Growing pains

A tiny, dense cloud whizzed across the hillside and resolved itself into a flock of starlings - around 100 of them, flying in a tight bunch in mobbing pursuit of a kestrel. The kestrel backed off and hovered in a furious flicker of wings. The starlings became a coiling snake, a rolling wave, a cannonball, and bowled themselves at the falcon. It backed off again, hesitated and hovered, then shrugged away up the hill.

The starlings were juveniles, fresh out of the nest, still drably brown but fully charged with instincts for the shape of a predator and how you organise to mob it. They are part of a wider pattern of dispersal of life from the egg, to be found among many species.

In the greenhouse, half-sunken in the ground on the stormy side of the house, baby frogs that could sit on my thumbnail have found the door open, descended the steps, and now skitter about beneath my feet. They, too, are in dispersal - from the overgrown marsh, a few metres away, that started spring as the garden pond. They, too, began as eggs.

Over the next couple of months we shall see caterpillars of various kinds in unexpected places: looping along the clothes-line, perhaps, or trundling up the wall of the house. They will have finished feeding and become restless, wandering as far as 100 metres from the food plant where they hatched from eggs.

It is a prelude to pupation: from now until autumn, check under window sills for their chrysalises, if the blue tits haven't picked them off first.

How do you feel about caterpillars? Most of us have held out a hand to them, slightly thrilled by the velvety procession of feet across the skin. I should, perhaps, have asked that differently: how, if at all, do you conceptualise caterpillars? Do you see them as a kind of primitive precursor to the butterfly or moth? Do you imagine a life-cycle of growth and development, from immaturity, through metamorphosis, to a higher, winged form?

Early in the last century, when the prime purpose of insects seemed to be to provide metaphors for the purple prose of clerical nature writing, the life-cycle of a butterfly was a favourite analogy for the passage of a soul: from its mortal carcass (the caterpillar), through death and entombment (the pupal chrysalis) to a transformed, winged resurrection.

Even in less exalted times, aesthetics have helped to sustain the idea that insect larvae, such as caterpillars, are immature creatures on their way to perfection. We can suppose this without knowing that "larva" derives from the Latin for "mask", or that scientists talk about the imago, the Latin word chosen early for the winged stage, meaning the image, or essential form, of the species. Linnaeus picked both names, plus "pupa", for the chrysalis (in the crisp sculpture of which may be imagined a girl-child, or doll, wrapped in swaddling clothes).

What we do suppose, generally, is that the is a finished insect in a way that the larva is not. But the caterpillar's job is to survive and eat and the butterfly's goal is to mate and lay eggs. The butterfly is adult in the sense of being old enough to die after reproduction - often in as little as three weeks.

The best-kept secret in the process is that a butterfly or moth - the - is not reconstructed or reorganised from the organs and muscles of the caterpillar. It is built from scratch within the chrysalis by the activity of isolated patches of cells - "imaginal discs" - working in a biochemical soup (any gardener who has squashed a glossy, chestnut-brown chrysalis of a moth, maturing under the soil, will know the soup's creamy colour).

The final emergence of the fully-formed and the slow unfurling of its silken wings, has much more meaning for the human spirit than a mere systems triumph for DNA. Recognising the caterpillar's virtually separate and equal existence sets us free to accord it proper respect.

In her glorious essay in Pilgrim at Tinker Creek, Annie Dillard writes of the hazards of getting too excited about the exuberance and extravagance of evolution: "I am horribly apt to approach some innocent at a gathering and, like the ancient mariner, fix him with a wild glittering eye and say, `Do you know that in the head of the caterpillar of the ordinary goat moth there are 228 separate muscles?' The poor wretch flees. I am not making chatter; I mean to change his life."

A caterpillar does, indeed, need thousands of muscles, most of them to hold its soft body in shape by keeping up a steady pressure on the blood. It uses blood-pressure to help other muscles move its legs, a bit like the mechanisms stimulated by Viagra - and there's a simile which could make for a good party line.

I could go on to tell about the puss-moth caterpillar, to be found in many ordinary sally bushes on the bog or at roadside ditches from now until September. Even in an equable mood it is quite a substantial insect - about 6 cm long, plump and bright green, with a sort of purple saddle. When disturbed, it uses its blood-pressure to rear up like a Macnas monster. Its head is swollen and terrifying, suddenly rimmed in red and with two black spots for "eyes". If that doesn't scare its attacker, it conjures two wicked looking scarlet filaments from prongs at the rear of its body and whirls them above its head. In the most desperate circumstances - for an enemy, say, like the dreaded ichneumon wasp, trying to lay eggs on its back - it squirts out a spray containing 40 per cent formic acid.

Now, may I get you a refill?