Butterfly-hawk?

I was perusing my current edition of NaturePhile – Balsam Mountain Preserve’s electronic newsletter – when I saw the photo below. Perhaps “dragon” fly is a more appropriate name for the mosquito hawk of my youth.

The photograph taken by BMP chief naturalist Michael Skinner shows a gray petaltail, Tachopteryx thoreyi, dispatching a just-captured eastern tiger swallowtail, Papilio glaucus. Dragonflies are noted for their voracious appetites. The bulk of their diet consists of mosquitoes, flies, gnats and other small insects.

However, the dragonfly is a very opportunistic predator and it is not uncommon for some of the larger species to take butterflies and/or moths. There are even records of common green darners, Anax junius, taking hummingbirds. Some take other dragonflies – even their own species.

Other eastern dragonflies noted for preying on larger insects include the swamp darner, Epiaeschna heros, the dragonhunter, Hagenius brevistylus and the eastern pondhawk, Erythemis simplicicollis.

A combination of incredible vision and extremely agile flight make the dragonfly a quite successful predator. In one study by neurobiologist and dragonfly expert professor Robert Olberg of Union College in Schenectady, N.Y., hunting dragonflies exhibited a 97-percent success rate.

The large, bulbous compound eyes of dragonflies give them a 360-degree field of vision. The dragonfly’s compound eye is composed of hexagonal planes or facets. Some dragonfly eyes contain as many as 30,000 separate facets. Each of these facets is oriented in a slightly different direction. According to researchers the dragonfly sees a mosaic of slightly overlapping images. And there is specialization in eye form and function among dragonflies.

Light-sensitive proteins called opsins allow dragonflies and humans to see color by absorbing a particular color. Humans have three opsins and absorb red, green and blue light. Day-flying dragonflies have as many as five opsins allowing them to see more colors including ultraviolet (UV) light. According to Olberg, in day-flying dragonflies the upward pointing facets gather blue and UV light while the downward pointing facets collect longer wavelengths like orange and green.

Researchers believe this scheme is quite advantageous for day-hunting dragonflies. Dragonflies capture their prey by rising up from below it. The blue and UV spectrum of the upper facing facets provide a crisp clear background for tracking their prey.

Dusk-hunting dragonflies have fewer but larger facets in their eyes, which allows for greater light collection. They also absorb only one spectrum of light – green, which provides the greatest range of light sensitivity.

Coupled with this amazing vision are exceptional flight capabilities. Dragonflies can approach speeds of 40 mph. Their four wings are capable of independent motion allowing for amazing maneuverability. Dragonflies can stop, hover and even fly backwards.

According to Olberg, dragonflies capture their prey by intercepting it in flight. Researchers believe dragonflies stay on target by maneuvering to keep the image of the prey in a particular retinal position. Olberg compares this interception with an outfielder’s ability to track down and catch fly balls. The fielder’s tactic, according to Olberg, is to properly position himself to restrict the retinal slip of the image of the ball to an upwardly projecting straight line, this allows the fielder to get to the proper point to intercept the ball – say, hey!

There are, of course, differences. The ball player is restricted to the playing field whereas the dragonfly operates in three dimensions. And with its amazing maneuverability is capable of restricting retinal slip in all directions. The other difference is that the human effort is a learned ability while Olberg believes the dragonfly is “hardwired.” He believes pairs of neurons from the brain that project to the flight control circuitry of the animal’s thoracic ganglia automatically guide the dragonfly to its prey. He and colleagues are presently studying this hypothesis.

There are fossil records of dragonflies from 300 million years ago. These creatures haven’t changed much from their ancestors except in regards to size. Today’s largest dragonflies, from South America, have a wingspan of about seven inches. The largest fossil record has a wingspan of two feet. One can only imagine what kind of prey this prehistoric predator might have intercepted.

(Don Hendershot can be reached at ddihen@earthlink.net)