Larva - larva

This refers to the period from when an insect hatches from an egg until it becomes a pupa or an adult (parent), and corresponds to the larvae of animals in general. In general, insect embryos develop inside eggs, and when the larvae are formed, they break the eggshell and come out. From this point onwards, they are in the larval stage, where they eat food and grow. This is what is meant by postembryonic development. Larvae molt several times as they grow, and the period up to the first molt is called the first instar, the period after the molt is called the second instar, and the final larval stage is called the final instar. In insects that undergo complete metamorphosis, the larvae molt for the last time and become pupae, but the period just before this, when they stop eating and become sluggish, is sometimes called prepupa. In insects that undergo incomplete metamorphosis, there is no pupal stage, and they become adults after the final molt; however, the difference in morphology between the larvae and adults is not that significant; in the latter half of the larval stage, wing primordia (buds) appear on both sides of the rear of the thorax and back, and they grow larger with each molt, so strictly speaking they are called nymphs to distinguish them from larvae in complete metamorphosis.

The larvae hatch at different developmental stages depending on the species, and take shapes adapted to their lifestyle, resulting in a wide variety of body shapes, but they can be broadly categorized into the following four types: (1) Polypodial: Caterpillars, green caterpillars, and hairy caterpillars have abdominal legs in addition to thoracic legs (Lepidoptera, sawflies, etc.). (2) Paucipodial: Long-horned and maggot-shaped types have only thoracic legs and no abdominal legs (Boleoptera, Neuroptera, etc.). (3) Apodial: Maggot-shaped types have no legs (Hymenoptera, Dipterans, etc.). (4) Protopodial: Embryonic type (some parasitic wasps).

Larvae of the same species usually do not change significantly in shape as they grow, but some, such as the ground beetle and the giant leaf beetle, change shape dramatically every time they molt, and are known as hypermetamorphic. Many aquatic larvae have tracheal gills on both sides of their bodies and at the end of their tails, and some even have a respiratory tube at the end of their tails.

[Takehiko Nakane]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

昆虫類が卵から孵化(ふか)してから蛹(さなぎ)または成虫（親）になるまでの子供の期間をいい、動物一般の幼生にあたる。昆虫は一般に卵内で胚(はい)が発育し、幼虫体ができあがると、卵殻を破って外に出てくるが、この時点以後が幼虫期であって、餌(えさ)を食べて成長する。後胚子(こうはいし)発生というのはこのことである。幼虫は成長する途中で何回か脱皮をするが、第1回の脱皮までが第一齢、脱皮後が第二齢、最後の幼虫期が終齢とよばれる。完全変態の昆虫では幼虫は最後の脱皮をして蛹になるが、その直前の、摂食をやめて動作が鈍くなった時期を前蛹(ぜんよう)prepupaとよぶことがある。不完全変態の昆虫では蛹の時期がなく、最後の脱皮で成虫になるが、幼虫と成虫の形態の差異がそれほど著しくなく、幼虫期後半になると胸背部後方の両側にはねの原基（芽）が現れ、脱皮するごとに大きくなるので、厳密にいうときは完全変態の場合の幼虫と区別して若虫(わかむし)（ニンフnymph）とよぶ。

　幼虫は種類によって異なった発生段階で孵化し、かなり生活に適応した形をとるため、体形も多様であるが、大別して次の四型がある。(1)多脚型　イモムシ、アオムシ、毛虫のように胸脚のほかに腹脚がある（鱗翅(りんし)類、ハバチ類など）。(2)少脚型　胸脚のみで腹脚がないナガコムシ型とジムシ型（甲虫類、脈翅類など）。(3)無脚型　ウジムシ型のように脚(あし)がない（膜翅類、双翅類など）。(4)原脚型　胚子型（一部の寄生バチ）。

　同じ種類の幼虫は、成長の過程で普通、体形に著しい変化がないが、なかには脱皮するたびに体形が著しく変わるツチハンミョウやオオハナノミのようなものもあり、過変態とよばれる。また、水生の幼虫には体の両側や尾端に気管鰓(さい)をもつものが少なくなく、尾端に呼吸管をもつものもある。

［中根猛彦］

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