LARVA (Lat., ghost, mask, given to the caterpillar because it was thought by the ancients to mask the form of the perfect insect or butter fly). When animals become free from the parent tissue or from the egg, they either have the form, if not the size, of the adult, or they differ very markedly from the adults both in form and structure. and spend more or less time as free individuals before maturity is gained. The lat ter method of development we denominate larval, in contrast to the former, which is foetal. Larval development may take place either slowly and by gradual stages, or by abrupt changes which we call metamorphosis (q.v.). The development of both the embryo and the larva is a series of onto genetic recapitulations of stages or conditions which the animal has passed through in its phylo genetic history. Secondary changes may fre quently have come in to complicate and obscure the phylogenetic inheritance. Such ehanges are brought about by variations in the larval, pupal, or adult stage. which prove of advantage to the organism and hence are perpetuated by inheri tance. A certain kind of secondary change— namely. shortening of development—is more pos sible in fictal than in larval development. Many organs that must be retained in larval develop ment, heeause useful for the individual during some stage in its free life, can be dropped alto gether in the foetus, because it is so well protect ed within the maternal body. Possibilities of variations are, however, greater in larval than in firtal development. Indeed. larva, such as those of many inseets, may exist with secondary characters only. This tendency to acquire see ondary characters is in a measure held in check by the necessity of retaining ancestral organs which are functionally useful to the larva at some stage in its transformations.
In order to allow every organ in its develop ment to repeat even in an abbreviated way its phylogenctie history. there is a tendency to put off the differentiation of the tissues into their definitive organs as long as possible. Thus when an organ is hatched certain organs are as yet wholly undifferentiated, certain others, such as muscles and nerves, arc histologically differentiated. The shorter the larval period the earlier certain organs must appear. and this fact sometimes necessitates their development out of their phylogenctie order, and hence obscures the ancestral history. :Nlany of the secondary modi fications which are produced in larva' are for the purpose of adapting the larva- to escape from their enemies; such are transparency, protective coloration, or nettling organs, and many of the spinous processes on larval crustaceans and fishes. l‘lost of the marine larva- are free-swim ming, and arc well provided with organs of locomotion for the purpose of scattering the larvae over a wide region and thus preventing undue crowding of the adults by enlarging their area of distribution.
Larval stages exist in the development of cer tain species in all the groups from Infusoria up to Amphibia. A number of these larvae differ so
much in form and habitat from the adult that their nature was often unsuspected by earlier observers, and they were given generic and spe cific names. This is the case with the zoom of crabs, the alima, erichthus, and squilleriehthus of the Squilla, the pilidium of certain neater tinea (q.v.) (at first named Pilidium gyrans), the seolex of the tapeworm, and the axolotl stage of the Amblystoma. Some of the larval forms, such as those of diplopod myriapods, crustaceans, and chordates, have few or no affinities to other than their own groups. There are a goodly num ber of larva', such as the cercaria of trematodes, about which we cannot make statements as yet, and there are a great many larva' that possess affinities with two or more groups. These stages or affinities we consider to be the representatives of a common ancestry. Thus the planula. with its simple double-layered and bilateral structure. is probably the ancestral form of the Oriente rata. The common ancestor of the vertebrates seems to have been a hydra-like organism, double layered, and with a central digestive cavity and with only one opening. In the ontogenetie de velopment of vertebrates this stage is represented by the gastrula.
The larva of insects, to which at first the term alone referred, differ very much in the degree of their developnwnt, depending on the order to which they belong. Some are almost like the adult, except for the lack of wings. as in the ease of the Orthoptera, and others are legless lame depending entirely for food on the provi sion made for them by their parent. The larva of beetles we call 'grubs? those of flies 'maggots; and of butterflies and moths 'caterpillars.' In general, the larva' of insects may be divided into two types, the campodcaform of Brauer and the cruciform of Packard. the latter being applied to the more or less worm-like, secondary larva', such as caterpillars and the maggots of flies, and of ants, wasps, and bees. The campodea - form larva is so called from its resemblance to the stem-form of insects, campodea; such are the nymphs of the white ant, dragon-fly, ephemera, etc. Active and voracious larve store up little reserve material; but the forms that 'awaits, such as the grubs, maggots. and caterpillars, store up a large amount of fat. The larval forms of certain species of flies are capable of reproduc ing their kind before they have reached maturity in all other respects. See PARTHENookxEsts, paragraph Pwilogencsis.
BIBLIOGRAPHY. Balfour, Comparative EmbryBibliography. Balfour, Comparative Embry- ology (London. 1880-81I ; Fritz Pacts for Darwin (London, ISM)) ; Packard. Text-Book of Entomology (New York, 189S) ; with the writing, of J, .711filler, Brauer, Chills. De Geer, Dohrn, Lyonnet, Faxon. llrooks, Riley, Hyatt, J. V. Thompson, and others.