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.
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 |
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.
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.
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]
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