Nature Journal

6. The Wheel of Life

47 ß chapter six The Wheel of Life This piece is linked to the previous one in that it’s based on an observation along the same stretch of the Cahaba River—again, something that I saw but didn’t immediately understand. Fortunately , this time I had Dr. Randy Haddock of the Cahaba River Society with me, and he very plainly and accurately described the wonderful wheel formation of damselflies. Snake doctors, devil’s darning needles, horse stingers, and mosquito hawks—these colorful names reflect the folklore and superstition of dragonflies and damselflies. Winged emblems of victory in Japan, their less-esteemed American cousins purportedly sew children’s ears together and resuscitate dead snakes. Biologists place the two closely related groups—ferocious , demonic dragons and refined, dainty damsels—in the order Odonata; the name refers to the tooth-like ends of their abdomens. (Sorry—no stingers or needles!) Fleeting streaks of iridescence, these slender cylinders sport tasteful (and perhaps tasty) pastels, harsh metallic 48 ß chapter six tints, or gaudy stripes and blazes. Two pairs of wings—perfectly equal in damselflies, not quite so in dragonflies—adorn the thorax. (Thanks to independent control of each appendage, the bearer can brake sharply, spin quickly, and even fly backwards.) At rest, the latter group keeps its wings perpendicular, ready for instant flight, while the former folds them neatly along the back. Damsels and dragons depend on water—in fact, they spend the majority of their lives as darkly camouflaged aquatic larvae. Skulking in the shallow bottoms of streams, ponds, lakes, and marshes, these insatiable nymphs or naiads bushwhack and dispatch an assortment of hapless critters—including brother/sister odonates and even small fish—thanks to a monstrous distensible lower jaw. This labium shoots out half the naiad’s body length, skewering a victim on pointy prongs before yanking it back toward churning teeth. In turn, unlucky little larvae serve as tasty snacks for bigger brethren. To respire underwater, damselfly larvae rely on a trio of caudal gills, fanning them out into the water column to gather much-needed dissolved oxygen. In contrast, dragonflies respire rectally, sucking in fresh water through the anus and—just like a rubber bulb syringe—expelling it by the same route. More than just good clean fun, the resulting jet propulsion hurtles a nymph toward waiting prey or scoots it away from danger. After many months (and molts), an odonate adult-to-be swims warily to the water surface, inspiring air for the first time while crawling up a stem or rock. As internal pressure increases, the larval “skin” or exoskeleton cracks open; the new adult slowly and eerily rises, a weak and the wheel of life ß 49 vulnerable ghost. Resting briefly to harden its body and inflate its wings, the imago soon flies away, leaving a lifeless case (exuvium) still tightly gripping the substrate. On this maiden flight, and while fully maturing, adult dragonflies may venture far from their watery birthplaces; damselflies stay much closer. But they both return, cruising the stream bank or pond margin, entangling mosquitoes, mayflies, and smaller kinfolk in their spinylegged “baskets” before savagely devouring them. Miniature hawks or eagles, odonates enjoy superior sight, with huge, protruding, goggle-like eyes taking up much of their heads. (A housefly has 4,000 units making up one compound eye; a dragonfly may have seven times as many.) And those heads swivel readily on a slender neck, engineering motion perception unparalleled in the insect world. Eyesight forms the basis for odonate sex—in big words, “visually mediated reproductive behavior.” A male stakes out a suitable territory and—goggles securely in place—fights Snoopy vs. Red Baron aerial battles to drive all competitors away. Anticipating that a willing partner will wander close, he curls his abdomen downward and forward, transferring sperm from true genitalia on segment 9 to a pouch containing secondary genitalia on the underside of segment 2. (That’s right, guys—he mates with himself, and has two sets of parts!) Hovering over a hovering female, he cradles her head and thorax with his legs, then grabs the back of her neck with anal claspers. Firmly connected, they fly off in tandem like two tiny airplanes, one being refueled by the other. But no “fuel” transfers until the bride, safe on a perch, imitates the groom’s previous contortion , looping her abdomen downward and forward to connect to his 50 ß chapter six accessory genitalia, completing...