The Sound of Wings

On a summer afternoon walk in the rain, I watched storm clouds rolling over Jackson Hole in massive clots of darkness.  Near a streetcorner, I passed the open back door of a dry cleaner’s.  Despite the rumble of thunder and the drone of industrial vents, I heard a soft whirring noise.  I immediately glanced around me for a hummingbird; I knew that unmistakable sound of wings.  Down near my feet, in a landscaped plot of drooping wet yellow lilies, I saw movement among the flowerheads.

It wasn’t a hummingbird.  Instead, a moth the size of a hummer, with intricate brown, pink and white lined wings, darted in between the lilies.  It hovered while delicately sipping from the flower throats, and I wondered at the immense weight of its body carried on fragile wings so aerodynamically.  I watched for a while, until suddenly the moth flew up higher and zipped through the dry cleaner’s door, disappearing among the rotating racks of clothing.

Later, I found a photograph of a White-Lined Sphinx Moth (Hyles lineata).  The sphinx, or hawk moths, of the family Sphingidae, sometimes fly by day and are mistaken for hummingbirds.  Not only are their wingspans approximately the same as the smaller birds, but these moths pollinate similar flowers, they’re often brightly colored, and their wings make the same humming noise.   Sphinx moths are among the masters of maneuvering flight in the winged world.  They can fly backwards, dip, and dive.

Their most remarkable feat is their ability to hover before huge blossoms while drinking nectar.   The majority of flying insects move their wings in true hovering flight.  Moths use more than the upstroke and downstroke typical of winged flight.   The Hummingbird Hawk Moth (Macroglossum stellatarum), like the Ruby Throated Hummingbird (Archilochus colubris), beats its wings at around 70 beats per second, the fastest of all birds and moths.  Hawk Moth wings can rotate almost 180 degrees as they sweep cyclically from front to back in a figure eight motion.   The naked eye sees a quick blur of color, but the scrutiny of high-speed photography and insect morphology allows us a better understanding.

In hummingbirds, all this is possible because the wing bones are short and nearly inflexible, and a loose shoulder joint allows for pivotal movement greater than any other bird.  But in insects, this phenomenon occurs with boneless wings.  At the base of each wing are several sets of powerful muscles.  Two sets of unattached, or “indirect” muscles contract alternately to cause surface changes on the insect’s body.  Because a wing is an outgrowth of an insect’s body wall, these surface changes can actually move a wing up and down.   Meanwhile, four sets of attached, or “direct” muscles tug a wing forward, backward, and in a rotary path.  Sphinx moth forewings are also narrow and tapered for greater speed.  These moth wings have the thickest veins along the front edge, providing the strength and rigidity necessary for their dexterous flight performance.

Insects are are the oldest winged beings.  From flower to flower, they have been pollinating the earth for about 350 million years, and their evolution is closely linked to that of flowering plants. Insect wings evolved from accessory appendages, unlike birds, whose wings are modified limbs.  A bird first takes to the sky through the teaching of parents, but when an insect flies for the first time, it is untaught flight.  While feathers are amazing and functional works of natural art, the wings of Lepidoptera (the butterflies and moths) have their own “feathers.”  The name of their order comes from the Greek for “scale wing.”  Each moth wing is layered with hundreds of infinitesimal scales which protect the wing’s membrane and refract light into color, much like the feathers of a hummingbird’s throat.  Linnaeus himself organized only the insect orders by the characteristics of their wings.

As I watched the sphinx moth fly into the dry cleaner’s, I reflexively and wrongly thought of moth-eaten sweaters and mothballs.  Now, I think of ancient wings, of creatures that inhabit the skies of night and storm, searching for flowers.  I will remember this the next time I hear the familiar sound of whirring wings, and wonder if I might indeed be mistaking a hummingbird for a sphinx moth.

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