Here is a leaf with small, medium, and large monarch caterpillars. I generally don't like to get a caterpillar as large as the one on the right--too much of a risk of a parasite but I needed cats for all sizes for the tv demo. Wasps and flies (and who knows what else) will lay their eggs in caterpillars. The wasp or fly larvae will feed on the inside of the caterpillar and about the time it forms a chrysalis, it will pop out. Yeah, it's as gross as it sounds.
After I finished the segment, I brought the milkweed home and set it in a glass full of water. I normally don't like to put milkweed in a glass of water, on the off chance that a caterpillar could fall into the water and drown, but this time, I'm glad I did.
The next day, one of the monarch caterpillars went on a "walk about"--a long journey to find the perfect place to form a chrysalis. It stopped at the top of our living room window frame and we thought, not the place we would pick, but safe enough in our apartment to chrysalize.
That night before we went to bed, he assumed the "J Position" and worked to shed his skin. We noticed another monarch caterpillar J-ing out on a monarch leaf. We tucked ourselves into bed, excited to find chrysalis the next morning.
Alas, I found one of our caterpillars shriveled up. It was dead--my fears were confirmed. If you look up at the third set of legs, you will see a bit of film--the left overs of fly larvae that had emerged from the caterpillar.
I looked up to the caterpillar that had been at the top of the window frame and found it had made it as far as forming an actual chrysalis, but the slimy rope hanging from it, showed that fly larvae had emerged--ew. All that milkweed chewing for nothing! The maggots emerge form the caterpillars when they are ready to pupate.
Since the larvae dropped straight down from the caterpillar and chrysalis, they ended up in the bottom of the glass of water and drown! Too late for the caterpillars I had, but at least they won't be getting future monarchs. Take that, you nasty maggot. I know you have to survive, just not on my monarchs.