Is it a sphinx moth or a hummingbird?

By Jeff Mitton

I was sitting at my campsite before dawn, enjoying a mug of coffee and watching the light accumulate in the eastern sky.

My eastern horizon was formed partially by distant mesas and partially by a rabbitbrush, fully six feet tall. As the light changed from blue to orange, I became more and more aware of activity at the edge of the rabbitbrush. When my curiosity exceeded the pleasure of sitting, sipping and gazing, I rose to investigate.

Three white-lined sphinx moths, Hyles lineata, attended the small yellow flowers that covered the full periphery of the rabbitbrush. They had wingspans of five inches, so they were easy to spot and follow, at least while they were maneuvering on a single bush.

A sphinx moth would hover silently before a cluster of open flowers, probing with a proboscis that was as long as its body. Once they arrived at a shrub, they investigated many of the flowers, then buzzed off to another rabbitbrush. When flying from shrub to shrub, they made a sound like a hummingbird.

Sphinx moths are a delight to watch, for they are agile flyers with bright colors. The head and thorax are brown with white stripes, while the abdomen is an ornate pattern of black, white, and salmon slashes and bars on a tan background. The upper forewings are overlapping bars and threads of white on a brown background, with a faint salmon trim on the outer edge. The upper side of the hind wings has a white bar, two black bars and a bright salmon bar, with a sharp white fringe around the entire wing.

White-lined sphinx moths are found throughout the 48 contiguous United States and into both Canada and Mexico.

I have the impression that they are more abundant in southwestern deserts; the Sonoran Desert has 17 species of night-blooming plants that they pollinate, probably more than anywhere else. Sphinx moths have two broods per year in the Sonoran and Chihuahuan Deserts, but only one brood per year in the Great Basin, which is cooler than the southern deserts.

Sphinx moths generally appear at dusk and are most active at dawn and dusk, though they have good eyesight in low light and are able to forage in the dark. I have seen them flying during the day, but daytime flights are uncommon.

Their most favored flowers open in the evening, produce fragrant plumes to attract sphinx moths in the dark, have long corollas and produce large volumes of nectar. But they are certainly generalists and in our area they pollinate giant eveningstars, scarlet gilia, evening primroses, morning glory, yucca, gardenia, and apparently, rabbitbrush.

Chilly weather can ground sphinx moths for they are ectotherms but need warmth to support the high metabolic rate demanded by flight. If they are perched and are cold, they will shiver to elevate their body temperatures. Once they are flying, their large bodies retain heat so that their body temperatures are markedly above the temperature of the air. As their internal temperature rises from 59 to 109 F, wing beat frequency increases from 15 to 55 beats per second.

Why are they called sphinx moths?

Apparently, the common name refers to a behavior of the caterpillar in response to a threat. The larvae are very large, perhaps longer than the body of the adult and they are called hornworms for a prominent horn that rises from the dorsal side of the hind end. They are usually bright green with spiraling, thin, bright yellow bands and white spots, but some are predominantly black. When they are threatened, they rear their heads and pause, reminding some people of the posture of the Great Sphinx of Giza.

If the threat posture does not dissuade the persistent predator, the hornworm vomits a disgusting green mass.

Jeff Mitton, mitton@colorado.edu, is a professor in the Department of Ecology and Evolutionary Biology at the University of Colorado. This column originally appeared in the Boulder Camera.

September 2013