I was walking through a garden this week and noticed that the meadow-rue (Thalictrum) was stripped of leaves, pictured below. The culprits were still at the scene, very striking caterpillars, pictured above. These are caterpillars of the Canadian Owlet (Calyptra canadensis), which, as an adult, is a very plain brown moth. The caterpillars are voracious. They continued to feed even after I plucked the plant stem and removed it to photograph them.
Note the jointed or claw-like aspect of the true legs, the first three pairs right behind the yellow head. These will develop into the adult moth’s legs. The four black middle bumps and the rear bumper (most clearly seen in the middle caterpillar) are called prolegs. They have little Velcro-like hooks on their tips to help them move about.
These garden foes are very picky eaters. They feed only on the meadow-rue, and they have evolved the intestinal machinery to enable them to acquire and store the bitter chemical compounds from the meadow-rue as they feed. Their bright black and yellow coloring advertise their foul taste to potential predators, providing an excellent example of the biological phenomenon of warning coloration.