Insect Metamorphosis

Metamorphosis is a term that comes from the Greek word metamorphoun, meaning ‘transform, change shape.’ The process of metamorphosis begins at birth and continues until the animal's death. Not only insects undergo these transformations; some amphibians, cnidarians, crustaceans, echinoderms, fishes, mollusks, and tunicates also undergo metamorphosis. There are two types of metamorphosis: partial (hemimetabolous) and complete (holometabolous). Animals that undergo no metamorphosis are called ametabolic.

Complete (Holometabolous) Metamorphosis = Four Stages

Complete metamorphosis consists of four stages: an egg stage, a larval stage (with several instars, or phases), a pupal stage, and an adult stage. During the larval stage, most species are worm-like in form. In order to grow and change form, larvae must molt, or shed, their skin. Upon the last (usually fifth) molt, the larva emerges from the old cuticle, called an exuviae. The different forms present in complete metamorphosis do not resemble each other and the larvae usually eat different foods than the adults.

Insects that undergo complete metamorphosis include:

alderflies, dobsonflies, and fishflies (Megaloptera)
ants, bees, sawflies, and wasps (Hymenoptera)
beetles (Coleoptera)
butterflies and moths (Lepidoptera)
caddisflies (Trichoptera)
fleas (Siphonaptera)
flies (Diptera)
lacewings and antlions (Neuroptera)
snakeflies (Raphidioptera)
twisted-winged parasites (Strepsiptera)

Above: The green lacewing undergoes complete metamorphosis: egg, larva, pupa, and adult.
Photos by Dinesh Valke, CC BY-SA 2.0); Katja Schulz, CC BY 2.0; Andrew Cannizzaro, CC BY 2.0; and Judy Gallagher, CC BY 2.0.

Partial (Hemimetabalous) Metamorphosis = Three Stages

Partial metamorphosis consists of three stages: an egg stage, a nymph stage (with several instars, or phases), and an adult stage. (Unlike complete metamorphosis, there is no resting, or pupal, stage.) In order to grow and change form, nymphs must molt, or shed, their skin. Upon the last (usually fifth) molt, the adult insect emerges from the old cuticle, called an exuviae. These insects may have wings, which if present, develop externally. Nymphs resemble adults and usually eat the same food, unlike insects that undergo complete metamorphosis.

Insects that undergo partial metamorphosis include:

aphids*, cicadas, leafhoppers, planthoppers, scale insects, and shield bugs (Hemiptera) [Note: a few aphid species undergo live birth (no egg stage) and thus have only two life stages - nymph and adult.]
cockroaches and termites (Blattodea)
crickets, grasshoppers, and katydids (Orthoptera)
damselflies and dragonflies (Odonata)
earwigs (Dermaptera)
praying mantises (Mantodea)
mayflies (Ephemeroptera)
silverfish (Zygentoma)

At left: The Scudder's bush katydid undergoes incomplete metamorphosis: egg, nymph, and adult. Photos by aecole2010, CC BY 2.0; Katja Schulz, CC BY 2.0 and Judy Gallagher, CC BY 2.0.