The Forest Unseen: A Year's Watch in Nature (19 page)

M
y body is tense as I pick through the misty air on my way to the mandala. I’m walking in the partial darkness of dusk. I place my feet carefully, straining my eyes through the gloom, searching for snakes in my path. Copperheads,
Agkistrodon
contortrix
, the “hooked-tooth twister,” concern me most. These snakes are particularly active on muggy summer evenings. Tonight, the copperheads’ favorite summer snack is emerging. Hundreds of cicadas crawl up from their underground larval burrows. Snakes are surely on the prowl. I am loath to scorch my eyes with the reflected glare of a flashlight, so I move slowly, seeing the copperhead’s leafy camouflage everywhere in the failing light.

My fear of predators was likely imprinted on my psyche by millions of years of natural selection. Tropical primates with poor night vision seldom live long if they have cocky attitudes toward the dark. Like all other living creatures, I am the descendant of survivors, so the fear in my head is the voice of my ancestors whispering their accumulated wisdom. My conscious mind chimes in with zoological fearmongering: long-hinged fangs, painful blood-destroying venom, a pit near the eye that catches minute changes in temperature, a strike that lashes out in a tenth of a second. I reach the mandala, and its familiarity eases my tensions. Another whisper from the family tree: what is known is safe.

A firefly’s flash greets me as I sit. The green light rises sharply several
inches, then holds steady for a second or two. The evening has just enough daylight for me to see the beetle as well as its lantern. After the green glow dims, the animal hangs motionless for three seconds, then swoops down and across the mandala. The beetle then repeats the quick glowing ascent, lightless pause, and plunging departure.

If I were a real firefly connoisseur, I could identify the firefly’s species by the distinctive rhythm and length of its flash, but such skills sadly elude me. During the day, I have used my field guide to identify fireflies from the genus
Photuris
clambering over the mandala’s vegetation. Nightfall is too far advanced for me to pick out whether or not this individual is a
Photuris
, but the rising flash identifies him as a male. His flash is the opening line to what he hopes is a conversation with a future mate. He throws the line across the leaf litter, casting for the response that too often fails to come. After his flash, the male scans the forest floor, holding steady to give the female a chance to respond, then flies off to continue his search. Occasionally a female will return a flash from her hiding place, and the male will fly to her, flashing again. The pair signal back and forth several times, then mate.

If the firefly over the mandala is a
Photuris
, his mate will have an extra trick of the light to perform after they have bred. Once a female
Photuris
has finished the unexceptional task of luring suitors and mating, she turns her attention to the males of other firefly species. The unique flashing sequence of each species usually keeps males and females of different species apart. Just as we have no interest in the sexual signals of gorillas, fireflies ignore flashes from species other than their own. But
Photuris
females mimic the answering signals of other species, drawing in hopeful but hapless males, then seizing and devouring them. After walking down the aisle, the grooms become the wedding feast; the bride who seemed so appealing from afar turns out to be a very hungry gorilla. The femme fatale uses her prey not just for food but as a source of defensive chemicals. She steals these noxious molecules from her victim, then redeploys them inside her own body. If a spider catches her, she bleeds out the chemicals, repelling her attacker.
The forest floor, it seems, is full of hooked-tooth danger on these warm summer evenings.

But danger is only part of the story. Fireflies bring delight also, enchanting us with their sparkle and glow. Like the brilliance and hue of flowers, or the exuberance of birdsong, the twinkle of fireflies opens a window, blowing away the mist that stands between us and a truer experience of the world. When laughing children chase after fireflies, they are not pursuing beetles but catching wonder.

When wonder matures, it peels back experience to seek deeper layers of marvel below. This is science’s highest purpose. And the firefly’s story is rich in hidden wonder. The beetle’s flash invites admiration for evolution’s ability to cobble together a masterpiece from unremarkable raw materials: the lantern at the tip of the firefly’s abdomen is made from standard-issue insect materials but assembled in such a way that the insect becomes a glowing forest sprite.

The insect’s light flashes from a substance called luciferin. Like many other molecules, luciferin will combine with oxygen and turn into a ball of energy. The ball eases its excitement by releasing a packet of energy in motion, a photon that we perceive as light. Luciferin is similar in structure to other household molecules in the cell but, presumably through several mutations, has become particularly susceptible to overexcitement and relief. The molecule is assisted by two other chemicals whose job it is to whip luciferin into an overstimulated state.

Fireflies have therefore supercharged their internal chemistry to turn a glimmer into a glow. But chemicals alone produce at best a weak, diffuse light. The firefly’s lantern is arranged to focus this potential into the flashes and counterflashes with which the courting fireflies so carefully time their prenuptial conversation. The lantern achieves this control by regulating the flow of oxygen to luciferin. Each cell in the lantern buries luciferin molecules in its core, then surrounds them with a thick mat of mitochondria. The usual function of mitochondria is to provide power for the cell, but the firefly’s lantern uses them as oxygen sponges. Under normal conditions, any oxygen that seeps into
these cells is quickly burned up in the mitochondria, leaving none to reach the core and stimulate luciferin. This layer of mitochondria is the firefly’s “off” switch. When the time comes to flash, a nerve signal shoots into the lantern and causes a gas, nitric oxide, to flush out of the cells at the nerve tips. The gas shuts down the mitochondria, and oxygen blasts into the interior, igniting the chemical glow.

The firefly’s flashing mechanism takes two ubiquitous features of animal physiology, mitochondria and nitric oxide, and combines them into an elegant and, as far as we know, unique light switch. The lantern’s architecture is likewise a tinker’s triumph, turning ordinary cells and the insect’s breathing tubes into an airy home for luciferin. The tinker’s work is no slipshod job. Over ninety-five percent of the energy that is used in the firefly’s flash is released as light, a reversal of the performance of human-designed lightbulbs that waste most of their energy as heat.

In the sky above me night’s darkness is complete. But as I stand to leave the mandala I see a forest full of lights. The fireflies stay within two or three feet of the ground, and from my standing position I look down on a swaying surface, a sea of glowing buoys. I light my path past imagined copperheads with my own lantern, pondering the contrast between my flashlight’s inefficient industrial design and the biological wonders that dance all around me. But this is an unfair contest. I am comparing an infant with a sage. Our flashlights have barely two hundred years of thought behind them and have developed in a sea of abundant fossil and chemical energy. Humans have applied little effort to improving the prototypes of our first electric lights. With limitless fuel, why should we? In contrast, millions of years of trial and error stand behind the firefly’s design. Energy has been in short supply for the beetles all along, producing a lamp that wastes little and uses beetle food, not mined chemicals, as its fuel.

I
t is midafternoon, but deep shade weighs on the mandala. The nadir of the year’s cycle of daytime brightness has arrived. Now that summer is at its peak, the mandala’s surface is darker than at any other time of the year. Even the winter solstice is brighter at ground level than is July’s gloom. Greedy layers of maple, hickory, and oak leaves suck the sun’s rays, stealing all but a fraction of a percent of the light that hits the canopy. Times are hard for the forest herbs; no wonder so many hurry through their yearly business in a few weeks of sunny springtime. Those low-growing plants that have not retreated into dormancy are adapted to lean living, scrounging light with leaves designed to persist on scraps. These forest herbs are the rangy desert goats of the plant world, with small appetites and thrifty flesh.

Suddenly, a column of intense light slants through the haze, beaming through a chink in the canopy and illuminating a single mayapple leaf in the mandala below. The mayapple shines in the spotlight for five minutes, then the beam’s slow swing picks out a maple seedling, then another. Over the course of an hour the circle of brightness crawls over a
Hepatica
’s tri-lobed glossy leaf, onto sweet cicely, up into the spicebush, then across the jagged leaves of leafcup seedlings.

No plant gets more than ten minutes in the sun’s eye before it is again covered by the blanket of shade. Yet fully half the plants’ daily ration of light may arrive during the sunfleck’s brief visit. The goats are given a few minutes at the feed trough before returning to the
desert. But a bonanza of food can bloat and kill a hungry goat. Likewise, this sudden illumination is a mixed blessing for the mandala’s plants. A dearth of light is a hardship that may eventually weaken a plant, but a sudden excess can wreck the leaf’s thrifty economy, permanently impairing its function. Leaves in a sunfleck must therefore speedily rearrange their bodies to accommodate the sun’s blast of energy.

Leaves are of course designed to snare light’s energy and put it to work. They do this by deploying light-harvesting molecules that catch sunbeams and turn them into excited electrons. These electrons are whisked away, and their sparkle is used to power the plants’ food-making machinery. But when too much light hits an unprepared leaf, energized electrons cannot be processed fast enough, and they wash around the delicate light-harvesting molecules, overwhelming them with their undirected agitation. Like a one-volt motor plugged into the wall, the leaf gets zapped. Plants adapted to shade are particularly vulnerable to damage from restless electrons. They have many more light-harvesting molecules than electron-processing molecules, so a sunfleck could easily overwhelm their inner architecture.

To cope with the arrival of a sunfleck, plants unplug some of their light-harvesting molecules before they can gather too much energy. At the first sign of trouble, an essential piece of the harvesting apparatus temporarily moves away from its usual location, returning only when the situation has calmed. This is like cutting a wire within an electric motor, stalling the motor’s operation, then rejoining the wire’s ends to start the motor again. A buildup of electrons also causes the stack of membranes that hold light harvesters to loosen, allowing energy to flow to the interior where electron processing takes place. The chloroplasts that contain all the photosynthetic machinery respond to the sunfleck by rolling to the edges of the cell, turning their faces away from the sun. In this way they protect the molecules within. When the sunfleck passes, the chloroplasts move back to the cell’s upper surface, basking like lily pads in the forest’s weak light.

The plants’ response to the sudden influx of light is paradoxical. They unplug and roll away, seeming to shun the very thing that they have been seeking. The mandala’s herbs sip at a mean trickle of light for most of the day, then hide their mouths under an umbrella when the deluge comes. But such is the force of the sunfleck’s downpour that water splashes under the umbrella’s rim, and plants receive a mouthful of life.

The sunfleck’s sweep across the mandala illuminates everything in its path. A spiderweb glows silver in the glare, its invisibility ruined by the bright light. The leaf litter turns sandy bright and jumps into relief as dark shadows emerge. Iridescent wasps and flies shine like metal shavings scattered across the mandala.

The mandala’s insects seem drawn to the circle of light, staying within its bounds as the sunfleck moves over the mandala. The most fastidiously loyal of these insect followers is a group of three ichneumon wasps. When a wasp steps out of the brightness, it immediately turns and scuttles back. The flies that also scuttle over the mandala have a looser attraction and make forays that last a minute or more into the darkness.

The sun-worshipping wasps overflow with nervous energy. They run frantically from side to side, constantly flicking their antennae and wings. They run their quivering antennae over and under every leaf in the sunfleck’s small world. Every minute or two the wasps flip onto their sides and shudder their legs together, cleaning away the silk that spiders have strewn over the mandala. After the rubdown, the wasps jump back onto their feet and start over on their tremulous way.

The wasps’ frenzy has a sharp purpose. They hunt for caterpillars on which to lay their eggs. Wasp larvae will creep out of the eggs and bore into the caterpillars’ flesh, then larva will eat caterpillar, slowly, from the inside out, leaving the vital organs until last. The caterpillars live stoically on, feeding and digesting leaves even as their lives
are stolen from within. These hollowed-out caterpillars therefore make excellent hosts, continually replenishing what the parasite robs.

The wasps’ parasitic life cycle inspired one of Charles Darwin’s more famous theological comments. He thought the ichneumon’s trade was particularly cruel. These wasps seemed incompatible with the God he knew from his Victorian Anglican training at Cambridge. He wrote Asa Gray, the Presbyterian botanist at Harvard, “I cannot persuade myself that a beneficent and omnipotent God would have designedly created the Ichneumonidae with the express intention of their feeding within the living bodies of caterpillars.” For Darwin, these wasps were the “problem of evil” writ in the script of the natural world. Gray was unconvinced by Darwin’s theological arguments. While he continued to support Darwin’s scientific ideas, he never abandoned his belief in the compatibility of evolution and traditional Christian theism. But suffering weighed heavily on Darwin; his body was always ill, and his spirit was bruised by the early death of his favorite daughter. As the dark years wore on, the weight of the world’s pain pushed him from vague deism to skeptical agnosticism. Ichneumons were a symbol of the suffering he carried within, and their existence made a mockery of the God whose providence the Victorians saw written all over the natural world.