I brought a pot tropical milkweed inside about a week ago to spend the winter on a sunny window sill. It's full of aphids and I was just about to hose it down in the sink tonight when I spotted a half dozen caterpillars. At first I thought they were lady bug larvae, but they don't have the legs all toward the front. These about a centimeter long, are brown and move like an inch worm. They rear up and swivel side to side, and are eating the aphids. What are these? I'll try to post a photo later.