One of the insects that can be found on Catalpa is the Catalpa Hornworm (Ceratomia catalpae). This caterpillar in the family Sphingidae can grow quite large (~3 inches) and goes through boom and bust cycles. During the summer of 2010, it was definitely a boom year for this caterpillar, which provided me with ample specimens.
There are also ample caterpillars for parasitoids to have a field day. Braconid wasps are especially prevalent, resulting in many of the caterpillars dying as they become vessels for broods of wasps.
Here you can see a Braconid wasp crawling on the cocoons.
This was a common sight, so I got used to seeing it after a few days. But once I found one that looked dramatically different (even from the cocooned ones), I took notice.
Yeah, I'd say that is dramatically different.
So....what happened? Good question! This guy lost the lottery in a spectacular way. He just exploded and that was that. Obviously, this is not like the parasitoidism of the Braconid wasps. If I were to venture a guess, I would blame a parasitoid fly maggot bursting out of the body: you can see a trail of liquid that comes out of the caterpillar's body and curves down by its head that looks like it could have been made by a maggot dragging itself along the leaf.
I haven't researched fly parasitoidism enough to put too much confidence behind that hypothesis, however. It also could have been a bird or other predator that decided to have a little snack, but wanted to leave a warning for the other caterpillars.
I would love to have the answer to the question, so I'll have to check for more caterpillars this summer. Maybe if I'm lucky, I'll find a maggot this time around, or see something else that explains it.