Yellow-Banded Underwing Caterpillar and Adult

2012 December 1

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[1], 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.

[1] 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.

4 Responses
  1. December 5, 2012

    Oh, darn. A couple of summers ago I discovered a long green fat caterpillar crawling out of the lilac bushes beside our screened porch, but it disappeared by the time I’d found my camera. I’d convinced myself it was a polyphemus moth (OK, for a while I thought it was a luna but polyphemous hang out around elms and maples and we have both, plus it looked longer than the 2.5 inches that seem standard for luna caterpillars). It was mint green and maybe 4 inches long.

    Now I’m thinking it’s much more likely that it was an underwing, since the moth you have pictured looks very much like the ones that keep sneaking under the screens and then can’t get back outside (or find their way into the house and flap madly in lampshades and curtains).

    Have you ever seen a luna or a polyphemus?

  2. December 6, 2012

    Anne: As a matter of fact, just this summer Sandy caught a polyphemus caterpillar crossing the road! I’ve got lots of pictures. It is currently cocooned up in a box stored in a cool cellar, and with any luck we will get an adult moth from it this spring so that I can get the full life cycle into the post about it.

    We’ve seen adult Luna moths around in the past, too, but no caterpillars yet.

    The polyphemus caterpillar was quite a lot bigger than this underwing (maybe 50% longer and twice as fat), but the color match is very close. If yours was 4 inches long, it was probably a polyphemus.

  3. ml kovalik permalink
    August 28, 2014

    trying to identify a 3 inch long moth- certainly an underwing, has rather large yellow spots down the side of his body- a huge surprise to find this beauty within city limits! he’s been here all day, now dark, but 2 arabesque spiders w/huge webs close by- hope he survives the night!

  4. lilly permalink
    June 10, 2020


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