Knotgrass (Acronicta rumicis)

Dear Readers, I have always loved caterpillars. When I was a youngster, living in a tiny house in Stratford, the garden was full of the woolly bear larvae of tiger moths and yellow and black-striped cinnabar moth caterpillars. Once, I even found a spitting puss moth caterpillar. But they seem rarer these days, or maybe I just don’t pay as much attention as I used to. Anyway, this wonderful chap had been found in my friend A’s garden, and she had brought ‘him’ round for me to try to identify.

As it turns out, this is the caterpillar of the Knotgrass moth, a member of the Noctuid family of small, greyish-brown moths that make up 70% of all my finds if I put out a moth trap. The caterpillars are spectacular creatures, usually hairy, often brightly coloured. It seems almost a shame from a human point of view that they end up being so inconspicuous, but of course this works well for the moth. The hairs on the caterpillars are an irritant for predators (and also sensitive humans) but the moth has no defence except camouflage.

Knotgrass moth

Caterpillars are curious creatures. The name literally means ‘hairy cat’ (from the Old French caterpelose’) although not all caterpillars are hairy. As any one who has ever looked after a caterpillar will tell you, they live to eat, and are perfectly adapted for the job. They have three pairs of ‘thoracic legs’ at the front, which they use for grasping the leaves that they eat: these are the ‘true legs’.Then there are a further five pairs of sucker-like legs, called prolegs,  for hanging on to things – if you hold a caterpillar you will notice how hard they are to remove from your skin.

Although the head looks as if it has two enormous eyes, these are actually what’s known as the head capsule. The true eyes are tiny spots underneath. A caterpillar also has two tiny antennae underneath the eyes, and two very fine jaws for munching.

Caterpillars need to breathe, but they have no lungs – spiracles, tiny openings along the side of the animal, enable oxygen to reach the tissues of the animal directly, without the need for a complicated blood system. Caterpillars have no veins or arteries, but their organs, such as they are, are bathed in green ‘blood’ called hemolymph – the fluid itself is clear, but may be pigmented green by the caterpillar’s food.

You can see the spiracles of ‘our’ caterpillar in the photo below: the spiracles are the tiny white spots. Behind each one are a series of branching tunnels called tracheoles, which deliver the oxygen into the hemolymph and hence to the rest of the body, including the 4000 muscles (compared to a human’s 629) and the digestive organs.

This simple body system means that there are first aid measures that work well on insects in general, and caterpillars in particular. If a caterpillar tumbles into a water butt, for example, rolling it in kitchen paper to pull the water out of the spiracles can often effect a near miraculous recovery. Similarly, if a caterpillar is slightly punctured due to some unfortunate accident, a tiny piece of sticking plaster may preserve it until its next change of skin.When I was a child I planned to write a book called ‘The Caterpillar Hospital’, but feared that it might not have many takers. Instead, at age 8, I wrote a masterpiece called ‘Riding and Stable Management’ even though I’d never actually touched a horse, let alone ridden them. I followed this up with the magisterial ‘Snakes of the World’, although I had never seen a snake. It’s nice, these days, to be writing about plants and animals that I actually have some acquaintance with.

The locomotion of caterpillars is, for me, part of their enduring charm. I love the way that they wave the front half of their bodies in larger and larger circles, reaching out with their little ‘arms’ in the hope of grasping something tasty. They have very poor sight (as we’ve seen, those ‘big eyes’ are fake) so they do everything by scent (using those tiny tiny antennae) and taste. They also have a good sense of touch, which is amplified by all those hairs (technically ‘setae’). When touched, many caterpillars will hunch up to make themselves appear bigger (like this one does) or roll up like a  Cumberland sausage and crash to the floor. After all, nearly every other creature, from the energetic blue tit to the near-invisible parasitic wasp, likes a tasty caterpillar.

Caterpillars don’t have a brain, as such – they have a line of nerves that resembles a vestigial spinal cord, but along their ventral surface. It’s safe to say that caterpillars need to be reactive when danger threatens, but they don’t need to mate, or indulge in social relationships, or bring up babies. They simply need to eat the right things, and as much of them as possible.

A butterfly, however, has a rudimentary brain, known as a cerebral ganglion, probably because its life has now gotten a lot more complicated – it needs to find a mate, maybe defend a territory, or even migrate for thousands of miles. However, recent research has shown that butterflies and moths remember what they learn as caterpillars. In the experiment, caterpillars were given a shock at the same time as being exposed to the smell of acetate, and as butterflies they would (very sensibly) avoid the smell.  I do wonder why, instead of being scared out of their wits, the caterpillars couldn’t be given something overwhelmingly tasty, so that at least they had a pleasant memory rather than one based on fear and pain. The poor things probably need therapy to get over the trauma. It does show, though that there is much more to caterpillars than we understand.

For many children caterpillars are their first introductions to the strange and wonderful natural world outside their back door. They are ideal in many ways – small, harmless (unless you pick a dermatitis-inducing one) and they are easy to rear. They are not so easy, however, to take right through to the butterfly stage. Plus, I remember being filled with horror when a caterpillar that I was looking after literally exploded into a mass of parasitic wasp larvae. What lessons they can teach us about the short, brutal lives of many creatures, and the moments of soaring beauty that pepper existence.

Alice talking to the Caterpillar from Alice in Wonderland, illustration by Arthur Rackham (Public Domain)

Photo Credits

Photo One (Knotgrass moth) – CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=387198

Photo Two (Caterpillar head) – http://www.monarchwatch.org/grafx/biol/cathead.gif