We received a reader question a while back about a brown worm that he found. The worm was thick, and about five centimeters long. Although the worm (or caterpillar, as we shall see) was primarily brown, it had a little red on its underside. Most notably, the worm’s exterior looked like a bottlebrush shrub (“bottlebrushes” is an informal term that refers to the shrub species that belong to the Callistemon genus). Not surprisingly, bottlebrushes resemble bottle brushes, which basically look like toilet cleaning brushes, with all the stiff bristles. This is a long way of saying that the worm our reader found had a lot of spiky bristles on it. Since the reader found the worm on his daughter’s bed, he was wondering if the worm was dangerous to children, and also where the worms tend to live.

The reader never asked what the worm was, which is nice because we can’t be sure what our reader found. We were given a nice physical description of the worm, but we don’t have a picture of the creature, and we also don’t know from what part of the world our reader is writing.

Given that the creature our reader found had tons of bristles on it, there is good chance that he didn’t find a worm at all. Rather, he might have found some type of caterpillar, which frequently have hairs (often called “setae”) on their bodies. To be sure, many worms have setae, but they are very hard to see. Earthworms, for example, have setae on their bodies that help them move, but they are hardly noticeable. Bristle worms, as their informal name implies (they are technically called “polychaetes”), have noticeable bristles on them, but polychaetes are overwhelming marine worms, although there are a few terrestrial species that can live in humid climates. Even so, it seems more likely that our reader found a caterpillar than one of the few species of terrestrial bristle worms.

We do not, however, know what kind of caterpillar he might have found. The reader’s description fairly well matches the caterpillar form (i.e., the larval form) of the yellow-based tussock moth, although the hairs on these caterpillars are often a whitish color. Another candidate is the ruby tiger moth caterpillar. These creatures are almost entirely covered with hairs, and their bodies are a tan, brownish color. Both caterpillars have a presence in North American, although the ruby tiger moth caterpillar can also be found in Europe. Unfortunately, we were unable to find out if these specific caterpillars are poisonous or otherwise harmful to humans. In general, though, humans can safely interact with caterpillars, so we suspect that neither of the two caterpillars listed above is dangerous. As a defense mechanism, many caterpillars look poisonous (at least to the predators they are trying to avoid), but this doesn’t mean that they are in fact poisonous, and in any case the reader obviously wants to make sure his daughter doesn’t eat any caterpillars she may find. When rubbed against the skin, some caterpillars can cause irritation, so you want to avoid touching some species.

We wish we could give our reader a definitive answer to his question, but we simply don’t know enough about the creature he found. All we can say is that he likely found a caterpillar, and that it is probably not dangerous.

• 
•