The other day my son in London sent me a rather good photo of a caterpillar: clearly a moth or butterfly caterpillar (I suspect the former), with a black head and a body in two shades of green, striped along the length of the body and a bit mottled too, and with hairy tufts, not great big ones, but clearly there, of a slightly orange colour.
To be honest, at first it looked a bit like the larva of a cabbage white, but it was on a twig, not a cabbage. Identifying what plant a caterpillar is on is a major step in identifying the caterpillar. On being told it was a beech tree, I was able fairly quickly to name it as the larva of the buff tip moth. There are huge numbers of caterpillars about through a British spring and summer; some are unmistakeable - the monstrous elephant hawk moth, for example, that you may find on a willow herb or a fuschia, or the little yellow and black mobile Wolves’ scarves that are cinnabar moth caterpillars, on groundsel or ragwort. Others are harder to spot, harder to identify.
But some caterpillars are found in such immense numbers you can’t ignore them. Processionary moth caterpillars will take over a complete hedgerow bush, covering the bush with protective silk. Large and small whites will decimate your cabbages or perhaps your nasturtiums. All caterpillars are basically eating machines, and - given a good supply of their food plant - they can increase in size very rapidly, shedding their skins on a regular basis as they do so. That in itself can make caterpillar identification difficult, as different caterpillar instars can vary in colour and pattern. When the caterpillar’s food plant is also yours, you have a problem!
Caterpillars, though, are also very nutritious food themselves, for birds especially but also for other small animals. They are fat and juicy - not much fun if one turns up in your salad, but ideal for a nest of growing blue tits. It’s probably just as well that such a lot get eaten, but obviously it’s not good for the caterpillar, and many have developed strategies for self preservation. I’ve already mentioned the elephant hawk moth caterpillar; this is one of our largest caterpillars, but it is able when approached to make itself look even larger and quite threatening, and that will see off most possible predators. And those stripy cinnabar moth caterpillars are dressed up in bright colours to advertise the fact that they are not good eating. As farmers know well, ragwort is a poisonous plant, and the caterpillars are able to absorb that poison. They’re not affected by it themselves, but instead are able to make use of it as a protection.
Other caterpillars have to rely on not being seen. They may be coloured to resemble the food plant on which they live, for example. Or there is the peppered moth, whose caterpillar is so like a twig you just don’t see it. Others may spin webs, as mentioned above, to protect themselves. Smooth caterpillars are nicer to eat than hairy ones, so the “woolly bears” that will grow into tiger moths are reasonably safe. And caterpillars may have urticating hairs, that release nasty chemicals. It’s good that some larvae at least get through, to appear as (often) beautiful flying adults!