Larva

LARVA (larva), a stage of postembryonic development in animals that possess transformation, or metamorphosis. In such animals, the developing embryo leaves the egg membrane in a state unlike the maternal organism and must undergo a certain transformation in order to produce an adult individual. Frequently, animals during metamorphosis successively pass through several different larval stages. For example, annelids pass through the following larval stages: blastula, gastrula, trochophore. Larvae are encountered in the development of the majority of animals, so that direct development without metamorphosis generally refers to rare phenomena. Development without larval stages takes place in ctenophores, a portion of roundworms and annelids, some higher crustaceans (e.g., the crayfish), the majority of arachnids, and higher vertebrates starting from reptiles. Larvae are especially widespread in those animals that lead a different lifestyle during the early stages of their postembryonic development than in the adult state. Thus, the larvae of many benthic crawling aquatic animals lead a swimming lifestyle; the larvae of some terrestrial animals, e.g., dragonflies, live in water, and so on. Larvae usually possess a number of morphological features that distinguish them from adult individuals and disappear at the end of metamorphosis. Such features and organs have received the special designation of larval or provisional, i.e., preliminary. Examples include the tracheal gills of dragonfly larvae, the external gills of frog tadpoles, and the like. Larval features may partly have an ancient origin and in such cases have great phylogenetic significance; for example, the gill slits of amphibian tadpoles indicate the origin of amphibians from fishes. Or these features may have a purely adaptive character, caused by the peculiarities of the larva's lifestyle, e.g., the long outgrowths of echinoderm larvae that help the larvae remain suspended in water. Very often not only the morphology but also the physiology of the larva differs radically from that of the adult animal; in butterfly larvae, the mouthparts are chewing, while in adults they are sucking; furthermore, larvae lack wings, antennae are undeveloped, the body is worm-like, there are significantly more legs than in the butterfly, and so on. The larvae of segmented animals have mostly an incomplete number of segments, and so on. The sex organs in larvae usually do not reach full development, excluding cases of paedogenesis, and therefore no external sexual differences are discovered in them. The duration of the larval period of life varies within very wide limits; there are cases, especially among insects, when this period exceeds the lifespan in the adult state many times over. V. Dogel.