The Shark and the Jellyfish: More Stories in Natural History

Eye of the Needle

39 Eye of the Needle The streamside trail flattens. The water quiets and opens onto a still, shallow pond. You leave the ferns and the salamanders to their deep shade and find a seat on a log among the reeds and sunflowers by the shore. If you are quiet, the calm will return and you will be accepted into the scene. Finally, you extend your finger, as if pointing at the far side, and you wait. And if you are patient enough, a dragonfly will alight right on your fingertip. Earlier in the season, it may be a powder blue or a flame orange skimmer—harbingers of the springtide. In the height of summer, perhaps it will be a big, aggressive green or blue darning needle. And if you have an unusually steady hand, you can start to ever so carefully draw your visitor in closer. Hold your breath and pull it right up faceto -face, and you will be rewarded with the rare chance to gaze squarely into eyes as hypnotic as a revolving mirror ball—but even better, for these entrancing spectacles of biological engineering are alive, and are looking back at you. The dragonfly’s eyes cover most of its head with a spherical latticework of thousands of crystal facets. As you gently rotate your finger, you will see patches of color that stay centered in each of its eyes, no matter which way it turns. Though these eyes have no moving parts, two spots—one a bright highlight, the other a thin, deep shadow—fixedly hold the centers of their revolving orbs. These two marks travel across the compound eye, moving against the direction of their rotation, and—from your perspective—staying centered. One of these spots, a bright glint that holds its place above the center point of each eye, is an image of the sun. It stays fixed at a constant angle of reflection. The other, darker patch that stays positioned just below the sun glint arises from within—it is an image of the dragonfly’s retina. It is as dark as a pupil—light that falls into that spot does not return but is absorbed and processed by the animal’s brain. These two points—one of light, one of shadow—are constant features of the surface of the compound lens. They don’t move as the dragonfly’s head moves, giving the eyes the illusion of depth. It’s as though you were watch- 40 ing spots that moved more slowly than the surface because they were not on the surface, but deeper within. Yet the textured globe of those eyes is opaque, and pigmented in the colors of the animal’s flanks and wings. You cannot see through it. The dark pupil is a fiber-optic projection onto the finely patterned surface. It is carried in transparent strands connected to the brain below. Those optical fibers radiate out from their center in all directions like the spokes of a dandelion seed head. The dark window through them is visible only on the few lens elements that are pointed directly toward you. The deep image appears, then disappears from each fiber tip as it rotates in, then out of alignment with your line of sight. The shape of this dark pupil evolves as your viewing angle changes. The light-carrying fibers have been arrayed to optimize the insect’s command of its air space. So as you gradually rotate the dragonfly to face you, aligning your line of sight with its line of flight, the dark pupil grows larger—like the eye of a predator dilating when prey comes into view. Rotate the creature further, and the dark region contracts to a thin cat’s eye that glances to the side and behind. Track the dark image of the retina up the mosaic lens to the top of the head, and the geometry of the matrix tightens until the pupil contracts into a single black point; from that angle, the insect is squinting at the noonday sun. These biological star sapphires take longer to mature than do the eyes of most insects. Though dragonflies live only briefly as adults, their lifespan is quite long by insect standards. Season after season during the ice-free days of spring and summer, dragonfly nymphs develop underwater, stalking the streambed. The nymph may grow for up to five years before it emerges through the surface to begin its week or two of adulthood. It will...