I’m gratified to see that the History Channel is branching out into prehistory, with a new series on the evolution of various organs and systems. They started off last week with the eye and did a reasonable job, although the program was shamelessly vertebrate-centric: no mention of the remarkable eyes of the mantis shrimp, or the sophisticated camera eye of the octopus, so much like our own. (Richard Dawkins says eyes have evolved independently at least 40 times in the animal kingdom; no hope of covering all that in an hour, less commercials.)
But there was a nice segment on the compound eye, starting with the pioneering trilobites and their calcite lenses. And I can’t quarrel with the choice of the dragonfly as a living exemplar of this evolutionary pathway.
Dragonflies show up early in the insect fossil record looking pretty much as they do today, except for scale. Remember the coal swamp diorama in the California Academy of Science’s Hall of Life through Time, in which a dragonfly with a two-foot wingspan hovered above a millipede the size of a Volkswagen? The oxygen-rich atmosphere of the Carboniferous Period may have allowed insects and other arthropods to attain larger sizes than ever before, or since.
It would be a mistake to consider dragonflies primitive, though. Compared with other early insects, like the lumpish springtails and scuttling silverfish, dragonflies are elegant monuments to the shaping force of natural selection. Science writer Colin Tudge calls them “glorious aerial predators, like bats or swifts or peregrines.”
They’re fine-tuned for the pursuit of other insects on the wing.
A dragonfly’s eye is an integral part of that package of adaptations. Like the compound eyes of other insects (and most crustaceans and a few other arthropods), it’s composed of a multitude of tiny units called ommatidia, each equipped with a cornea, a lens, and photoreceptor cells that distinguish brightness and color. Up to 30,000 ommatidia are packed into each of a dragonfly’s eyes; a honeybee has only 4,500. A zone of specialized facets serves as the equivalent of the fovea in the vertebrate eye, enhancing visual acuity.
Somehow the dragonfly’s minuscule brain integrates all the incoming images, with simultaneous processing allowing it to track rapidly moving targets. The compound eyes wrap around the insect’s head, sometimes meeting at the top, providing a 360 degree field of vision. Dragonflies not only see all the colors we can; they can also see into the ultraviolet and detect polarized light, which they use to home in on ponds.
I supposed it was inevitable that someone would try to build a dragonfly eye. It was done a couple of years ago at UC Berkeley, in fact, by Luke Lee of the Bioengineering Department’s Bio-Poets (Biomolecular Polymer Opto-Electronic Technology and Science) project. His pinhead-sized creation has 8,370 crystalline ommatidia, each linked by a microscopic length of plastic to a retina-like detector. It can detect multidirectional light signals and rapid movement. Possible applications, according to Dr. Lee, include covert surveillance devices. Thanks. Just what we needed.
You would think the compound eyes would provide all the visual information a dragonfly would reasonably want. But no. It also has three simple eyes, the two lateral and single median ocelli. Scientists at the Australian National University in Canberra have been trying to figure out what they’re good for, beyond sensing general light levels. In the median ocellus, at least, the photoceptors feed into neurons that travel down the dragonfly’s neck to its motor centers. Bypassing the brain enables near-instantaneous physical responses to visual stimuli. The whole system seems to work as a flight stabilizer.
Spinoffs from this research may include better designs for robotic microaerial vehicles. It should not come as a surprise that the work was partially funded by the U.S. Air Force.
I don’t want to sound like a Luddite here, but this kind of thing can make you just a tad ambivalent about bioengineering. Yes, it’s extremely cool to be able to understand how dragonfly eyes and other complex natural mechanisms work. Yes, it’s an achievement to be able to mimic their function with manmade devices. But I wish we could do something with our knowledge of dragonfly optics other than building a better robotic spy plane.