Found in Cambridge, Ontario. Late August. Laying on ground plants, seemingly doing nothing. Black with yellow spots, red face and red tail, approximately 3″ inches long.
Your caterpillar is the Galium sphinx moth caterpillar. Yours looks like it might be big enough that it is roaming to find a place to pupate.
Caterpillars of this moth do vary some in color, but like most sphinx caterpillars they have a ‘horn’ or tail on the rear end. This insects horn is either red or black.
The moths look a little like hummingbirds as they fly from flower to flower gathering nectar. The straw like mouthparts are long tubes for sucking up nectar…it is fun to see them do so. This particular sphinx moth does fly during the day as well as at night.
While the moths do feed on nectar the caterpillars feed on a wide variety of plants like willow-herb/Fireweed (Epilobium spp.), Bedstraw (Galium sp…hence the name), and other plants in the evening primrose family. They are not considered a pest of any crop.
This insect is found in the northern most areas of North America.