The caterpillar of P. zelicaon changes
in appearance from instar to instar, and is somewhat variable in the final instar,
although the very black example shown here is quite exceptional. The caterpillar
is conspicuous, presumably to warn birds that it is distasteful, but I have
seen robins eat a zelicaon caterpillar on more than one occasion. When
disturbed, like other swallowtails it will protrude an orange forked osmeterium
from behind its head, and a strong smell of rotten fruit appears. Although this
may be intended to deter tachinid flies, which often lay their eggs behind the
head of a caterpillar, it is by no means 100% successful, and zelicaon
seems to be just as vulnerable to tachinid attacks as other caterpillars.
The foodplant is generally given as various Apiaceae (Umbelliferae), but I have found the caterpillars to be slightly selective. The most usual foodplants in this area are Lomatium nudicaule and Oenanthe sarmentosa, although I have also found them on Foeniculum vulgare, Perideridea gairdneri, Levisticum officinale, Angelica lucida, Petroselinum crispum, Pastinaca sativa and, a little surprisingly, on Glehnia leiocarpa. On the other hand I have never found them on Daucus carota or Heracleum. When ready to pupate, the caterpillar spins a silk pad on a twig, and embeds its anal segment into the pad with a structure of small hooks called a cremaster. It spins a silken girdle around it between segments five and six. The skin splits behind the head, and gradually peels off, revealing the chrysalis, of which there are two colour forms, green and brown.
For the most part the species is univoltine - i.e. there is one generation per year, winter being spent as a pupa. A few butterflies, however, will emerge and try to start another generation. I have found caterpillars as late as November.
There is a second insect in the photograph of the adult butterfly. See if you can find it.