What the species is and where it came from
Metamorphosis is a concept in biology: a process in which an animal undergoes a conspicuous and relatively abrupt change in its body structure as part of its development. Some insects, fish, amphibians, mollusks, crustaceans, cnidarians, echinoderms and tunicates undergo it, often along with a change in nutrition or behaviour. Organisms with larval stages undergo metamorphosis to reach the adult stage. The word comes from the Ancient Greek for transformation.
How it works, in terms someone would actually use
Species differ in how much changes. Some undergo complete metamorphosis, called holometaboly, between radically different stages. Others undergo incomplete metamorphosis, hemimetaboly, between somewhat similar stages. Some have none at all, called ametaboly.
In insects, hormones control growth and metamorphosis. Brain cells secrete PTTH, which activates the prothoracic glands to make ecdysone, a hormone that induces shedding of the exoskeleton. PTTH also prompts the corpora allata to make juvenile hormone, which prevents adult characteristics from developing at a moult. In holometabolous insects the moults between larval stages have a high level of juvenile hormone, the moult to the pupa has a low level, and the final moult has none. In chordates, metamorphosis is induced by iodothyronines, and in amphibians thyroxin stimulates it while prolactin counteracts it.
What it gets right
It accounts for some of the most striking changes in an animal’s life. A hemimetabolous insect is a nymph that resembles a small adult without wings, and external wing buds form in later stages. A holometabolous insect passes through a larva and an inactive pupa, called a chrysalis in butterflies, before it emerges as an adult. In frogs, the long spiral gut shortens, the mouth is resorbed, the gills disappear, a tongue forms and the eyes and legs grow quickly. All of this can happen in about a day. The tail is reabsorbed a few days later, because that needs a higher thyroxin concentration. Many flatfish begin symmetrical, with an eye on each side, and one eye later moves to join the other.
What it does not
Not every animal does it. Bristletails and silverfish are insects that still develop directly, with very little difference between larva and adult. Most caecilians do not undergo a frog-like one. The axolotl, a salamander, keeps its larval form and never becomes a terrestrial adult. The material also notes that the origin of complete metamorphosis is open: several hypotheses ask whether the stages of incomplete metamorphosis are the same in origin as the pupa.
What it changed
The earliest insects developed directly, and the evolution of metamorphosis is thought to have fuelled their dramatic radiation. Research from 2008 found that an adult Manduca sexta retains behaviour learned as a caterpillar. The ornate moth caterpillar, for its part, carries diet toxins through metamorphosis into adulthood, where they still protect against predators. A 2009 study found that temperature matters: individual insect species have thermal windows in which they can progress through their stages.
Who it is for, and who it is not
It is for anyone who wants to understand how a tadpole becomes a frog or a larva becomes an adult insect. It is not for anyone who wants a single rule, since the material shows different hormones, different timings and different outcomes across groups.
Is it worth your time
Yes. Metamorphosis is how one animal can look like two, a tadpole and a frog or a larva and an adult insect. The material also shows that each group does it in its own way.
