Collected from a stand of three Goldenrod plants growing along a pathside under hemlock in the Blue Hills, Milton MA 10/17/2008. I worked hard today to find any caterpillars and to have one of these guys fall in my lap just as I was getting back to the car was a real pick me up. It had defoliated much of its Goldenrod plant so it really stuck out (and yeah, its colored like a circus clown). When agitated - poked - it curled its head backwards to its side.
Most of these I have seen have an orange or red mid-dorsal stripe. This one clearly has a yellow one. Natural variation, age, sex, something else?