Yellow-Banded Underwing Caterpillar and Adult
I spotted this big, lurid green caterpillar crossing the road in front of our house on June 4, 2012. It alternated between playing dead, and thrashing violently in my hand.
I thought a big caterpillar like that, with distinctive-looking bumps on its rump, would be identifiable, but it was harder than I thought it would be. There were several similar pictures on BugGuide, but none of them had been identified beyond being some kind of moth. The face turned out to be pretty lacking in distinctive features. Although, if you look at the neck just behind the head, there is a white object that looks a lot like the ones on this tachnid-fly-infested caterpillar, which I coincidentally found the same day in about the same place.
The underside didn’t have any particular markings, either.
This is a good opportunity to see the crochets on the caterpillar’s feet. These are little hooks, kind of like velcro hooks, that can grab onto leaf surfaces. Most leaf-feeding caterpillars can use these to hook onto a leaf so tightly that it’s practically impossible to pull them off without ripping off their foot, but then they can release the hooks at will to let go and walk. This is a very important ability when you are twenty feet off the ground, hanging on to a swaying, shaking twig in a tree and trying not to fall off.
I figured that, with a large caterpillar crawling around like that, it was almost certainly either looking for a place to pupate, or infected by parasites. Either way, I wanted to see what it would turn into, so I put it in a container with a moistened paper towel. It promptly burrowed into the towel and coccooned up, and that was the last we saw of it for about a month.
Then, on July 13, this great big moth came out.
It was an underwing. With a bit of teasing, it was possible to get glimpses of the pattern on the hindwings, which were black with a brown border, and a yellow-orange stripe running across horizontally.
Hey, I think we’ve seen this one before! It looks like a slightly darker version of the Yellow-Banded Underwing that Sam caught for me last August, in which case this is Catocala cerogama. Only one problem, though; the caterpillars shown for this species on BugGuide are very different from the one we have here. They have rows of tiny black dots all over their bodies, they have the fleshy whiskers along their bellies, and they are not even similar in color. But, they do have the black tubercles on their rumps, like this one does (and which the majority of underwing caterpillars don’t have). Looking it up in Wagner’s book, he says that the yellow-banded underwing caterpillars are “extremely variable in color”, so I don’t know. Maybe this is an extreme color form. Or maybe I have the wrong moth.
 I bet when I started talking about tachnid flies, you thought this was going to turn out to be another caterpillar parasite posting, didn’t you? Nope! Not this time! While I suspect that the white object stuck to it is a tachnid fly egg, it looks like it never hatched for one reason or another. If that’s what it is, the fact that this one grew up OK shows that not all caterpillars that get attacked by parasitic flies end up being killed by them. Maybe tachnid fly eggs don’t have that high of a hatching rate, or maybe the caterpillars have some sort of immune response that can keep them from hatching as long as the egg load isn’t too large.