The Caterpillar That Kept Me Coming Back

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This week, I've been visiting the outdoor plant collections at the University of Illinois Plant Sciences Laboratory (PSL) for short, lunch-hour photo shoots. The motivation behind my first visit was to see what was in bloom, since it had been about a month since I last took the five-minute walk from my office to PSL. The outdoor landscape beds that are nestled between greenhouses and serve as a living classroom for plant science students looked bedraggled from the excessive heat we've endured this summer. There were a scattering of flowers to photograph, but I quickly realized that the insect life was going to be the prime focus on my lens.

A variety of moths, butterflies, bees, wasps and other insects were on full display -- munching, flitting, and buzzing all about.

But the critters that kept me coming back each day were the Black Swallowtail caterpillars (Papilio polyxenes) that infested the two dill plants in one of the flower beds. The first day, these caterpillars were so well hidden, I completely overlooked them until a friend pointed them out to me.

In the bright sun, their yellow, green and black markings blend well with the dill stems and flowers. Once I knew what I was looking for, 20-30 caterpillars suddenly appeared on the plants.

Over the course of two days, the caterpillars easily doubled in size. As they grew larger, it was easier to see their camouflage and decoy markings. The black spots on their posterior end (left in the photo above) look just like a face, serving as a decoy for predators that might try to attack their heads.

The true head of the caterpillar stays hidden most of the time, as it faces down toward the stem on which it's feeding.

When the caterpillar is disturbed, it rares up and extends an orange tentacle from a small slit between its head and thorax. This forked appendage is called an osmeterium and contains a foul-odored substance made of 2-methy-buteric acid and isobuteric acid (Source: Richard Fox, Lander University). I can only assume this foul smell is reserved to repel potential predators that are unfortunate enough to attack the caterpillar. The strike of the osmeterium was instantaneous, and equally quick in its retraction.

This week's trips over to PSL have been more than just fodder for my photographic impulses; they've been true learning experiences and a chance to see the life cycle of Papilio polyxenes in action. I hope that the greenhouse staff don't try to dispose of the caterpillars before they have a chance to go through their metamorphosis into Black Swallowtail butterflies. You can be sure I'll be there to witness the transformation if it occurs.

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