxillæ. The second pair of maxillæ (Lm) are coalesced, as usual, and form the spinneret. The mouth-parts of the Caterpillar do not therefore in all respects represent a universal stage of development, but show important adaptive modifications. The mandibles are rapidly pushed forward, and attain their full development in the larva; the first pair of maxillæ are temporarily arrested in their growth, and persist for a long time in a condition which Orthopterous embryos quickly pass through; the maxillæ of the second pair are not only arrested in their growth, but converted to a special use, which seems to stop all further progress. The labial palps, indeed, which are not at all developed in the caterpillar, survive, and become important parts in the moth; but the greater part of the labium disappears when the time for spinning the cocoon is over.
Fig. 23.—Mouth-parts of Honey Bee.
Fig. 23A.—Diagram of Mouth-parts of Honey Bee.