Congratulations to Joyce F. for correctly identifying last week’s “What Is It!” on the Ames Free Library’s Facebook page. This surrealistic creature is, indeed, a spicebush swallowtail caterpillar.
From this angle, the caterpillar has a giant head with intimidating eyes. We are looking at false eyespots on the animal’s thorax. Its tucked-under head is small, not to mention its minute eyes. From a predator’s perspective, the larval swallowtail presumably looks like a small snake, especially when its primary color is green. Our caterpillar had recently changed color because it was almost ready to pupate.
Spicebush Swallowtail Larva on a Spicebush Leaf
Ryan Hagerty, Public domain, via Wikimedia Commons
At this stage, the insect hides by day and feeds at night. It creates a safe daytime shelter by making a leaf tube held together by silk. According to Caterpillars of Eastern North America by David Wagner, the caterpillar always rests “with the head up.” A potential predator such as a bird would see those startling “eyes” – and think twice. Wagner suggests that this species might be “the best snake mimic in North America.”
Soft-bodied and rich in protein, caterpillars must avoid becoming someone’s dinner. The spicebush swallowtail uses a different protective coloration at hatching and through its first few molts. The early “instars,” the stages between molts, look like bird droppings: brown, white, and unappealing. This convincing disguise allows the baby caterpillars to live unhindered on the leaf’s upper surface. Visit “Featured Creatures” by the University of Florida to view images of the entire life cycle.
After receiving the “What Is It!” photo from our reader, Lorraine P., I really wanted to see one of those snake-mimic caterpillars, but time was running out. Busy with work, I never found a chance to visit appropriate habitats; and then I left for vacation. I made one last attempt yesterday by visiting the Hockomock swamp, where spicebush, one of its primary host plants, grows.
Wearing office clothes and sandals, mobbed by mosquitoes and biting flies, I examined spicebush leaves for caterpillars – without success. My efforts were soon rewarded with the sight of a large, dark butterfly. It traveled along the cart path, landing on the roadside vegetation: arrowwood viburnum, winterberry holly, and Virginia creeper, as well as spicebush. I saw an arc of green iridescence on its hind wings and pale spots along the margins. Could it be? Yes, and the photos confirmed my hopes. This lovely male spicebush swallowtail appeared just in time!
Like all caterpillars, this species eats, grows, sheds its skin, and eats some more until it transitions to adulthood through pupation. The adult butterfly is a brief phase in the insect’s life cycle, but it is most conspicuous and perhaps most delightful to humans.
If you wish to observe these insects as larvae or adults, there is still time, for they produce two broods here in eastern Massachusetts. In our area, the caterpillars feed almost exclusively on two aromatic plants: spicebush and sassafras. Spicebush grows in damp forests and floodplains. I often find it along streams. “Sassafras is a pioneer tree that spreads to semi-open areas with reduced competition,” according to Illinois Wildflowers. Look for it along forest edges, roadsides, abandoned farm fields, and similarly disturbed habitats.
Learn to identify sassafras and spicebush in the off-season to scout them out as possible larval feeding grounds when warm weather arrives. The female butterflies will return to these woody plants to lay their eggs. Otherwise, the adults (butterflies) will be nectaring in open woodlands and borders. Some of their preferred nectar sources – jewelweed, Joe-Pye weed, and sweet pepperbush – grow near the Hockomock Wildlife Management area’s entrance.
Sassafras albidum – unlobed, bilobed, trilobed leaves
KATHERINE WAGNER-REISS, CC BY-SA 4.0
Once you do find swallowtails, look closely. Better yet, use binoculars. The butterflies you observe may differ from the image above. Female spicebush swallowtails have a bluish tone to their hindwings; males exhibit a greenish hue. Moreover, several other swallowtail species have light spots on a dark background. Consult a good field guide like Butterflies Through Binoculars before heading out, and bring some photos home to ID. You may not find your quarry, but you will discover something fascinating and beautiful.
Thanks again to Lorraine P. for her photos and curiosity.