Hi What’s That Bug,
I found this caterpillar in my back yard on the chain link fence. What is it and is it poisonous?
Theresa
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Hi Theresa,
That’s a great-looking caterpillar, isn’t it? I love the bizarre-looking ones the most. This caterpillar turns into a pipevine swallowtail butterfly. Since they feed on pipevine, they are indeed poisonous. However, that’s only a problem if you decide to eat one!
One of the neat things about swallowtail caterpillars is a gland on the front of their head that can protrude if touched or threatened. You can see it in the fence photo you took. It’s the orange “Y”-shaped thing that is sticking out its head.
This is called an osmeterium, and it’s used as a defense mechanism. It also emits a smelly odor that you don’t really want to get on your hands (don’t ask me how I know this!).
Pipevine swallowtail caterpillars are picky about the plants they munch on. Adult females will seek out pipevine plants to lay their eggs on, and the caterpillars will only eat that plant. Since the plant is poisonous, the caterpillars are able to store some of those toxins in their bodies as they grow. Their scary-looking colors and spikes warn others that they shouldn’t be eaten!
I suspect this caterpillar is ready to pupate and form a chrysalis. That’s probably why he was hanging out on the chain link fence.
Dr. Bolton, PhD
