Assigment: Entomology

Insect development and metamorphosis

Metamorphosis

Each time an insect molts, it gets a little larger. It may also change physically in other ways -- depending on its type of metamorphosis: ametabola, hemimetabola, or holometabola.

Ametabolous insects undergo little or no structural change as they grow older. Immatures are called young; they are physically similar to adults in every way except size and sexual maturity. Other than size, there is no external manifestation of their age or reproductive state.

Hemimetabolous insects exhibit gradual changes in body form during morphogenesis. Immatures are called nymphs or, if aquatic, naiads. Maturation of wings, external genitalia, and other adult structures occurs in small steps from molt to molt. Wings may be completely absent during the first instar, appear in the second or third instar as short wing buds, and grow with each molt until they are fully developed and functional in the adult stage. Developmental changes that occur during gradual metamorphosis are usually visible externally as the insect grows, but adults retain the same organs and appendages as nymphs (eyes, legs, mouthparts, etc.).

Holometabolous insects have immature forms (larvae) that are very different from adults. Larvae are "feeding machines", adapted mostly for consuming food and growing in size. They become larger at each molt but do not acquire any adult-like characteristics. When fully grown, larvae molt to an immobile pupal stage and undergo a complete transformation. Larval organs and appendages are broken down (digested internally) and replaced with new adult structures that grow from imaginal discs, clusters of undifferentiated (embryonic) tissue that form during embryogenesis but remain dormant throughout the larval instars. The adult stage, which usually bears wings, is mainly adapted for dispersal and reproduction.

Most larvae can be grouped into one of five categories based on physical appearance:

Appearance	Larval Type	Common Name	Description	Examples
	Eruciform	Caterpillar	Body cylindrical with short thoracic legs and 2-10 pairs of fleshy abdominal prolegs	Moths and butterflies
	Campodeiform	Crawler	Elongated, flattened body with prominent antennae and/or cerci. Thoracic legs adapted for running	Lady beetle, lacewing
	Scarabaeiform	White grub	Body robust and "C"-shaped with no abdominal prolegs and short thoracic legs	June beetle, dung beetle
	Elateriform	Wireworm	Body long, smooth, and cylindrical with hard exoskeleton and very short thoracic legs	Click beetle, Flour beetle
	Vermiform	Maggot	Body fleshy, worm-like. No head capsule or walking legs	House fly, flesh fly

Pupae can be grouped into one of three categories based on physical appearance:

Appearance	Pupal Type	Common Name	Description	Examples
	Obtect	Chrysalis	Developing appendages (antennae, wings, legs, etc.) held tightly against the body by a shell-like casing. Often found enclosed within a silken cocoon.	Butterflies and moths
	Exarate	None	All developing appendages free and visible externally	Beetles, Lacewings
	Coarctate	Puparium	Body encased within the hard exoskeleton of the next-to-last larval instar	Flies