Last week I found this little guy crawling on the ground outside the screen porch. I picked it up and it thrashed around until I placed it on a nearby hickory sapling. I recognized it as the larva of the Walnut Sphinx Moth from the unusual angled head capsule and the numerous raised spots arranged in rings around the body. This individual is a mid-instar as the head is very elongated.
A few years ago I found the late instar larva pictured above. The head is less elongated and the white stripe alongside the head capsule is more pronounced as pictured in the field guides (most field guides showing caterpillars show only the last instar which can be quite different from the earlier ones, but, in this case, Wagner’s Caterpillars of Eastern North America also has an inset showing the pronounced head of the early instar). These caterpillars, as is the case for many others, are easier to find at night by flashlight when they are actively feeding. During the day, they often rest on the underside of a leaf along the midrib.
The thing I find incredible about this species is it is one of the few caterpillars that makes sounds. When touched it thrashes about violently and can hiss or whistle by forcing air out of the spiracles (the small openings of the respiratory system found along the side of the caterpillar). How cool is that! So, if you see me along the woods edge with my ear down at a hickory or walnut leaf, just assume I am listening for a larval tune.