A Parasitized Chrysalis
I was sorry to see this sight, a kindergarten class had enjoyed watching the caterpillar turn into this chrysalis just 2 days earlier. The dark hole on the side was made by a tachnid fly larva. While the caterpillar was still small, a female fly had laid an egg on it. The emerging maggot buried into the caterpillar and ate it alive. It is not possible to tell if a caterpillar has been parasitized until it becomes a chrysalis because it eats and grows as though it is unaffected. However, a couple of days after the caterpillar pupates, the maggot crawls through the chrysalis, falls to the ground and becomes a pupa itself. If this happens in a captive chrysalis, you will see a small brown oval-shaped pupa on the bottom of the cage.
In August and September 2006 nearly every classroom at my school had a caterpillar because we have a very convenient supply of milkweed in the Secret Garden just outside my door. I explained to the teachers that upsetting as it may be to the students if this happens, this is a part of nature. All creatures must eat, it just happens that the tachnid fly larvae eat caterpillars (and other insects).