One hormone keeps caterpillars young; when it vanishes, the butterfly emerges
Whether an insect moults into a bigger larva, a pupa or an adult comes down to a single chemical, juvenile hormone. Plenty of it during a moult keeps the insect juvenile; a little triggers pupation; none at all produces the adult. Metamorphosis itself may be why insects became so astonishingly diverse.
Metamorphosis is an abrupt, conspicuous change in body structure during an animal's development, often paired with a change in diet or behaviour. It is found in some insects, fish, amphibians, molluscs, crustaceans, cnidarians, echinoderms and tunicates. The word comes from Greek roots meaning after and form. In chordates it is driven by thyroid-type hormones and is thought to be an ancestral trait of the whole group.
Insects show all three possible patterns. The earliest insects developed directly, with young looking much like adults, and a few such lineages survive, including silverfish and bristletails. Cockroaches, grasshoppers, dragonflies and true bugs have incomplete metamorphosis: their young, called nymphs, resemble small adults without wings or genitalia and grow through a series of moults called instars, sprouting wing buds late on. Complete metamorphosis adds a resting pupal stage, called a chrysalis in butterflies, between a larva that looks nothing like the adult and the final winged form.
The hormonal machinery starts in the brain. Nerve cells release prothoracicotropic hormone, which activates glands that produce ecdysone, the signal to shed the exoskeleton. The same brain hormone prompts a pair of organs behind the brain, the corpora allata, to make juvenile hormone, which blocks adult features from developing. Experiments on firebugs showed juvenile hormone can even change the number of nymph stages.
Evolving metamorphosis is thought to have fuelled the insects' dramatic radiation. How complete metamorphosis arose from the incomplete kind is still debated, largely over whether the pupa corresponds to some intermediate stage in insects without one. Temperature matters too: a 2009 study found that each species has a specific thermal window for passing through its stages, shaped by its evolutionary history rather than its ecological traits.
Source: Metamorphosis
