Field Notes from the Beiser Field Station: October 7, 2008

The Spicebush Swallowtail

The Amazing Snake-Mimicking Caterpillar

Walking through the woods in late summer at the Beiser Field Station, one may be rewarded by examining carefully the many spicebush trees on the forested slopes. In particular, it's useful to look for leaves that are rolled over as the one to the left is. What could be lurking in this rolled-over leaf? Many insects and spiders can spin silk and use this to fashion a retreat. These retreats are quite useful as they protect the inhabitants from the sharp eyes of insectivorous birds or the touch of predatory insects. But what would be feeding on spicebush, a tree loaded with defensive chemicals?

If we peel back the leaf a bit (right) we are confronted - much as a bird would be - with a large eye staring back at us. Snake!

Actually, of course, it's not a snake at all but the larva of the Spicebush Swallowtail Butterfly. The snake-head illusion hopefully gives a bird enough pause to leave the caterpillar alone.

The Spicebush Swallowtail Caterpillar is one of the best mimics known. The caterpillar, in its early stages, resembles a bird dropping (brown caterpillar in image to left) and is found exposed during the day on the upper side of the leaves. After molting to the 4th instar (I believe there are 5 total instars in this species) the caterpillar turns green (large caterpillar). Both the early instars and the later ones display the snake's head mimicry, with large spots on the swollen thorax giving the appearance of a snake's head. The spots look like eyes, even to the point of having a white "highlight" to simulate moisture (below). The later instars roll up leaves, holding them together with silk. They stay in these retreats by day; a bird investigating the rolled up leaf will be confronted by a "snake" peering back at it.

Behaviorally, they will rear up and retract the actual caterpillar head to increase the illusion (below)

Another view of the caterpillar in its "bird-dropping" disguise.

A second line of defense peculiar to this family of butterflies is the presence of an osmeterium, an eversible structure normally folded up inside the body. When threatened, the caterpillar will extend the structure, which is branched and covered with an aromatic chemical which serves to repel many insect and even vertebrate attackers. Apparently, in this species the osmeterium has the added benefit of looking like the forked tongue of a snake, although on our specimen the "tongue" is yellow instead of black, and I had to press on the thorax to get it to evert the osmeterium at all. Thus the picture with the everted osmeterium is obviously posed (pressing on the thorax also causes the head to protrude). The aroma of the deterrent chemical persisted in my lab for quite some time. It was actually a pleasant smell, probably derived from some of the defensive chemicals which give the spicebush its fragrance and its name . As they prepare to pupate, they turn yellow.

We have seen the caterpillars at the station from early August through early October, but they are never common and the season may in fact be much broader. In the fall of 2010, we found a good number of the caterpillars at the field station. We brought two back to try to rear out to the adult stage. As of this writing, they are in the chrysalis stage. To the right is the 2010 Zoology class on a night hike at the station, where we found a number of interesting species of caterpillars.

Transformation - above right, a mature caterpillar searches for a good spot to locate its chrysalis. It then deposits a patch of silk on the branch using its salivary glands - the same glands that produce the silk in silkworms. It then revereses itself so it is facing upwards and spins more silk around the branch, including a "safety harness" that encircles its body (above left). You can also see the patch of silk attaching the bottom of the caterpillar in the image above left. Below, the transformation to the chrysalis is completed. The left two images show the chrysalis is dorsal and lateral view, with the "safety belt" still intact and the characteristic "horns" of this species in evidence. I think these chrysalises strongly resemble dried leaves on the spicebush; the picture to the right is of one such leaf taken at the same time the chrysalis pictured was present in the lab. A final act of mimicry in this extraordinary butterfly's life? It is remarkable that as it ages it goes from resembing a brid dropping to a snake to a leaf. That's flexibility!