METAMORPHOSIS (in animals). A change of form in the post-embryonic life of an indi vidual animal. The term is also applied to the change in form of homologous parts in different species.
The young of many animals pass through a series of changes of form, in each of which the animal is adapted to changes in its surround ings, involving alterations in its mode of life, slight if the change of body-form is slight. thor ough-going- and radical if its body becomes pro foundly modified. As examples of a complete metamorphosis may be cited the life-histories of the jellyfish (q.v.), the starfish, mollusks, crus taceans, insects, and also the salamanders. toads, and frogs. Most shrimps and crabs undergo a complicated metamorphosis, for in the different they lead different lives and are subjected to different environments, the large for the most part being free-swimming and living near the surface of the water, while the parents are sta tionary. The result of this eluinge of habits and form undoubtedly is to prevent the extinction of the species, since if at a given moment the par ents were swept out of existence, the young, living under very different circumstances, would sur vive, develop, and represent the species. Again in the marhie species of Nvorms,erustacea.etc..the free-swimming young (larva') are borne about by oceanic and tidal currents, and in this way what in adult life are the most sedentary forms be come widely distributed from one part of the world to another. On the other hand, the larval forms of fixed marine animals serve as food for fishes, especially young fishes. and numerous invertebrates. Thus were it not for the meta morphoses of animals, many species would be come extinct sooner than they do. while the great overphis of larval forms gives to many other spe cies of animals a secure hold on existence.
As an example of metamorphosis we may cite that of a butterfly, fly, or bee. Their life is di vided into four stages, the embryo passed within the egg, the larva, pupa or chrysalis, and imago. _An insect after hatching lives, so to speak, three different lives. having distinct bodily st rue t u resand exist ing under very different conditions as regards food, enemies. etc. The caterpillar, for example, has big jaws, Nvideli in the winged or adult state are entirely want ing. Other radieal changes are observable in the body and appendages. and also in the inter nal organs. The term 'larva' (q.v.). as applied to the first stage of animals, is 0 very variable and indefinite one, that of insects in general be ing a ranch more highly organized animal than the larva of a worm, sta rfish. or crustacean. Wing less insects (synaptera) do not pass through a metamorphosis. That of winged insects is said to be 'incomplete' or`eomplete.' An example of incom plete metamorphosis is that of locusts and grass hoppers. In these insects the freshly hatched young differs from the adult Only in hying with out wings. The different stages of metamor phosis are not primitive. inherited from sonic early form, hut are acquired characters: the min plius stage of most erustamea, and the caterpillar. maggot. or grub of insects. are forms which were adaptations to changed modes of life, inducing use or disuse of certain organs. At first insects were ametabolous, and it was not until perhaps the middle of the Paleozoic era that insects with a metamorphosis began to exist.