Hanging by a Thread

Have you ever run into a small caterpillar dangling from a silk thread in the forest? I have, many times. I’ve wondered why they do this. Are these caterpillars preparing to pupate? Are they dispersing from their natal tree?

I photographed this hanging caterpillar in the forest along the Salmon River, OR. It was dangling from a Bigleaf Maple tree. When I got home that day, I was motivated to do some research on this behavior. I think I understand it now… and the answer isn’t what I had guessed.

My first task was to identify the insect itself. It looks like an “inchworm,” a moth larva that lacks prolegs on the middle segments of its body and has a familiar, rather adorable mode of locomotion. It first extends forward with the front part of its body, then draws up the back end, forming a little arch or loop in the process.

What we colloquially call inchworms are larvae of moths in the family Geometridae. So, my best guess is that this is a geometrid moth larva.

Many species of geometrid caterpillars are excellent mimics: when threatened, they straighten out and stiffen their bodies so that they look like leafless twigs. Most species are either brown or green, and are smooth-textured. The species I saw was green, perhaps because that color provides good camouflage among maple leaves.

Another defense that these caterpillars have is “bungee jumping” behavior. If a wasp or spider gets too close to a geometrid caterpillar, the caterpillar jumps off its tree branch and is suspended in the air by a silk thread it left attached to the tree. The caterpillar eventually hauls itself back up into the foliage.

The caterpillar I photographed was motionless while hanging; it didn’t seem to be climbing back up yet.

I’m going to keep an eye out for these caterpillars when I am in the woods, to see if I can see more of this interesting behavior.

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