Many of us have collected caterpillars at least once in our lives, and watched them transform into adult butterflies. It is such a common childhood experience that we take it for granted. But nature offers few phenomena as bizarre as metamorphosis.
My daughters have kept a monarch (Danaus plexippus) caterpillar in a jar all week, once or twice providing a fresh branch of milkweed, the insect's preferred food. Last night it began to spin a nearly invisible pad of silk and hang upside down from its back end. This morning, within a matter of minutes, we watched it shrug off the skin with its garish stripes and become a glistening, helpless chrysalis, pale green delicately jeweled with gold.
Four organisms in one
There could scarcely be four creatures as dissimilar as the stages of a butterfly's life cycle: the hard and tiny egg, the larval eating machine, the almost featureless pupa, and the migratory adult with its capacity for flight and taste for nectar. And yet such a complex existence is common among living organisms.We are so used to the relatively large and obvious vertebrate animals which go through most of their development within the egg or, in mammals' case, inside the uterus.
Mammalian simplicity
The other day at a small city zoo we saw four harbor seals (Phoca vitulina) sprawling in the sun. A sign near their pool said the two youngsters had been born less than two weeks earlier. The facts took a minute to sink into my mind: that the two seals which were only slightly larger, not 50 per cent heavier, were the mothers. Like many other familiar animals-ducks, horses, snakes-these babies have scarcely left their embryonic form yet are practically ready to join their parents' lifestyles.Most of Earth's creatures don't grow this simply.
Variations on a larva
Many of the most successful insects share the complete four-staged metamorphosis of butterflies and moths: notably beetles, flies, bees, wasps, ants, and mosquitoes. Others go through slightly simpler incomplete metamorphosis: cockroaches, grasshoppers, dragonflies and damselflies.Echinoderms (the phylum containing starfish, sea urchins and sea cucumbers) have their own approach to the life cycle. The larvae typically go through two stages, called bipinnaria and brachiolaria. They are planktonic and exhibit the bilateral symmetry (having a distinct left and right side) typical of most animals. At metamorphosis, they settle to the sea floor and adopt their distinctive five-part or radial symmetry.
Molluscs (including clams, snails, slugs, squid, octopi, limpets and other organisms) have their own
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