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Adult buck moths earned the name by flying during fall deer hunting season

A caterpillar trundled across a dirt road on the Uncompahgre Plateau, and although its tiny legs were stepping right along, its progress was slow. Two others were visible, crossing in the same direction, and by watching for a few minutes I learned that this was a slow motion parade — caterpillars were continuously visible. What prevented, I wondered, a predator such as a vole, deer mouse, chipmunk, or bird from standing in the road and filling its belly with caterpillars?

Each caterpillar was 1.5 to 2 inches long and conspicuous against the earth tones of the road. Its background color was black with white lines along the lower edge of the sides and broken lines higher on the back. A circle of short yellow bristles highlighted each tall, black, branched spine, and spines were in four rows running the entire length of the caterpillar. Spines had many branches, some white and some black, tipped with stiff, slender quills pointing upward and to the sides, forming a bristling defense system.

Poison glands at the base of the spines synthesize a poison that flows into the spines. Quills are adorned with microscopic barbs pointing backwards, causing them to work deeper when they become embedded in flesh. These spines and quills inject a poison that causes a painful sting that persists for hours, sometimes into the following day.

My colleague Deane Bowers, an entomologist, identified the caterpillars to the genus Hemileuca, the buck moths. At least three buck moth species are native to Colorado. Based on online photos of caterpillars, I suspect that these were Hera buck moths, Hemileuca hera. This species is also called sagebrush sheep moths, because the eggs are usually placed on sagebrush, the favored larval food host. Sagebrush was common where I found the caterpillars.

For the first three instars, the caterpillars feed cooperatively, increasing both their visual impact and their defenses. When a predator threatens a gang of small caterpillars, they all thrash to become a miniature forest of pulsing spines. Fourth and fifth instar caterpillars carry more and larger spines and are thus better defended, safe to assume a solitary existence.

Adult Hera buck moths, which fly during the day, have wingspans ranging from 2.5 to 3.5 inches. Wings have a background color of white marked with black lines, large dots, and prominent triangles reaching from the outer margins of both forewings and hind wings.

Adults lack mouthparts and consequently are unable to feed. They emerge from eggs in the early morning and usually mate by late morning. Females neatly arrange eggs around sagebrush branches later that day or early next morning, and adults die approximately one day after they emerged. The eggs overwinter on branches, and caterpillars appear from April through June, while adults emerge from July to September. Adult moths in this group earned their common name, buck moths, by flying during fall deer hunting season. Because adults live only long enough to court, mate and lay eggs, adult Hera buck moths are rarely seen — I have seen one.

In contrast with adults that live about one day and can fly to flee, caterpillars are present and conspicuous for about a month. Caterpillars eat, grow and then molt or shed their skin four times to reach the fifth instar, or larval stage. Adult moths can survive one day by being furtive, but slow, conspicuous caterpillars must develop poison glands attached to branched spines tipped with quills. A buck moth caterpillar is a mobile fortress of hypodermic needles.

Don’t pick up a caterpillar bristling with spines. They will move to jab with their spines and the poison is painful, persistent and memorable. Furthermore, if you see one of these caterpillars, others are undoubtedly around, so vigilance is warranted. You would regret sitting on or resting your hand on one. I enjoy watching them feed, and taking a close look at those formidable defenses.