This is a Monarch caterpillar. It is eating climbing milkweed outside the schoolroom window. The larva is about 3 inches long and is gobbling the leaves at an amazing rate. The milkweed has taken over the flower bed which is bad for the hydrangeas but good for the butterflies which have been in abundance this summer. Milkweed is the favorite food of the Monarch caterpillars. When Monarch larvae ingest milkweed, they also ingest the plants' toxins, called cardiac glycosides. They store these compounds in their wings and exoskeletons, making the larvae and adults toxic to many potential predators. Vertebrate predators may avoid Monarchs because of their terrible taste. We're hoping the caterpillar will turn into a chrysalis. If it does, we'll bring it into our butterfly hatchery and observe its metamorphosis.