The Forest Unseen: A Year's Watch in Nature

by DavidGeorge Haskell

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.

  July 27th—Sunfleck

  It 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 molec
ules, 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.

  Theologians have tried to answer Darwin’s challenge, but theistic philosophers have, perhaps unsurprisingly, little insight into the lives of caterpillars. Caterpillars are assumed to have no souls or consciousness, so their suffering cannot be a mechanism for their spiritual growth or a consequence of their free wills. Another argument claims that caterpillars don’t really feel anything or, if they do, their lack of consciousness means that they cannot think about their pain, so the pain isn’t true suffering.

  These arguments miss the point. Indeed, they are not arguments but restatements of the assumptions that are being challenged. Darwin’s claim is that all life is made from the same cloth, so we cannot dismiss the effects of jangling nerves in caterpillars by claiming that only our nerves cause real pain. If we accept the evolutionary continuity of life, we can no longer close the door to empathy with other animals. Our flesh is their flesh. Our nerves are built on the same plan as insect nerves. Descent from a common ancestor implies that caterpillar pain and human pain are similar, just as caterpillar nerves and human nerves are similar. Certainly, caterpillar pain may differ in texture or quantity from our own, just as caterpillar skin or eyes differ, but we have no reason to believe that the weight of suffering is any lighter for nonhuman animals.

  The idea that consciousness is a humans-only gift likewise has no empirical basis: it is an assumption. But even if the assumption were correct, it would not resolve Darwin’s ichneumon challenge. Is suffering greater when pain is embedded in a mind that can see beyond the present moment? Or, would it be worse to be locked in an unconscious world where pain is the only reality? A matter of taste, perhaps, but the latter option strikes me as the poorer one.

  The sunfleck has swung across the mandala and now shines on my legs and feet. It moves on and beams directly on my head and shoulders, like a caricature of divine inspiration. The Sun Goddess unfortunately sends no sudden insight into the knots of philosophy; rather, she starts the sweat running down my face and neck. I’m feeling the energy that sustains the wasps’ fidgeting dance across the forest floor. Their bodies are so slight that even a few seconds in the sun will raise their temperature by several degrees. To keep from roasting, the wasps send air currents flowing over their bodies, keeping a second-by-second balance between the inpouring of the sun’s rays and the outflow of heat by convection. My own oozing sweat is the sluggardly response of a bulky mammal for whom heat balance is measured in hours, not seconds.

  The sunfleck finally falls off my right shoulder, leaving the mandala as it travels east. The troubling wasps move with it. As the sunfleck flows away, dimness returns to the mandala, and I find that my senses have been changed by the experience of the sunfleck’s passing. Now as I gaze around the forest I see not the uniformity I knew before but constellations moving over a dark sky.

  August 1st—Eft and Coyote

  Rain has drawn the leaf litter’s humid world into the open. The litter’s inhabitants scuttle exposed on the mandala’s water-glazed leaves. The largest of these explorers is a salamander, a red eft, which stands on a mossy boulder, peering into the haze.

  The eft’s belly and tail rest on the rock. The animal’s chest curves up, held by a push-up of spread front legs. The head is level and still. Eyes like droplets of gold stare unmoving across the mandala. Unlike the skin of most salamanders, the eft’s looks dry, like crimson velvet, even in the heavy mist.

  Two rows of bright orange spots run down the eft’s back. These spots beam warnings to birds and other predators: stay away, toxins! The eft’s skin is impregnated with poisons, giving the animal a shield against predation that most other salamanders la
ck. Efts are therefore confident, sauntering aboveground while most salamanders skulk below. This boldness explains the eft’s unusually dry skin. Unlike their timid, light-fearing cousins, efts have thick, relatively waterproof skin that can withstand the daylight glare.

  The eft holds still for a couple of minutes, breaks its trance with five steps across the moss, then halts and freezes again. Most likely it is searching for gnats, springtails, or other small invertebrates, using alternating bouts of quiet watching and surging movement to sneak up on, flush, and grab its prey. This is a common hunting tactic. Watch a robin on a lawn or a human searching for a lost cat and you’ll see the same pattern of movement.

  The eft’s walking style is clumsy. Legs sprawl away from the body and oar the ground. A back leg swings out and forward, then the front leg on the opposite side, then the other back leg. The spine curves from side to side as the legs move, throwing the legs out and forward. This horizontal sway of the spine is like a fish swimming. Although the bones and muscles of efts are adapted to a terrestrial existence, their overall walking style is a fishy wiggle. This sideways twist works well for animals swimming against the all-encompassing solidity of water or soil, but on two-dimensional surfaces the writhe is inefficient—salamanders have to balance on three legs (or on their bellies) as they swing out one leg at a time. A panicked, running salamander is a whir of flailing limbs.

  Terrestrial vertebrates whose lives require speed have reworked the fishes’ ancient architecture at least three separate times. The ancestors of mammals and two lines of dinosaurs each came up with modifications to the sprawling inefficiency of the fish-on-land. Legs moved in and under, putting the animal’s weight directly over its feet. This made it easier to balance and, therefore, to run without toppling over. The spine’s side-sway was replaced with an up-and-down flex. Mammals are masters of this flex and can reach forward with both forelegs while pushing off with the combined power of both hind legs, then curve the spine down and stuff their forelegs back while swinging the hind legs forward to plant them ready for the next push-off. No salamander can match the bounding gait of a mouse, let alone the enormous leaps of a running cheetah. This newfangled spine has, ironically, returned to the ocean to compete with the old fishy spine. Whales move their tails up and down, rather than side to side, revealing their terrestrial ancestry. Mermaids, it seems, do the same.