A reader approached us with a question about a literal brush with nature. While outside, she accidentally brushed her ankle against a caterpillar. Very quickly, she noticed her ankle swelling. She’s curious what type of caterpillar this might be, and has sent in a picture to help us identify it.

First of all, we want to recommend that anyone who is worried about an adverse reaction to an encounter with nature see a doctor. It is always better to be safe than sorry.

Next, we want to compliment our reader on her photograph. This is a great picture, and the little critter is just cute as a button! It’s just too bad that our reader found him to be so irritating.

Since our reader did not report having been bitten, we can narrow this down to the caterpillars that have a defense mechanism knows as “urticating hairs”. These “hairs” are not actually hairs at all, but are very small, hollow tubes that sit on top of specific glands that produce an irritating fluid. When something touches those “hairs,” the irritating fluid is delivered through the tube. Interestingly, this exact mechanism can also be found on some plants, where it is a bit easier to see.

 

In both plants and caterpillars, this is a defense mechanism developed to convince hungry predators to stay away. If swelling on the ankle is uncomfortable, just imagine how it would feel in the mouth!

There are many caterpillars that have this particular defense mechanism, and many of them are lepidopteran caterpillars. Lepidoptera is an order of insect that includes several hundred-thousand species of moths and caterpillars, and many of the caterpillars are irritating when touched. These include the Walnut Caterpillar (Datana integerrima) and the Hickory Tussuck Moth (Lophocampa Caryae), both of which have larvae that resemble the reader’s picture.

The good news is that these irritations are rarely dangerous. Typically, touching these urticating hairs will result in swelling or a rash that will go away on its own in a relatively short amount of time. Of course, some people are more sensitive than others and may have a more painful or lingering reaction. If that case, it is important to consult their doctor and follow the doctor’s orders.

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