Home & Garden Columns

Wild Neighbors: It’s a Bird! It’s a Bee! No…

By Joe Eaton
Monday August 23, 2010 - 08:04:00 PM
Snowberry clearwing, a day-flying sphinx moth.
Cody Hough
Snowberry clearwing, a day-flying sphinx moth.

A couple of weeks ago I ran into an old acquaintance from the South: a dayflying moth variously known as the bumblebee moth or the snowberry clearwing (Hemaris diffinis.) Unfortunately, the moth was in dire straits, having blundered into a spiderweb in some kind of exotic Asian maple. It was intact but not moving. If the web had been within reach, I would have been tempted to free the moth. Its height saved me from violating the Prime Directive. 

Diurnal moths of any kind are uncommon, and I believe Hemaris is the only dayflying genus in the huge sphinx month family. Representatives occur across North America and Europe. In general they’re big burly moths with, as one of the names implies, mostly scaleless wings. Their bodies are patterned in more or less bee-like bands of green, black, and yellow. 

I used to see them in action when I lived in Georgia, and they were striking things to watch. The first impression was as much hummingbird as bumblebee. The moths hovered over flowers in an upright hummer-like posture, deploying their extraordinarily long tongues. Their vibrating wings made appropriate humming noises. 

Entomologists have speculated for a long time that Hemaris moths were bee mimics, the logic being that if you resemble something that’s capable of delivering a wicked sting, predators are more likely to leave you alone. Bees seemed a more probable model than hummingbirds since the moths occur in hummerless Europe. The recent discovery of fossil hummingbirds in, if I recall correctly, Germany suggests the possibility of dual models. 

There are several kinds of model-mimic relationships in nature. The most widely known is called Batesian mimicry, after Henry Walter Bates, Alfred Russel Wallace’s traveling companion in Amazonia. A Batesian mimic gains protection from its resemblance to a chemically defended species. The stock example is the relationship between the monarch butterfly, which sequesters toxins from the milkweeds its larva feeds on, and the viceroy (but recent research suggests the viceroy itself is unpalatable.) The Lorquin’s admiral, a relative of the viceroy, appears to mimic the oak-eating California sister. The pipevine swallowtail, loaded with aristolochic acid, has mimics in the eastern part of its range, although not in California. 

The other type of mimicry is Muellerian, named for another 19th-century naturalist, Fritz Mueller. In this version both species are toxic or unpalatable, and their close resemblance is a way of amplifying the don’t-touch-me message—a kind of branding. Among the best known Muellerian mimics are the tropical longwing butterflies you can encounter in the California Academy of Science’s rainforest exhibit. Some widespread longwing species have local races that match counterparts with more restricted distribution. 

It appears that at least one bumblebee sphinx moth, the European H. fuciformis, is distasteful to predators, thus arguably a Meullerian rather than Batesian mimic. Its larva feeds on honeysuckle, some species of which berries that are toxic to humans. That’s also true of snowberry, the primary food source of our local clearwing diffinis. I don’t know if anyone has done palatability studies with likely predators of this insect. 

Bumblebee moth larvae don’t wear obvious warning colors: they’re just plain green caterpillars, not unlike tomato hornworms in appearance. Maybe their leaf-matching camouflage is sufficient. 

When they reach their full growth, they drop to the ground, spin a cocoon, and pupate in the leaf litter under their food plants. 

Adaptive mimicry is a wonderful phenomenon. Even plants do it—there are species that lure pollinators to their nectarless flowers which resemble nectar-rich models. But it’s no guarantee against rotten luck. Looking like a bee won’t do you much good if you don’t watch where you’re flying.