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Larva

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A larva is a distinct juvenile form that many animals undergo before metamorphosis. Metamorphosis is the process of transforming from one life stage into another. Animals that use this method are said to have indirect development. This life cycle stage is common in insects, some arachnids, amphibians, and cnidarians. A larva usually looks very different from its adult form. It may possess unique structures or organs that the adult does not have.

Papilio xuthus Larva 2011-10-15
Papilio xuthus Larva 2011-10-15

The primary purpose of the larval stage is to consume food. This energy fuels the transition into the adult form. Larvae often live in different environments than the adults. For example, some tadpoles live almost exclusively in water. However, adult frogs can live on land. This separation provides two major advantages for the species. It offers the larvae shelter from predators. It also reduces competition for food between the larvae and the adults.

In some species, the larval stage is used for dispersal. This means the larvae move to spread the population to new areas. In organisms like barnacles and polychaetes, the adults are immobile. However, their larvae are mobile and can swim. These dispersing larvae are categorized into two types. Planktotrophic larvae are those that feed on food. Lecithotrophic larvae are non-feeding and rely on stored nutrients.

Eurosta solidaginis larva
Eurosta solidaginis larva

Some larvae rely on adults for survival. In many eusocial Hymenoptera, which include certain ants and bees, female workers feed the larvae. In the paper wasp species Ropalidia marginata, males can also feed the larvae. However, the males are much less efficient. They spend more time and receive less food while feeding the young.

Hercules beetle (larva)
Hercules beetle (larva)

In the world of insects, only Endopterygotes show complete metamorphosis. This means they have a very distinct larval stage. In 1913, Antonio Berlese created a classification for these larvae. He grouped them based on their physical structures. One group is called apodous larvae, which have no legs at all. These are often poorly sclerotized, meaning their outer skin is not hard.

Micromus.aphids.2
Micromus.aphids.2

Berlese also identified other specific types of insect larvae. Protopod larvae often look very different from the adult insect. They usually hatch from eggs that contain very little yolk. Polypod larvae, such as caterpillars, have abdominal prolegs. These are extra legs on the belly in addition to the usual thoracic legs. Finally, oligopod larvae have six legs and well-developed mouthparts. These mouthparts are similar to the adult, but they lack compound eyes. These larvae can be campodeiform, which are long-legged predators, or scarabaeiform, which are burrowing forms.

There are many specialized names for larvae across different groups. In the insect world, we call butterfly larvae caterpillars and fly larvae maggots. Mosquito larvae are often called wrigglers. In the ocean, many crustaceans have larval stages called nauplius or zoea. Even fish have a larval stage known as ichthyoplankton.

Micromus.aphids.2
Micromus.aphids.2

Understanding larvae helps scientists study evolutionary history and ecology. It was once thought that the larval form always reflected an animal's evolutionary past. However, this is not always true. In many insects, the larval stage evolved secondarily. This means the larva may actually differ more from the common ancestor than the adult does. Some organisms even exhibit neoteny. This occurs when larvae, such as certain newt species, become pubescent and do not develop into adults. This shows how diverse and complex the journey from juvenile to adult can be.

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File:Papilio xuthus Larva 2011-10-15.jpg
Papilio xuthus Larva 2011-10-15.jpg
File:Eurosta solidaginis larva.jpg
Eurosta solidaginis larva.jpg
File:Hercules beetle (larva).jpg
Hercules beetle (larva).jpg
File:Micromus.aphids.2.jpg
Micromus.aphids.2.jpg
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