This is my third year of raising caterpillars on my screened-in porch, and over the past two days I've witnessed a significant die-off among different instars. I've never had this problem before, so I don't think it's my methodology.
Up here in northeastern Illinois we've experienced a heat wave with the heat index repeatedly reported about 100 degrees. I was wondering if caterpillars have difficulty coping with extreme heat/humidity?
I'm sorry I don't have any pictures -- mine do not look like the one in Dave's post (where it looks "burnt"). Instead, I just find them lying at the bottom of the jar as if they've just keeled over.
Not all of them have died, but I hate to keep finding bodies. I have stopped looking for caterpillars "in the wild" in case they have a better coping mechanism.