Worms (or Caterpillars) on Trees

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What type of woodland worm clings to trees with smooth bark and excretes purple feces? This is the question that recently confronted us in our inbox. We can’t seem to catch a break. All the worm (or caterpillar) questions we have received as of late have been – how should we put it? – extremely bizarre. First there was the worm that looked like a goldfish, and then there was the worm that had gold writing on its back, writing that appeared to be in some Asian language. The worm question we are presently concerned with seemed straightforward enough at first, and then we got to the part about purple feces, which has us thoroughly perplexed.

We’ll start with the obvious: we don’t have nearly enough information to answer this question with any specificity. We don’t know what the worm looks like – its color, size, etc. – and we don’t know where the worm was found (other than on a tree, which is actually quite helpful, as we shall see). It is also always helpful to have a picture, which the reader did not supply in this case.

While we can’t identify this worm specifically, what we can say is that it is likely not a worm at all. Why? Because the reader found it on a tree, which would be a strange place to find a worm. Worms bodies need to stay moist, so they tend to live in soil, where they are protected from the drying effects of the sun. (This is why nightcrawlers come out at night, and the fact that they come out at night is why nightcrawlers are called “nightcrawlers.”) Sometimes “worms” are found on trees – for example, Catawba worms are known to infest catalpa trees – but in most cases the alleged worms are not worms at all, and this is true of the example we just cited: Catawba worms are not worms, but the larva of the sphinx moth, which makes them caterpillars.

This is a long way of saying that our reader probably found some type of caterpillar, not a worm. As far as what caterpillar he found, we don’t, to be perfectly frank, have much of a clue at all. The color of a caterpillar’s feces is generally not the criterion we use to identify a given caterpillar, and in any case we aren’t aware of any caterpillars that excrete purple waste specifically. Perhaps our reader found a type of caterpillar that eats purple organic matter, like a purple flower, for example. Since we always like to throw out a specific suggestion, here is our best guess: our reader found the larval form of the ruddy daggerwing
butterfly (i.e., the caterpillar that will one day become this butterfly). We saw pictures of this butterfly on a purple flower, and the butterfly lays eggs on plants (the leaves of plants, specifically). Once the caterpillar hatches, it starts eating what is around it and leaves behind a trail of frass (caterpillar waste). So, perhaps the larval form of the ruddy daggerwing butterfly was born near a purple flower, ate it, and then moved on to a tree with smooth bark to excrete his flower meal. That’s as far as we are willing to speculate.


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