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The caterpillar of Proserpinus proserpina, an insect larva

A larva (Latin; plural larvae) is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle.

The larva's appearance is generally very different from the adult form (e.g. caterpillars and butterflies), and a larva often has unique structures and larval organs that do not occur in the adult form. A larva's diet can be considerably different from its adult form.

Larvae are frequently adapted to environments separate from adults. For example, some larvae such as tadpoles live exclusively in aquatic environments, but as adults can live outside water as frogs. By living in distinct environments, larvae may be given shelter from predators and reduce competition for resources with the adult population.

Animals in the larval stage will consume food to fuel their transition into the adult form. Some species such as barnacles are immobile as adults, and use their mobile larvae form to distribute themselves.

The larvae of some species can become pubescent and not further develop into the adult form (for example, in some newts). This is a type of neoteny.

Eurosta solidaginis Goldenrod Gall Fly larva

It is a misunderstanding that the larval form always reflects the group's evolutionary history. It could be the case, but often the larval stage has evolved secondarily, as in insects. In these cases the larval form might differ more from the group's common origin than the adult form.

Names of various kinds of larvae:

Animal Name of larva
Hydrozoa planula
Many crustaceans nauplius
Decapoda zoea
Mayflies, Grasshoppers, True Bugs, etc. nymph
Dragonflies, Damselflies naiad, nymph
Butterflies and moths caterpillar
Beetles, Bees, Wasps grub
Flies maggot
Mosquitos wriggler
Certain molluscs, annelids trochophore
Certain molluscs veliger
Freshwater mussels glochidium
Lamprey ammocoete
Fish (generally) larva
Eels leptocephalus
Amphibians tadpole, polliwog
Sea stars Bipinnaria

See also

External links


Simple English

File:Haliotis asinina
A trochophore larva

Many animals develop in separate stages. A larva (plural: larvae) develops from the egg in those animals. It is a separate life stage from the adult reproductive stage. A larva does not look like the adult animal, and changes shape (known as metamorphosis) as it grows up. There may be several larval stages before the adult form. Tadpoles, maggots and caterpillars are larvae.

Marine organisms with a larval stage often release large numbers of eggs and sperms into the water column. After fertilisation, the eggs develop into tiny larvae. The larvae develop and grow for some time before metamorphosing into adults. Most marine invertebrates and many fish have a pelagic larval stage or pelagic eggs. These live in the plankton, and may be transported long distances.

Evolutionary theory

Probably the most widely accepted theory explaining the evolution of larval stages is the need for dispersal. Sessile organisms such as barnacles and tunicates, and sea-floor groups like mussels and crabs, need some way to move their young into new territory, since they cannot move long distances as adults. Many species have relatively long pelagic larval stages (how long a larva is in the water column).[1][2] During this time, larvae feed and grow, and many species move through several stages of development. For example, most barnacles moult through six nauplius larva stages before moulting to a cipris, when they look to settle. The larvae eat different food from the adults, and disperse.

The other consideration is the small size of the eggs. If animals lay many small eggs (and most do), then the young stages cannot live the life the adults lead. They must live a separate life until they have the size and capability to live as an adult. This is what the larvae do.

Origin of vertebrates

A long-standing theory is that in transformed larvae of sea-squirts (tunicates) lies the origin of vertebrates. Tunicates are sessile, but their larvae are mobile, and have some features found in early vertebrates. The process of paedomorphosis, where juvenile features are retained in the adult, is the proposed mechanism.[3][4][5] Genome analysis does show that the tunicates are the closest living relatives of vertebrates.[6]

References

  1. Brothers E.B., D.M. Williams, and P.F. Sale. 1983. Length of larval life n twelve families of fishes at “One Tree Lagoon,” Great Barrier Reef, Australia. Marine Biol. 76: 319-324.
  2. Scheltema R.S. 1986. On dispersal and planktonic larvae of benthic invertebrates: an eclectic overview and summary of problems. Bull. Mar. Sci. 39: 290-322.
  3. Garstang, Walter 1894. Preliminary notes on a new theory of the phylogeny of the chordates [chordates are vertebrates + a few closely related invertebrates]. Zoologischer Anzeiger 17, p122.
  4. Garstang, Walter 1928. The morphology of the tunicata, and its bearing on the phylogeny of the Chordata. Quarterly Journal of Microscopical Science 72, p51.
  5. de Beer, Gavin 1951. Embryos and ancestors. 3rd ed, Oxfor, The evolution of chordates, p76.
  6. Delsuc, Frédéric et al 2006. Tunicates and not cephalochordates are the closest living relatives of vertebrates. Nature 439, 965-968








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