On Looking: Eleven Walks With Expert Eyes (14 page)

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.

Saccading and searching are normal behaviors, done every visual moment of every eyes-open day. Search images are for the masters. That is not to say that the masters always know how they use these search images to solve the puzzles before them. Interviews with expert radiologists, satellite image analysts, and fishermen show that though they recognize their own expertise, they often cannot tell you exactly how they identify the hidden malignancy, detect a target from a bird’s-eye view, or spot the school of sardines as it surfaces. Me, I felt one step closer to being a gall search master myself—without knowing how.

 • • • 

As our walk wound down, I was reluctant to let Eiseman go. I slowed my pace to get him to talk about the insect sign on a limp
plant by the side of a dusty road. It turned out to be a winding trail of one of two known grapeleaf moth miners. Following it, we found sweet purple grapes. We gobbled a handful. A melancholy thought occurred to me. Most people are not going to have an invertebrate tracker on their walks with them, I worried out loud. Eiseman reflected for a moment, and then quoted one of his tracking teachers, Susan Morse: “Half of tracking is knowing where to look, and the other half is looking.” If you understand even the most superficial elements of the life histories of different animals—such as what kinds of things they are attracted to—once you start looking, you are going to find them everywhere. If you want to find otters, find a peninsula in a big body of water where they might scent-mark and loiter; if you’re looking for rats, try the alleyways behind restaurants, where overflowing bags of trash are left, kitchen floor mats are shaken out, and busboys eat hasty meals on break. Hence the ease at finding gulls around Dumpsters, raccoons sheltered in the holes in a stone wall, and, we discovered, parasitized flies on the bottoms of leaves in Springfield.

A small bit of knowledge goes a long way when thinking about “where to look”: “Every kind of mammal has a particular landscape feature or microfeature that it keys in on, where it does its marking behaviors,” Eiseman explained. “With insects it’s just the same, just smaller: microhabitats.” You need only know those habitats. Once you have an eye for those features, whether alley, leaf, or wall, as long as your eyes are open, you are likely to see the animal or the traces it has left. Eiseman described coming across the sign of a palmetto tortoise beetle while on his cross-country insect voyage: “I had seen a picture in a book of this beautiful structure that a palmetto tortoise beetle makes—it’s a beetle larva that only feeds on palmetto leaves. It extrudes this long strand of brown segmented excrement [which might only be beautiful to palmetto beetles and Eiseman], this twisted mass of segmented
straw. I didn’t know what size it would be. My brother and I were walking along a trail in Georgia and I—I wasn’t specifically looking for it—I just spotted one out of the corner of my eye; it was that big”—he held his fingertips almost touching—“about eight feet away. . . . Somehow it jumped out even though it seems physically impossible to see it.”