When disturbed, the caterpillar extends a smelly appendage called an osmeterium from a slot in the top of its head. The strong smell is supposed to discourage predators. I was able to smell something different than the overpowering odor of dill, but it was not that obnoxious. Here, the osmeterium is partially extended.
Here is another closeup of the osmeterium extended out further. As it extends, it behaves like two fingers of an inside-out latex glove that invert as you inflate the glove.
Assuming the birds don't sneak in and eat them, perhaps we will see the metamorphosis later this year.