When the Black Swallowtail caterpillars are ready to pupate, they leave the foliage of the food plants (fennel, parsley) and wander about seeking twigs on sticks. Having found a suitable one, they proceed to anchor themselves with a silk attachement at the base and a remarkable silk strand enclosing the upper thoracic region, so that the pupa is suspended like a mountain climber hanging from a cliffside in a sleeping bag. How they manage to spin a silken cord like that around their bodies is a process that I haven't been able to observe or capture on film.
For several days the caterpillar hangs like that, while undergoing some subtle color changes on the surface. Then it becomes very active with a wriggling, shimmying motion, and proceeds to shed its skin. Emerging from inside the skin is the pupa in the second image above, with the residual skin still visible at the base. Incredibly, the silken cord remains intact, supporting the creature in its new guise.
Sometimes the pupa/chrysalis remains green, but most of the ones we've had have gradually turned to a brown color as in the bottom photo; at that point they look ever so much like an excrescence on the stick itself, and that camoflage undoubtedly helps prevent predation. The caterpillars that reach this stage by late August - early September will emerge ("eclose") about two weeks later. Those that pupate later may remain inside the brown chrysalis over the winter - through temperatures of 30 below zero. Incredible. And then on a warm spring day the adult Black Swallowtail appears.
I'll have some photos of our late summer hatchlings in a day or two.
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