There were a total of 15 Black Swallowtail caterpillars munching on my dill patch this past couple of weeks — not much dill is left. Now, the mature larvae are about 1 1/2 inches long, and their little round bodies look like the perfect food for the insectivorous birds around here. But the birds leave them alone. Why?
True to that old saying, “you are what you eat”, swallowtail and other caterpillars sequester their host plant’s noxious chemicals in their own bodies, which makes them unpalatable. (It’s not just the Monarch Butterfly that can do this!) And they have an added defense just under the skin of the back of their head where two fleshy projections emerge, when the head is touched lightly, to spray bad-tasting terpenes in the face of the would-be predator. I discovered this interesting feature by accident and managed to photograph the 1 second deployment of the caterpillar’s “horns”.
Unfortunately (for the photographer), the wind was blowing and it was rather cold for caterpillars, so these warning displays were not as active as they might have been.
The younger larvae of this caterpillar apparently don’t use this chemical defense, and are not as unpalatable because they haven’t eaten enough of the host plant. So they mimic a bird dropping instead, hoping to avoid detection. In addition, spines projecting from their skin are irritating to tender bird mouths.
However, by far the most aggressive defense by caterpillars I have read about is that of Lonomia obliqua, the exceptionally hairy larva of the Giant Silkworm Moth. if the hairs break off and become embedded in human skin, they cause a chain reaction that basically causes the individual to bleed to death internally.
As more tourists visit the Amazon region where this species lives, there are more reports of envenomation by stepping or brushing against the caterpillar, with subsequent hemorrhagic shock and death just days later. You can read more about this beast here.