Skip to content
Hackberry emperors use their long proboscis to imbibe minerals from mud, rotting fruit, and human sweat. (Stephen Jones courtesy photo)
Hackberry emperors use their long proboscis to imbibe minerals from mud, rotting fruit, and human sweat. (Stephen Jones courtesy photo)

There are a few shallow ravines in the foothills north of Boulder where sprawling hackberry trees thrive among knee-high grasses and scattered shrubs.

Standing quietly under these low-growing trees on a sunny morning, you may see dozens of butterflies flitting among the surrounding skunkbrush, snowberries, and late summer wildflowers.

A light touch on the arm will alert you to the presence of a hackberry emperor, one of our most alluring and approachable butterflies. We’re learning to identify this locally uncommon species by its touch, alone.

These brazen beauties imbibe our sodium-rich sweat and may use their senses of taste and touch to help determine who we are. Once the ritual is complete and they’ve landed on a nearby flower or leaf, they often allow us to approach to within a couple of feet.

Hackberry emperors lay their eggs almost exclusively on hackberry leaves. Hackberries grow predominately east and south of the Rockies, and these emperors reach the northwestern limit of their continental range in shrubby canyons of the Colorado Front Range and southeastern Wyoming.

Hackberry emperors are fast and erratic fliers with distinctive yellow and midnight-blue eyespots on their wing margins. Erratic flight helps the butterflies to elude predatory birds, and the false eyespots may direct pecking movements away from their fragile bodies.

Virtually all butterflies react evasively to sudden movements that signal the approach of a predator. But they have no particular reason to fear humans. If you stand quietly and give nectaring butterflies time to check you out, some individuals may eventually alight on your clothing or the top of your head.

However, the emperors’ habit of repeatedly brushing against our bare skin and getting to know us by taste seems unusual if not unique. On her “Natural Web” website, Mary Anne Borge explains that hackberry emperors derive much of their nutrition and needed minerals from sap and rotting fruits, but they show a particular fondness for human sweat.

“It is the species of butterfly I’ve most often seen landing on people to get a tasty snack.”

The fast-flying butterflies gather our sweat with a quick flick of their long proboscis. Unlike a straw, which it’s often compared to, the proboscis is a sponge-like organ that absorbs nectar and other sweet liquids through osmosis. Seen through a microscope, the tip of a hackberry emperor proboscis resembles a hairbrush with rows of soft spines.

Emperors and other butterflies use nectars to fuel their flight muscles and reproductive organs. In Colorado, hackberry emperors produce 1-3 broods each summer. The eggs hatch after just a few days, and caterpillars pupate within less than two weeks. So the offspring of the butterfly who imbibed our sweat in late July may be flitting around in September, carrying faint traces of our DNA.

Stephen Jones and Ruth Carol Cushman are authors of Wild Boulder County and the North American Prairie. Jones is co-author with Janet Chu of Butterflies of the Colorado Front Range.

Other September Events

A new generation of particularly large monarch butterflies emerges, ready to undertake a 1500-mile migration to the high mountains of central Mexico.

Beautiful buckeye butterflies appear in foothills grasslands.

Boulder’s earliest September snow occurred on September 12, 2014, with 0.5 inches falling.

Join the Conversation

We invite you to use our commenting platform to engage in insightful conversations about issues in our community. We reserve the right at all times to remove any information or materials that are unlawful, threatening, abusive, libelous, defamatory, obscene, vulgar, pornographic, profane, indecent or otherwise objectionable to us, and to disclose any information necessary to satisfy the law, regulation, or government request. We might permanently block any user who abuses these conditions.