On Looking: Eleven Walks with Expert Eyes, page 10
Other leaves were intact but unusually shiny. A resolute shiny streak indicated that a slug had been sliming along the leaf overnight. A slap-dash shining job was probably a wash of “honeydew,” the clear excrement of aphids, which itself draws other bugs and birds to feed on the sticky stuff. Holes and slime are only the beginning. A single leaf, nonchalantly fanning itself in the breeze, might be the repository of one of dozens of types of sign.
“Here’s something,” Eiseman said—and repeated on our walk together. There was always something. In this case, it was a browned, curving scribble on a leaf. “This is a leaf mine of a fly larva,” he explained, ending with a period. He must have sensed that I thought at least ellipses were due, and elaborated: “The fly inserted its egg there”—he pointed at the base of the trail—“and the larva is living between the epidermal layers, and is munching along and making a wider trail as it grows bigger.”
My mind boggled a small boggle. The strip of denuded leaf we were looking at was a path cleared by a young fly who was growing up sufficiently quickly that the path he left in his wake had widened over the course of his living on that one leaf.
These leaf mines, I learned, are left mostly by moths, beetles, and flies, whose larva create a visible trail as they plow along, chomping leaf tissue. They spend their whole larval life in one leaf and then emerge as an adult. Many of the mines are very particular to the species of insect: the female always inserts her egg in a particular place, such as at the leaf’s edge or base. As a result, by looking at the starting point of the mine one can (if one knows quite a bit about insects) determine the larval species that is living there. The type of trail is species-specific, too. Some leave serpentine trails that draw an inscrutable image along the canvas of the leaf. Other mines cruise along the veins of the leaf. Still others are more blotch than trail, growing pools of ungreen leaf. We saw mines that followed a serpentine course and ones that hugged the leaf ’s perimeter: leaf-as-jogging-track.
It is not mines but galls that are the crowd-pleasers of the insect-track-and-sign world. “Some of the galls are what draw people in first,” Eiseman said with a straight face. This was initially hard to believe. A gall is a growth, a plant tumor, caused by a critter burrowing into the tissue of the plant when it is developing. The small lump, fold, or pouch that results serves as shelter and often food for the nymphs that the midge, sawfly, moth, aphid, wasp, or mite (not technically an insect) lay in it. To my eye, many galls looked a bit cancerous, the leaf blighted or with an embarrassing skin condition.
“How do the plants endure all this?” I wondered aloud. “Most of it seems so destructive.”
“Being deciduous helps: they get to refresh their leaves every year. Galls are a sort of agreement that’s been worked out, concentrating the damage to one spot. It gives the insect shelter and food.”
“The tree doesn’t get anything.”
“It gets less damage.”
Most galls do no serious harm to their tree hosts. Some galls are almost picturesque—and adorably named. I could imagine trying to hunt for the fuzzy red “hedgehog” gall; the flamboyant “sea urchin” gall, boldly pink and spiky; or the “wool sower gall,” a soft dotted pom-pom that reaches outward like a flower.
Each of these winsome galls actually holds tiny wasp larvae that reside on oak trees. If your city has oaks, you could find galls down the street. When I returned to New York City, I was barely out of the train before I come across an oak and quickly found a gall, green and pealike, on a leaf.
I started to wonder about wasp aesthetics: their galls are architectural beauties. If not inspiration for humans, there is a theory that galls might have served as a sort of inspiration for trees. As Eiseman described it, the theory suggests that galls kick-started trees into bearing us the fleshy fruits we now cultivate greedily, such as peaches and plums. If that were the case, fruits are really just evolutionary extensions of galls, developing because of wasps laying their eggs in the plant’s flowers, eventually altering the plant’s genome to produce galls that become bigger and more nutritious over time.
Galls are also a very particular sign: gall insects usually choose a specific place to induce a gall, as at the midrib, edge, or underside of a leaf. Most are on leaves, but some insects choose branches, twigs, or even flowers. You can open one up to reveal the tightly packed, sleeping larval species inside—or you can just infer the species from the shape and location of the nub.
• • •
Surprisingly, those leaves that have no sign, no holes, no smattering of excrement, are themselves sign of something else. They indicate that the tree is probably not from around here. Although some bugs are generalists, knocking about in our faces or homes or wherever they can find nourishment, many are extremely plant-specific. They might be born, grow, eat, mate, and die on the same plant. One type of plant—or even one individual plant—may be their entire universe. Plants native to North America have their own community of North American bugs that have evolved to live on and with the plant. However, non-native—what are increasingly called invasive1—plants are often new enough to the area that no native bug has yet evolved to specialize in them. As a result, the invasive plants do not need to put any of their resources into defense chemicals or strategies; they can put all their energy into growing and reproducing. That is why invasive plants invade so well: they can spread quickly while native plants, struggling against the bugs, are lucky to maintain their numbers.
You can, thus, make a reasonable guess as to whether a tree is native by checking out its insect population. Walking by the lovely Norway maples, five-fingered leaves robust and hole-free, I suddenly realized that the “Norway” was not a rhetorical turn. The tree I was accustomed to seeing throughout New York City is an immigrant. Even the city Parks Department logo is a silhouette of what is probably a maple (or London plane, another non-native tree) leaf. The trees we passed looked gorgeous, each leaf custom-printed, dry-cleaned, and pressed. And largely sign-free.
• • •
Though Eiseman had not given any urban insect-sign tours, I was starting to think there may be a call for them. One might imagine that a typical city does not host too many bugs (outside of cockroaches and bedbugs); this walk was convincing me otherwise. Eiseman flipped over a stone and rocked a log with the toe of his shoe, causing shiny dark bugs to move quickly under the nearest leaf litter. In some ways, he suggested, cities are not unnatural so much as they are concentrated nature. On his cross-country tour, highway rest areas turned out to be gold mines for bug-tracking. “It almost seems like in someplace like that, where there is just a little scrap of nature, life is more condensed.” Upon entering Texas, they discovered the Texas leaf-cutter ant at the parking lot where they had pulled over for a pit stop. Then, on going into national parkland, “we barely saw anything,” he said. The density of insect life was not the same, or maybe the method of exploring—wandering along waiting for something to pop up, rather than poking into every crevice at the rest area—is not conducive to finding sign.
Certainly if a rest area provides opportunity to find sign of insect life, a city must. Eiseman cataloged some urban elements that lead to good insect-sign hunting—beginning with the city’s tendency to never shut down. When Wabash, Indiana, was the first town to light itself up at night with electric light, onlookers were flabbergasted: when the lights went on, “people stood overwhelmed with awe,” the local paper reported. “Men fell on their knees, groans were uttered at the sight, and many were dumb with amazement.” We have grown accustomed to the ordinariness of nighttime lighting, but many insects are still in its thrall. Lit all the time, cities attract insects whose compound eyes are specially tuned to the short wavelengths of UV light, found in incandescent lamps and many fluorescent lamps. City streetlights are now usually a more energy-efficient sort, such as high-pressure sodium vapor lamps, which emit less UV light but are still a siren call for various fliers. Insects use these light waves to find and choose mates, navigate, hunt, even migrate. So they must get very excited to find UV radiating toward them at every address and along every street. The UV-seekers include moths, of course, but also beetles, lacewings, things lacewings eat (aphids), various flies (caddis-, crane-, hover-, true-, scorpion-, damsel-, dragon-, even butter-), and wasps. About a third of them will perish in their excitement to touch the light. They hit the hot light, circle around it until exhausted, or make enough of a ruckus that a predator (bird or bat or other insect) gobbles them up.
Other urban elements are conducive to the insect hunt. If there is a waterway near a city, you can expect that there are all kinds of mayflies and stoneflies nearby, who lay their eggs on the lampposts, and, as we saw on our walk, often molt their white filamentous skin and leave it there, quivering in the wind like the clarinet lessons Maira Kalman plucked from their flyer. Walls, especially brick walls, provide nooks that house cocoons or nests. On the first brick wall we approached, we saw jumping spiders—and jumping spider “retreats,” which sound like summer homes for spiders but are just the places where they hide but do not lay eggs. We found an egg sac of a common house spider. We found bee and wasp nests punched into tiny holes in a wall. If you are lucky, you might see a leafcutter bee nest, which features the packets of leaf bits that they have collected, each with a ball of pollen, nectar, and an egg inside. If you are less lucky, you might find a paralyzed cricket stashed by a wasp in a hole stoppered with grass or mud.
Abandoned places, something cities provide in abundance, are “promising” in Eiseman’s eyes. A dusty, unpeopled overpass is a great substrate for insect tracks. Old Dumpsters attract cocoons and spiderwebs. “I did see a grasshopper munching on the paint on the corner of my house one time,” he added. We were poking around a Dumpster together. Eiseman naturally got much closer than I would. It was only when perched on its wall that he spied a downy woodpecker on a nearby tree making a rhythmic racket.
Sidewalks have sidewalk-crack ants. A graduate student who recently cataloged the ants in the medians of the long avenues in New York City found cornfield ants, thief ants, pavement ants (a warmongering, pavement-loving species), and a Chinese needle ant, a stinging ant that is not from around here. He also found that the ant diversity on the Upper West Side of the city was greater than on the Upper East Side—presumably a function of that great environmental condition, “number of trash cans.”
Even fallen twigs can reveal insect sign. Surely you have twigs on the ground in your area? “Neatly severed twigs,” as one of Eiseman’s book sections is headed, can reveal the presence of a beetle girdler, a pruner, or a borer. While galls might happen to cause a twig to weaken and break, there are various beetles who intentionally weaken a twig: the girdler, after laying her eggs toward the tip, moves toward the tree’s trunk and plows a tidy path around the circumference of the twig. When a wind comes along, the twig snaps off cooperatively and the growing larvae in the twig feed on the aged or dying tissue. In other beetle species, it is the larvae themselves who burrow, when they are old enough to begin burrowing, into the twig and chew their way to the surface, creating characteristic sign in the process. All you have to do is look at the end of the fallen twig to see the spiral formed by a hickory spiral borer, or the expertly cut burrow around the perimeter, caused by a different wasp.
• • •
The discovery of the day was not the downy woodpecker cruising up and down the hackberry in the corner of a vacant lot—leaving sign in the form of beak marks, itself sign of some tasty beetle under the bark. It was not the adorable pupa of a ladybug, sitting calmly square in the middle of a catalpa leaf, its head tucked under its body and its abdomen folded protectively.
It was the sign on wood by a most unlikely creature.
We had just found some slug slime on a birch leaf. “There’s a slug among us,” Eiseman said. I did not think of slugs as critters that might want to be on trees. But Eiseman described to me how some slugs eat the film of algae on the bark of a tree, and in scraping their teeth against the bark leave a series of kisses with jagged lips. The resultant mark shows up clearly on light-colored backgrounds, like a birch tree—or on a white propane tank, or an abandoned car covered with weather and detritus.
“It’s a feathery pattern, this back-and-forth S-shaped pattern,” he was saying, before, “oh, here we go.”
On the broad trunk of the tree was a sinuous pattern of spikey footsteps, a series of stamps of a sharpened fern frond. Slug teeth marks.
Eiseman looked entirely satisfied. “I had always suspected it was slugs who were doing it”—leaving this kind of track—“but I couldn’t figure out how, because I didn’t realize slugs had teeth.” It took seeing a slug in action to confirm his suspicion.
In truth, slugs do not really have teeth; they have radula, a finely toothed kind of tongue that only mollusks have. It allows them to graze, rasping their body against a surface to sop up whatever they are gliding over.
Sign of slug. It was pretty, delicate—even more so for being the unlikely result of a gelatinous, lumbering creature. We gazed admiringly at its path tattooed on the tree. I fumbled through my bag for a camera and snapped a photo of it, surely one of the only extant images of slug sign outside of Eiseman’s and other slug enthusiasts’ collections.
• • •
While we idled down a broad, newly laid sidewalk, hardly a sidewalk ant in its cracks, I began thinking about Eiseman’s brain. What was it that allowed him to notice these mystery objects, where most of us see just plain leaves and twig and wall? How was it that lace bugs “jump out at” him while they left me staring blankly at a tree?
The difference between how Eiseman sees and how I see is traceable to a concept popularized in the early twentieth century by Luuk Tinbergen, brother of the Nobel Prize–winning animal behavior researcher Niko, and a noted bird-watcher in his own right. Tinbergen noticed that songbirds did not prey on just any insect that had recently hatched in the vicinity; instead, they tended to prefer one kind of bug—say, a particular species of beetle—at a time. As the numbers of young beetles rose through a season, the birds gorged on these beetlettes, ignoring any other available young insects nearby. Tinbergen suggested that, once the birds found a food they liked, they began to look just for that food, ignoring all others. He called this a search image: a mental image of a beetle—with its characteristic beetly shape, size, and colors—with which the bird scans her environment.
The concept of a search image has now been widely studied in the animal world and is used to help explain the efficiency many predators have in finding their prey, despite the best efforts of the prey to be unfindable. In the lab, blue jays trained to look for camouflaged moths initially have trouble seeing them—they blend in so well with the speckled bark on which they alight. But after a number of attempts, the birds get preposterously good at finding even the most well-concealed moths. Dogs, skunks, and spiders have been found to have olfactory search images: they are more concerned with the smell of their food than its shape, and can find, say, that dry dog food (dogs and skunks) or that particularly yummy mosquito (spider) among a riot of smells in the environment, by searching out its characteristic smell.
Search images are not just used or useful for finding prey or avoiding capture; they are the way we find our car keys, spot our friends in a crowd, and even find patterns that we had never seen before. The neurologist Oliver Sacks writes about a splendid, human example of this phenomenon from his own experience. At a time when Tourette’s syndrome was not widely recognized, Sacks saw his first Tourette’s patient, exhibiting the tics that define the syndrome. The following day, he says, “I saw three [ticcing] people on the streets of New York and another two the next day. And I thought, ‘If my eyes are not deceiving me, this must be a thousand times commoner than it’s supposed to be . . . why haven’t I noticed this before?’” He had acquired a Tourette-tic search image.
Everyone needs a mechanism to select what, out of all the things in the world, they should both look for and at, and what they should ignore. Having a search image in mind is what makes finding your friend among the crowds of people disembarking trains at Grand Central Terminal possible at all: it is the visual form of the expectation that allows you to find meaning in chaos. At the same time, if you are searching for your friend who you last saw twenty years ago in high school, she may no longer look quite like your search image representation of her. Jakob von Uexküll, the German biologist, wrote about this with his own search for a pitcher of water, which he expected to find at his table at lunch. Though he was assured that the pitcher was in its usual place, he could not see it right in front of his face—for the clay pitcher he had expected had been replaced by a glass pitcher. The “clay pitcher search image” obliterated the perceptual image of the glass pitcher. Von Uexküll recognized this as the same mechanism that led animals to mistake harmless objects as fearsome. He described a jackdaw (a kind of crow) flying above bathers at the beach, fooled into attacking an innocent person carrying his bathing suit over his arm: the jackdaw had a search image for a jackdaw-in-a-cat’s-mouth, and the wet, drooping trunks mimicked it. The jackdaw unreflectingly set to attack the feline killer of his brethren. Presumably our German biologist emerged with only minor peck marks.
Eiseman has an insect sign search image. He has got galls bumpily imprinted on his mind; bug footprints etched in his brain. And in his eyes: neuroscientists who look at “visual search” find not only expected areas of the brain involved—a layer of the visual cortex called V4, the frontal eye fields (in the frontal cortex), areas in the brainstem and other areas involved in eye movement—but also the retinal ganglion cells in the eye itself. Our visual system has what researchers call inhomogeneous processing; this is a fancy, and slightly unflattering, way of saying that even when we want to, we cannot see everything at once. We see best right in front of us, in the center of vision made crystal clear by the abundance of photoreceptor cells in the center of our eyes, the foveal area of the retina. The periphery of our vision? Not so much. Our eyes simply are not designed to focus on what is to our sides: that is why we have nicely swiveling heads (presumably evolution was not concerned with what was behind our heads). Once our eyes are open, we automatically begin scanning the environment, flitting our gaze to and fro in short saccades—quick, automatic hops of our gaze back and forth to move our two degrees of central vision across the fifty or so degrees of our future path. We gaze-hop to scan a scene and we gaze-hop simply to stay looking at the object in front of our noses. You cannot stop saccading (except with anesthesia to the eye), nor would you want to: if you looked steadily ahead without saccading to and fro, the image you were looking at would seem to disappear. After constant stimulation, our sensory receptors get tired and stop firing. The result is that we become inured to constant sensory input: we stop noticing the foul odor in a room (but it is still there for a while, as evinced by the expression of others entering the room after you) or the heat of the steam room air on our skin (though it is still the same hot temperature). Saccades are the eyes’ way to avoid having the lion, mouth agape, disappear from our vision while we stand in front of him, frozen in fear.
