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Two lives, one animal

Why Do Some Animals Grow a Second Body?

A caterpillar and a butterfly are the same animal wearing two bodies that share almost nothing. Building the second one from scratch seems mad — until you ask what each is for.

A cricket Hatches as a smallversion of the adultA butterfly Hatches as somethingwith nothing incommon
Two ways of growing up, and insects use both.

  1. Step 1 of 5

    Two bodies, because there are two jobs.

    One animal, twodifferent jobsGrowing wants a soft bag of legsBreeding wants wings and eyes
    No one body is excellent at both, so it builds each in turn

    Eating and growing wants a soft bag with legs that never has to go anywhere. Finding a mate wants wings, eyes and no interest in food at all. One body cannot be excellent at both, so some animals build each one separately.

  2. Step 2 of 5

    About half of insects do it gently.

    1 An egg hatchesinto a small copyof the adult2 It moults,slightly largereach time3 Wings grow on theoutside all theway through

    A cricket, a grasshopper or a dragonfly hatches as a nymph, which is roughly the adult in miniature. It moults through a series of larger copies of itself, growing its wings on the outside as it goes.

  3. Step 3 of 5

    The other half take the whole thing apart.

    Insects that moult graduallyBugs, crickets,dragonfliesInsects that rebuild entirelyBeetles, flies,bees, moths
    Which makes rebuilding the ordinary way to be an animal.

    Beetles, flies, bees, ants, moths and butterflies seal themselves away and rebuild almost from scratch. It sounds like the rare, exotic option, and it is the opposite — most insect species alive do it this way, and most animal species are insects.

  4. Step 4 of 5

    A tadpole rebuilds too, and it is not an insect.

    1 Gills, a tail,and a long gutfor algae2 Lungs, legs,and a short gutfor insects3 The tail isabsorbed, notshed

    A frog swaps gills for lungs, grows legs, and rewires a long plant-eater's gut into a short meat-eater's one. The tail is not dropped off — it is dismantled and the material is used to build the rest.

  5. Step 5 of 5

    The real prize is never competing with your own young.

    CaterpillarEatsleavesButterflyDrinks nectarTadpoleGrazesalgaeFrogSnaps up flies
    So a butterfly can lay a hundred eggs on a plant it will never eat itself.

    A caterpillar eats leaves and a butterfly drinks nectar. A tadpole eats algae and a frog eats flies. Parent and offspring live in the same place and never once want the same meal, which means a good year for one is not a bad year for the other.

    The axolotl simply refuses. It breeds while still wearing its larval body, gills and all, and never grows the second one at all.

The short version

An animal grows a second body when eating and breeding need different machines — and the bonus is that its young never compete with it for food.

Try it yourself

Look for a shed insect skin on a plant stem. A split down the back means a nymph climbed out and moulted rather than rebuilding.

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