Larva, << LAHR vuh, >> is an active, immature stage of an animal. It differs from the adult in such characteristics as structure, behavior, food habits, and environment. An animal’s development through egg, larval, and often pupal stages to the adult is called metamorphosis.
Larvae occur in the metamorphoses of many groups of animals. They are especially common in the insects and in animals that live in the water. The larva of the sponge is a tiny, oval creature that swims about by means of short, hairlike cilia. Eventually, it attaches itself to a solid object, and then develops into an almost immobile, adult sponge. Flukes, tapeworms, and roundworms usually have one or two parasitic larval stages.
Marine annelid worms have larvae called trochophores that swim about by means of cilia. Many mollusks, including clams, oysters, scallops, snails, and periwinkles, have a free-swimming larva called a veliger. Some also have another larval stage, a trochophore similar to that of the annelids. The larvae of freshwater mussels, called glochidia, fasten themselves to the gills or skin of fishes and ride about on them. Then they drop off and burrow in the bottom. Many crustaceans, including lobsters and crabs, have active, free-swimming, large-eyed larvae. Barnacles have distinct larval stages called nauplii and cyprids. These larvae swim and drift great distances before fastening themselves to solid objects and becoming adults. Larvae of many water animals are important food for fishes. They float about as part of the mass of tiny, drifting aquatic organisms called plankton (see Plankton).
Most insects have larvae more or less distinct from the adults. Larvae that undergo incomplete metamorphosis change directly to the adult stage. Such larvae are often called naiads if they belong to water insects. If they belong to land insects, they are called nymphs. The caterpillar of a moth or butterfly, the grub of a beetle, the hellgrammite of a dobsonfly, and the maggot of a fly are insect larvae with complete metamorphosis. Such insects pass through a pupal stage before becoming adults. Among the vertebrates, or animals with backbones, many fishes have distinct larvae. For example, eels have ribbon-shaped transparent larvae. Frogs, which are amphibians, have larvae called tadpoles or polliwogs.
