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Hold that swat—maybe it’s a hover fly!

by Timber Press on March 7, 2017

in Natural History

Photo by Doug Waylett.

You feel an insect on your arm. You look down, see a yellowjacket—hold that swat! Don’t let a case of mistaken identity turn deadly. Learn about the clever disguise of the hover fly in this species profile from Natural History of the Pacific Northwest Mountains: Plants, Animals, Fungi, Geology, Climate by Daniel Mathew.

Hover Fly

Family Syrphidae
Also called “flower fly.”

Yellowjackets—a kind of wasp—crave your salami or your protein bar, but rarely land on your skin except to sting you. If this one hasn’t stung, trust it. Chances are it is a hover fly, a stingless insect that mimics bees or wasps in order to ward off predators.

Look closer: are the two antennae stubby and short, much smaller than the eyes? Hover fly. Or are they long, slender, and turned down? Bee or wasp. Is there a short, foot-like dark proboscis stepping around on your skin? Hover fly tongue, lapping up the salty sweat that attracted it to you. Flies also have two wings (the meaning of Diptera), and bees and wasps four, but that is less helpful; the narrow hindwings of bees and wasps (Hymenoptera: “membrane wings”) are translucent and hard to see in a live situation. As for those long, downturned yellowjacket antennae, some hover flies heighten their mimicry with a “dance” behavior, waving front legs that are darker than the other four to make them look like wasp antennae.

Do hover flies do more to earn our friendship than just slurp on us, look pretty, and decline to pack heat? Absolutely. They rank high among insects beneficial to plants, and hence to farmers. The larvae of many hover flies are the leading predators of aphids. In adulthood, hover flies are second only to bees as pollinators, globally; in our high mountain meadows, flies may do more pollinating than bees. Their pollinating style has one advantage over that of bees—carrying pollen farther—and one disadvantage— being less faithful to a plant species on any given day. Whereas bees use their long tongues to draw nectar out of deep flowers, flies have short tongues, visit shallower flowers, and eat more pollen than nectar. Both males and females visit flowers,the females gorging on pollen proteins they require to develop their eggs, the males snacking while patrolling territory in hopes of a chance at a receptive female.

Illustration by Eric Eaton.

Since they seek sweat and resemble bees, hover flies are sometimes conflated with “sweat bees,” a family of actual bees that earn that tag. Sweat bees are typically black or green all over and just 1/4-3/8 inch long. Our hover flies tend to measure 3/8-5/8 inch. A few thousand syrphid species have black-and-yellow-banded abdomens to mimic this or that well-armed bee or wasp. On some of our commonest ones, the thorax shines like polished brass and the huge eyes are maroon. Male eyes are plastered to each other on the top of the head; a narrow but distinct forehead separates female eyes.

Some entomologists campaign for the name “flower flies” as opposed to “hover flies,” wishing to raise the family’s favorables in the public eye. But hovering is more distinctive of this family: many insects pollinate, but few can compete with a hover fly at hovering. Bumble bees sashay from side to side as they descend upon a blossom, whereas male hover flies spend minutes at a time hovering perfectly. They are the insects you see maintaining a fixed position in a beam of light above a forest trail. Watch for the fly to zoom abruptly off to the side, then retake the same spot it held before. This is a male hover fly defending aerial territory where a female may show up. When he darted off , he was bouncing a rival. (Or was that the rival that won the confrontation and usurped the midair post? If you can tell which one of them came back, you’ve got quicker eyes than I do.)

A position in a beam of light can be maintained longer than one in the shade because the fly absorbs solar heat, supplanting calories he would otherwise burn to maintain optimum body heat for efficient hovering. This allows him more minutes hovering before he has to go find the next snack. Cost-benefit analysis confirms that the sunlit fly comes out ahead.

 

Daniel Mathews comes from a line of botanically knowledgeable forebears, who began teaching him the names of trailside plants at an early age. His writing is informed by literally thousands of scientific papers as well as five decades on and off hiking trails in the Pacific Northwest.

 

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