The wonderful world of caterpillars

Environmental writer Elizabeth Kolbert had an interesting article in the New Yorker about the little-known world of caterpillars and other insects. I have to admit, caterpillars had not figured much in my consciousness but was impressed to learn that they undergo a dizzying array of transformations in their very short lives.

From a caterpillar’s perspective, humans are boring. The young they squeeze out of their bodies are just miniature versions of themselves, with all the limbs and appendages they’ll ever have. As they mature, babies get bigger and stronger and hairier, but that’s about it.

Caterpillars, for their part, are continually reinventing themselves. They emerge from tiny, jewel-like eggs and for their first meal often eat their own egg cases. Once they reach a certain size, they sprout a second head, just behind the first. They then wriggle free of their old skin, the way a diver might wriggle out of a wetsuit. (In the process, the old head drops off.) In the course of their development, they will complete this exercise three, four, in some species sixteen times, often trying out a new look along the way. The spicebush swallowtail, for example, which is found throughout the eastern U.S., emerges from its egg mottled in black and white. This color scheme allows it to pass itself off as a bird dropping. After its third molt, as a so-called fourth instar, it turns green (or brown), with two yellow-and-black spots on its head. The spots, which look uncannily like a pair of eyes, enable the swallowtail to pretend it’s a snake.

After running through its allotment of instars, a caterpillar ceases to be itself and becomes a pupa. It sheds its skin one last time and develops a hardened shell. Inside this shell, its body dissolves. Then, from bundles of cells known as imaginal disks, a new body takes form. Some disks develop into legs, some wings, some genitalia, and so on. The creature that emerges retains almost nothing of its juvenile self except, weirdly, its memories.

As a way of life, this radical, whole-body transformation is ancient. It arose some three hundred and fifty million years ago, during the Carboniferous period. How, exactly, the process evolved is still debated, but it has proved wildly popular. Not only moths and butterflies undergo complete metamorphosis; so, too, do beetles, flies, wasps, fleas, and lacewings.

There were lots of other things that interested me, such as this one.

There are roughly sixty-five hundred species of mammals, nine thousand species of amphibians, and eleven thousand species of birds. These are what people tend to think of when they picture the world’s biodiversity. But the planet’s real diversity lies mostly beneath our regard. The largest family of beetles, the Curculionidae, commonly known as weevils, contains some sixty thousand described species; another beetle family, the Tenebrionidae, comprises twenty thousand species. It is estimated that in one family of parasitic wasps, the Ichneumonidae, there are nearly a hundred thousand species, which is more than there are of vertebrates of all kinds. (Ichneumonids inject their eggs into the larvae of other insects. Darwin adduced their existence as a powerful argument against intelligent design; he could not, he wrote, imagine a “beneficent and omnipotent God” purposely creating such a fiendish creature.) There are, in fact, so many insect species—at least two million and possibly as many as ten million—that Robert May, an Australian physicist turned theoretical ecologist, once joked, “To a good approximation, all species are insects!

In keeping with their variety, insects play a vital role in virtually every terrestrial ecosystem. Roughly three-quarters of the world’s flowering plants depend on insects for pollination. Insects are also crucial seed dispersers; many plants stud their seeds with tiny treats to entice ants to carry them off. And they’re key decomposers. (When a person dies, blowflies arrive on the scene within minutes; in warm weather, blowfly maggots can eat through most of a corpse within a week.)

Legions of other creatures, meanwhile, depend on insects for food. Insectivorous mammals include hedgehogs, shrews, and most species of bats. Just about all amphibians consume insects, as do many species of reptiles and freshwater fish. Lots of birds rely on insects, particularly during breeding season: before they fledge, a clutch of young chickadees will consume as many as six thousand caterpillars. Collectively, insects transfer more energy from plants to animals than any other group. They are the solder that holds food chains together.

I guess it makes sense that those species that have shorter life spans are likely to have more diversity in their species, since they have so many more opportunities to evolve from generation to generation.

I was also intrigued to learn that “Most caterpillars have twelve eyes, six on each side.” That really surprised me. I wondered why they might have evolved to have so many and how their brain processes so many images at once. This article explains that all these eyes just help with light detection but do not give great vision.

Another fascinating fact about caterpillars is that they have about a dozen eyes. On either side of a caterpillar’s head are 6 tiny eyelets known as stemmata. Some other species have 5 or 7 on each side.

One would expect that a creature with 12 eyes would enjoy incredibly excellent vision. Well, not so. The eyelets only help them distinguish light and dark. However, when it comes to identifying colors or just seeing stuff, caterpillars are naturally handicapped.

This article goes into more depth about caterpillars.


  1. machintelligence says

    To a first approximation all animals can fly.
    To a first approximation all animals are insects.

  2. Matt G says

    It bears keeping in mind that this is the same creator who has an inordinate fondness for beetles.

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