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All animals change as they get older. Typically these changes are slow and subtle, but many animals famously undergo sudden and dramatic changes from young to adult – caterpillars to butterflies; fry to fish; or tadpoles to frogs. In this episode, we discuss the diversity and history of the incredible phenomenon called Metamorphosis.
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Metamorphosis
Metamorphosis is extremely diverse and – like so many things in nature – can be difficult to define, but the general idea is a life cycle organized into distinct phases separated by rapid transitions. These transitions often include shifts in the animal’s behavior, habitat, and even diet. Perhaps the most famous metamorphosing animals are insects and frogs, but it’s also commonly seen in other amphibians, fish, and crustaceans.
Insects are also the best-studied of these groups, and researchers have identified three classic categories of metamorphosis among insects:
Holometaboly refers to “complete” metamorphosis. Holometabolous insects include butterflies, flies, wasps, and beetles, among others. These insects typically start life as wriggly larvae that focus their energy on eating and growing before entering a pupal phase, an inactive period during which they metamorphose into their adult form. As adults, they’re usually winged and ready to mate.
As we mentioned in the episode, some research suggests these insects may even retain memories from their time as a larva.
Hemimetaboly refers to “incomplete” or “partial” metamorphosis. This is seen, for example, in cockroaches, grasshoppers, and dragonflies. These spend their early days (or years) as nymphs or naiads, physically similar to adults but lacking special features like wings and reproductive organs until they metamorphose.
Ametaboly is a term used for insects that don’t metamorphose at all, changing only slightly and gradually as they grow (like us). These are the most basal of insects like silverfish and bristletails, and it’s thought that the earliest insects did not metamorphose.
Among other animals, crustaceans often start life as tiny plankton that disperse across the sea before transitioning into adults, and many amphibians from frogs to salamanders start life as aquatic larvae that metamorphose into terrestrial adults. These groups, too, show lots of variety.
Metamorphosis is also common in fish, whose larvae are often called fry. Fish that migrate between salt and fresh water can have especially complicated life cycles, such as salmon and the European eel.
Fossils and Evolution
Fossil evidence of metamorphosis goes back to some of the earliest members of arthropods and chordates. Larvae, being generally soft and squishy, don’t always fossilize well, but some incredible examples have been discovered:
520-Million-Year-Old Fossil Larva Preserved in 3D
A 100-million-year old predator: a fossil neuropteran larva with unusually elongated mouthparts
Bizarre Jurassic blood sucking fly larva
Parasites discovered in fossil fly pupae
Earliest-known lamprey larva fossils unearthed in Inner Mongolia
Metamorphosis appears to have evolved multiple times, but exactly how, when, and why are still open questions, though research has found evidence that – among insects at least – “complete” metamorphosis evolved from “incomplete” metamorphosis. There have also been some more creative takes on the subject.
One of the major factors in the incredible success of this growth strategy might be how it separates young from old. Larvae and adults that live in different habitats eating different foods can exist alongside each other without competing for resources, and with larvae focusing their efforts on growth and eating, adults can concentrate on finding mates and reproducing.
If you enjoyed this topic and want more like it, check out these related episodes:
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COMMON DESCENT 
I realize that egg-laying is a far more common method of reproduction than live birth, but is it exclusive to oviparous animals or does it occur in ovoviviparous animals, too, possibly pre-birth?
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Some animals classified as ovoviviparous (a term that isn’t used as much these days, largely since there’s just so much diversity in birth methods) seem to form an egg – or parts of the egg, like a yolk – inside the body to nourish the embryo before giving birth to live young.
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