DIGRESSIONS NOTWITHSTANDING, READERS MIGHT well inquire why they are being offered an account of the mosquito life at 106 Parkside Trail, Michiana Shores, Indiana. If we must have mosquitoes, readers might say, why not mosquitoes of a more general stripe, such as mosquitoes of the Midwest, or of North America? Why not urban, suburban, or rural mosquitoes, or just mosquitoes, period?
The problem is that the word mosquito is not very susceptible to generalization. The term—which comes from the Spanish for “little fly,” and seems to have displaced the pithier Old English word gnat over the past few centuries—can properly be given to any member of the insect family Culicidae, a group that comprises some 3,000 species and subspecies and ranges over virtually the entire globe. In equatorial Africa, some insects called mosquitoes breed in small tree holes filled with rainwater; they do not take blood from other animals but feed solely on plant nectar and other sources of sugar. In Arctic Canada, other insects called mosquitoes breed in large, open ponds of melted snow; they attack warm-blooded animals in such numbers and with such avidity that they could, if left to do so, suck a man dry in four hours. Between these extremes, mosquitoes conduct their essential business—eating, growing, reproducing, dispersing—in such a variety of ways that one can scarcely cite a rule of mosquito behavior that lacks an exception. For example, one of the most basic facts of life for mosquitoes is that in their immature, aquatic stages—the larva, which hatches from the egg, and the pupa, which metamorphoses into the flying adult—they breathe via organs that protrude through the water’s surface. But the larva of a North American species called Psorophora discolor can stay under water almost indefinitely; it apparently takes dissolved oxygen directly from the water, through its cuticle and perhaps through a set of overdeveloped gills. Another general rule is that female mosquitoes must have a meal of blood if their eggs are to develop; but, in addition to the above-mentioned nectar feeders, several species of blood feeders have autogenous strains that can reproduce successfully without blood.
Nearly everyone knows that mosquitoes flourish in wet weather, but some species that favor stagnant pools do much better in dry times, when streams slow to a trickle or stop altogether. Some species prefer open sunlight, some shade. Some breed in the foulest of latrine pits, others in saltwater marshes, and some exhibit almost no tolerance for pollution or salinity. Some like to bite cows, others prefer humans, and still others concentrate on birds or reptiles; one genus—Malaya—inserts its proboscis into the mouth of the Cremastogaster ant and, without hurting the ant in any readily apparent way, helps itself to the food that the ant has gathered patiently into its crop. All these creatures are called mosquitoes.
This being the case, the reader will perhaps appreciate that the chronicler of mosquito life has to draw the line somewhere. To me, 106 Parkside Trail seemed to be as good a place as any. The property, which lies very close to the border between Michigan and Indiana, a few hundred yards southeast of the Lake Michigan shore, covers seven eighths of an acre of mature sand dune, heavily populated with tall oaks, numerous wild grasses and weeds, and—it has often seemed to me—enough mosquitoes to feed roughly half the world’s birds. The address is ideal for another reason as well: scientists are encouraged, by tradition and economics, to conduct their inquiries close to home, or, when possible, close to their favorite vacation spots. I am not a scientist, but I do enjoy pretending, and I happen to own part of the summer house that sits on the hill.
ON A COOL, SUN-DAPPLED MORNING LAST AUGUST, as the wind off the lake and the leaves in the trees combined to produce a soothing wash of rustling white noise, I ascended the hill at 106 Parkside carrying a bag of small glass vials, each a little less than an inch in diameter. Together with my own flesh and blood, these vials gave me the makings of the adult-mosquito trap used by generations of entomologists. Thanks to the efforts of those researchers, and my experiences during numerous twilight barbecues on the hilltop, I knew I did not really need to look for mosquitoes. The mosquitoes, I knew, would come looking for me.
How they would find me is a question still open to speculation, even though mosquitoes have been studied intensively for decades. Laboratory tests have shown that mosquitoes are attracted to certain ranges of temperature and humidity, and especially to carbon dioxide. Entomologists know that simply exhaling into a jar of mosquitoes can rouse them to such a frenzy that they will attempt to draw blood from the glass. For this reason, presumably, some species show a decided preference for animals’ heads as opposed to other body parts; one species is said to go only for the nose. Color also seems to play a role in attraction: the chief yellow-fever mosquito, Aedes aegypti, has been shown to prefer black guinea pigs to white, and other species exhibit a similar preference for dark-colored objects. Certain chemical smells may come into play as well. Although the blood and sweat of various animals have not been found particularly attractive, some of the components of blood and sweat—for example, hemoglobin and certain amino acids—do attract. This makes sense from the hungry mosquito’s point of view: better to have a taste for these basic substances, which are common to a wide range of animals, than to insist on the more specific smells of one kind of host. Picky eaters often go hungry.
These attractants, however, are effective over only very short distances—a few feet, at best. Light is the only thing known to attract mosquitoes over great distances (clouds of migrating mosquitoes have been observed making for the lights of a city skyline at night), but not all species respond to light, and light is obviously not necessary to attract a biting mosquito. What, then, brings the mosquito within close range of its victim? Although most experts are convinced that something does, they cannot say for sure what that something might be. The best explanation offered so far is that the mosquito simply flies upwind rather randomly until it stumbles upon evidence of its dinner.
As anyone who has tried to swat one in midair can testify, mosquitoes are expert fliers. Like all their relatives in the fly order, they have two wings instead of the four possessed by other flying insects. (The name of the order, Diptera, means “two wings.”) Over the eons, the other two wings have been modified into “halteres,” which appear to act like gyroscopes, providing stability in flight. Mosquitoes can hover, move vertically, and even fly backward. Though they can reach speeds of up to seven kilometers an hour, they typically cruise into the wind at a land speed of about half a kilometer per hour, judging and adjusting their speed visually, according to the apparent progression of the landscape below them. (For this reason, a high-flying mosquito can be fooled by appearances into flying faster than normal.) Flying into the wind is probably helpful in finding blood. The generally accepted theory is that the mosquito’s potential animal victim exudes a “host beam” of warm, moist air, heavily laden with carbon dioxide. When the mosquito, flying into the wind but going nowhere in particular, encounters such a beam, it cannot determine the source, so it keeps flying straight ahead until it leaves the beam, then turns back into it. Zigzagging back and forth, into and out of the narrowing beam, the mosquito homes in to within a few inches of the victim, whereupon its senses of sight and smell apparently take over to identify the creature as palatable or not. Motion on the victim’s part may help, somehow, to rouse the mosquito or guide it to the victim’s general vicinity—a phenomenon noticed in the forests of Uganda by investigators who found that the best way to capture certain species was to move around, then halt briefly with collecting equipment at the ready. My own experience in the forest-like environs of 106 Parkside Trail confirms this. I spent hours sitting calmly, waiting for mosquitoes to call, but caught almost as many while doing something else.
Once, pausing briefly after walking up the hill, I noticed a mosquito developing an interest in me, and watched as it darted and hovered around my head; with its long legs bent at the knees and partially drawn up, it looked as though it were daintily pulling up its skirt before settling down to dine. Eventually it chose the back of my hand and went to work, burying almost its entire proboscis—which was about three millimeters long, roughly half the length of the mosquito’s body—beneath my skin.
THE MOSQUITO’S PROBOSCIS CONSISTS OF SIX DIFFERENT shafts surrounded by an outer sheath. Four of the shafts are cutting and piercing tools; a fifth, whose tip can bend in different directions as the insect searches for a capillary, transports blood from host to mosquito; the sixth transports saliva the other way. Entomologists have long believed that in some mosquito species the saliva acts as an anticoagulant, ensuring a smooth flow of blood (though recent work indicates that this may not be so); in some species, it also serves to transmit malaria, yellow fever, dengue, and most of the other diseases for which mosquitoes are notorious. When you sense a biting mosquito on your body, you are not necessarily feeling the piercing of the skin or the weight of the mosquito (it weighs only about a ten thousandth of an ounce, though it will weigh two or three times that after it has tapped your blood supply). What you usually feel is an allergic reaction to the mosquito’s saliva, which also causes the characteristic swelling and itch. That this reaction is allergic in nature helps explain why some people seem to suffer more when bitten than others—just as some people get hay fever and others do not. Often, two or three minutes pass before the reaction becomes noticeable. If the mosquito has not filled up and flown off by this time, it ends up spread across the palm of your hand.
The mosquito on the back of my hand took about two minutes and thirty-five seconds to finish its meal. After about a minute of feeding, it appeared to withdraw, then reinsert, its proboscis; this was its only motion other than a few barely perceptible twitches of one leg and the gradual swelling of its gut, which turned bright red before my eyes. The mosquito was so intent on its task that it did not notice when I slipped one of my glass vials over it; it continued feeding for a full minute, discovering the trap only as it tried to fly away, at which point I turned the vial over and stoppered it with cotton.
My purpose in bottling the mosquitoes that came to me was to identify the species that inhabit the hill. In order to collect a representative sample, I had to offer myself at various hours of the day, because different species are active at different times. In this regard, mosquitoes are often classified as nocturnal, diurnal, or crepuscular. Diurnal species are generally active throughout the daylight hours, but with certain well-defined peaks of activity—at midday, say, or in early morning and late afternoon. These species characteristically inhabit the cool, shady environment of forests. Crepuscular species are most active at dawn and twilight. Researchers studying yellow-fever mosquitoes in the tropical forests of Africa found one species, Aedes africanus, whose feeding activity was highly concentrated in a twenty-minute period near sunset. In this study, mosquito catchers were stationed in the trees, at various heights, because different species are also known to prefer different altitudes. Some of the mosquito species that carry yellow fever, for example, stick almost exclusively to the forest canopy. Indeed, the cycles of yellow-fever transmission require mechanisms other than mosquitoes to bring the disease from infected monkeys in the treetops to human victims below; mosquitoes transmit the virus from monkey to monkey in the canopy and from human to human on the ground, but they cannot normally carry it from one level to the other. In Africa, the monkeys shuttle the virus down, receiving it from one type of mosquito in the canopy and delivering it to another type on the ground. In Central and South America, yellow fever is known as an occupational hazard of lumbermen, because their efforts bring the infected mosquito down to the forest floor; left to itself, this mosquito — Haemagogus spegazzinii — would remain in the trees and bother only monkeys.
I did not climb any trees in my mosquito hunting, but I did manage, after a couple of days, to assemble a modest collection of bottled specimens, and these I took to Dr. George Craig, director of. the Vector Biology Laboratory at the University of Notre Dame, for identification. Craig’s office is located in a third-floor corner of Notre Dame’s biology building, behind a set of special doors that can be sealed to foil mosquito escape attempts. (The doors, Craig told me, were part of a ludicrous government effort to eliminate the chief yellow-fever mosquito, Aedes aegypti, from the Western Hemisphere; the government apparently feared that if it got the job done—which of course it never did, despite the expenditure of about $60 million—an escaped mosquito might ruin everything.) Mounted on one wall of Craig’s office is a collection of handmade metal butterflies fashioned by his wife, Betty, each one engraved with the name of a graduate student who received his or her doctorate under Craig’s tutelage. I asked why butterflies rather than mosquitoes—Craig’s laboratory, manned by a staff of about twenty-five teachers, students, and technicians, is devoted solely to mosquito research—and he answered that he found the butterfly more metaphorically appropriate to his purpose. Besides, where would you engrave a name on a model mosquito?
Craig took my bottled mosquitoes into his lab and, one by one, unceremoniously spilled them onto a counter and placed them under a microscope. At his side he had a book on Indiana mosquitoes, filled with drawings of identifying morphological characters, but he didn’t bother to consult it. “Aedes vexans,” he said of the mosquito that had taken a leisurely meal from the back of my hand. He recognized it by the narrow bands of light-colored tissue on its legs. This species, nearly ubiquitous in the forty-eight states, is most active at dawn and dusk. It characteristically breeds in temporary accumulations of water such as rain pools, ditches, and wheel ruts, but it is quite adaptable; it does well in sunlight or shade, in clean or polluted water. Because of its ubiquity and the huge numbers in which it breeds, Aedes vexans is considered the most troublesome pest mosquito in many parts of the northern U.S.
Another of my glass vials contained Coquillettidia perturbans, which must be something of a rarity at 106 Parkside. In its aquatic stages, this mosquito—which breeds in reedy lakes and swamps—has an ingenious method of obtaining oxygen. While most immature mosquitoes must spend the bulk of their time at the water’s surface in order to breathe, Coquillettidia larvae and pupae, along with those of a few other genera, stay under water, boring into the submerged stalks of aquatic plants and using them as snorkels. They are thus safe from many types of predator, including the human type—the mosquito-control worker—whose most effective weapon is often the spreading of a larvicide over the water’s surface. The Coquillettidia perturbans specimen I caught evidently had traveled quite a way to 106 Parkside; as far as I know, the closest suitable habitat is a cattail-lined swamp several miles from the hill. Craig told me that the species is able to fly as far as ten miles from its breeding place, which qualifies it as a long-distance flier among mosquitoes, though not, by far, the longest-distance flier. Aedes sollicitans, the obnoxious salt-marsh mosquito of the eastern seaboard, often migrates in swarms and can normally range up to forty miles from its birthplace; a considerable population, perhaps aided by high-altitude winds, was once found on a ship 110 miles out to sea off the coast of southern Virginia.
(By the way, though not all mosquitoes have such evocative names as vexans and perturbans, many do, including Aedes tormentor, Aedes excrucians, Culex territans, Aedes implacabilis, and Haemagogus lucifer. Several such species were named by the British entomologist Francis Walker, who seems to have spent most of the time between 1847 and 1866 cataloguing a vast collection of Diptera specimens accumulated by the BritishMuseum. Apparently, he found ways to amuse himself.)
Dr. Craig identified all the rest of my mosquitoes as Aedes triseriatus, a species in which he is particularly interested. He invited me to admire one of the magnified corpses, and I was shocked by its brilliant silver coloring; it looked like something that had emerged from a metalworker’s shop, rather than from a pool of dirty water. Aedes triseriatus breeds most often in tree holes, the various notches, cracks, and cavities in trees that collect rainwater. The species seems to prefer its tree holes close to the ground; had I ventured into the treetops on my mosquito hunt, I probably would have found a closely related species, Aedes hendersoni, which prefers the holes at canopy level. Aedes triseriatus, a diurnal species, almost never ventures into the “light traps” that entomologists commonly use to monitor mosquito populations. Craig believes that because of this, and because of the near impossibility of finding and eradicating its breeding places, the species is far more dangerous than many mosquito-control workers realize. It is the chief vector of La Crosse encephalitis, also called California encephalitis, of which it is actually a subtype. This type of encephalitis is less severe than most other mosquito-borne types that affect humans, but in most years it accounts for the vast majority of cases reported in the United States. Craig seemed pleased that I had found Aedes triseriatus on the hill. I feigned enthusiasm.
ALL THE ADULT MOSQUITOES I CAPTURED ON THE hill were females. Male mosquitoes do not suck the blood of other animals; incapable of piercing skin, they get by on plant nectar instead. Therefore, they do not patrol for animal hosts and do not transmit disease; therefore, they do not much interest entomologists. Like the males of many other insect families—ants and honeybees are probably the best examples—male mosquitoes are important for just one reason, and after they have served that purpose, they become superfluous. The female of Culex pipiens, the common species chiefly responsible for the misconception that all mosquitoes breed in polluted water, survives northern winters by resting in an attic, cellar, or similar man-made structure, usually with a load of sperm that she will use to fertilize eggs in the spring. The male of the species, having bestowed his seed, merely dies at the onset of cold weather.
If you have ever seen a congregation of mosquitoes flying busily in circles over a chimney, over a light-colored object on the ground, or, as sometimes happens, over a person’s head, you have probably witnessed a swarm of males preparing for their one opportunity to do something useful. Swarming is closely associated with mating in many flying insects. In the North American mayfly Hexagenia bilineata, for example, females clearly attract mates by flying into the swarm of males. The purpose or advantage of mosquito swarming is probably similar, but conclusive proof has been difficult to obtain, partly because mosquito swarms often occur in dim light and are easily broken up by outside influences. In any case, most species swarm before mating; however, some species of the genus Aedes need not swarm, and males of these species can sometimes be found skulking about near a human or some other host animal, waiting for a hungry female to happen by.
In the mayfly mating ritual, the female flies straight through the swarm, and her unequivocal flight path is the chief means by which the males recognize her; if an observer throws a stick or cigarette butt into a swarm of Hexagenia bilineata males, some in the swarm will make straight for it, and others will make straight for those straight-flying males. When mosquitoes mate, male finds female—and is stimulated to amorous behavior—chiefly by sound: the hum produced as the female beats her wings. The antennae of most male mosquitoes are equipped with bushy hairs that vibrate in sympathy with this hum, which is usually called the wing-beat frequency or flight tone. Typically, the antennal hairs do not become functional until the male is about a day old. By this time, the abdominal section bearing his genitalia, originally arranged in such
a way as to make mating impossible, has usually rotated round through 180 degrees, so the young mosquito does not generally want to mate until he is also able to. Mating usually begins, and sometimes ends, in flight, the male hanging upside down beneath the female.
Biologists once believed that the male antennal hairs of any one species would respond only to the unique female wing-beat frequency of the same species, and that this was
the primary means by which mosquitoes recognized suitable mates. The compelling aesthetic appeal of this theory seems to have kept it alive for a time, despite the knowledge that the wing-beat frequency in any species can vary according to the female’s size, age, and whether or not she is carrying blood or mature eggs. H. Frederik Nijhout, then an undergraduate student of George Craig’s at Notre Dame, helped lay this idea to rest in 1970, with a classically structured series of experiments. Nijhout exposed several species of Aedes males to females of their own and other species, to artificially produced flight tones, and to dead females, females whose wings had been immobilized, and females that had been glued to the head of a pin. He found that the female flight tone served only to excite the males and bring them into the females’ vicinity, and that the same frequency would do the trick for several different species. (So would a tuning fork.) Copulation, however, did not generally occur until after the males had fondled their prospective mates and had somehow judged them suitable. Many insects are known to recognize each other by means of chemical substances called pheromones, though usually pheromones are perceived by smell, not touch. For mosquitoes, Nijhout postulated the existence of a “contact” pheromone, one that is received by touch, and suggested that in the absence of olfactory attraction, the sonic stimulation of the flight tone is necessary to initiate the fondling. The pheromone has not been isolated or synthesized, but the Notre Dame entomologists have given it a name: caressone.
THOUGH AN AEDES AEGYPTI FEMALE WAS ONCE observed mating forty times in succession, with one male right after another, the normal female mosquito needs to mate only once for life. Like many insects, she stores sperm in her body and fertilizes her eggs at the moment of laying; although she may lay more than a thousand eggs in her lifetime—which typically lasts about a month, though longevity varies widely with species and environment—one encounter with a male gives her all the seed she will ever need.
Shortly before or after obtaining it, she takes a meal of blood to provide the eggs with the protein they must have to develop. When the eggs are fully mature and ready to be fertilized—usually a matter of a few days—the female lights out in search of a place to lay them.
Because the female’s choice of laying site determines the environment in which her offspring will hatch and grow—or fail to—the choice is an important one, though of course the criteria are different for different species. Culex pipiens can lay in almost any available container of still water. On the other hand, Wyeomyia smithii, which is found in the eastern half of the United States, seems to lay only in water accumulated in the pitcher plant (the larva spends the winter encased in ice and becomes active again when the water melts); Sabethes chloropterus, of Central America, prefers the holes in bamboo stems, hovering beside them and flinging her eggs in with a flick of the abdomen. The means by which females find the appropriate site has been much studied in the laboratory, but is little understood. By and large, the choice seems to rely heavily on visual clues, the females of many species gravitating toward dark, reflective sites that may appear to be pools of moisture but sometimes are mirrors or other inappropriate objects. Some females take a sip of water before laying, presumably to taste for salts and other chemicals. In at least one species, smell also seems to be important: the female of Aedes atropalpus, found in the northeastern and midwestern U.S., lays in rock holes alongside running rivers; she evidently can recognize the smell of pupal skins left by previous generations, and thereby “knows” which holes have in the past been deep or hospitable enough to support hatching eggs to adulthood.
At 106 Parkside Trail, many different species lay in a fifty-five-gallon drum that is sunk into the ground beside the house—the remains, apparently, of a decorative fountain enjoyed by a previous owner. I have long imagined this drum, which is always filled with rainwater and decaying leaves, to be some sort of mosquito heaven, but before I knew what to look for, and how to look, I would peer at the water for minutes at a time without spying anything that might qualify as a mosquito larva. I consulted Dr. Craig at Notre Dame and quickly learned what I had been doing wrong. Every time I approached the fountain, I would hear a couple of gentle kerplops; toads, resting on the rocks around the perimeter of the fountain, were diving into the water to escape the danger I represented to them. In Craig’s laboratory, I learned that mosquito larvae do much the same thing. When Roger Nasci, a postdoctoral research associate at the lab, took me into an insectary and pulled a pan of larvae from a covered shelf, the larvae dove for the bottom of the pan immediately upon sensing the motion of the water and the change in light intensity. Many species will dive for cover in this way no matter how the light changes, from dim to bright or vice versa. The larvae in my hilltop fountain were apparently diving as a result of the shadow that I cast as I loomed over them; perhaps they were also reacting to the splashes made by the frightened toads. In any case, I knew now that although mosquito larvae spend most of their time at the water’s surface, the surface is no place to look for them. Dr. Craig equipped me with the standard larvae hunter’s tool, a long-handled white-enamel ladle that holds about two cups of water; it can be plunged into a pool to intercept diving larvae, and, upon being removed, it provides a light background against which to view them. Returning to the hill, I dipped the ladle into the fountain and, with a single flick of the wrist, brought up more than seventy small, wormlike larvae.
I also hunted with another tool that Craig had given me, a plastic pipette with a collapsible top, which worked like an eyedropper or a turkey baster. This tool was for drawing water out of small, inaccessible places, and I poked it into objects all over the hill, including hollow pipes and old beer bottles—to no avail—and several times into a ground-level hole in the oak tree that supports one end of the household hammock. This looked to me like a classic tree hole, and I was disappointed when I failed to find a larva in it—so disappointed that I kept returning to it until I eventually succeeded in extracting a few large, lethargic-looking larvae, obviously of a sort different from those in the fountain, which was about ten yards away.
FOR SEVERAL WEEKS AFTER THIS EXPEDITION, I always had a pan or two of mosquito larvae close at hand. I kept several pans at the house on the hill, a couple more at my apartment in Chicago, and, for a while, a jar full of larvae in my car. Watching them was an agreeable pastime. They were capable swimmers, able to propel themselves backward through the water with quick, whipping contractions of the body; in the larger, slower larvae that I had found in the tree hole, the motion was revealed to be rather serpentine, the larvae often taking the shape of the letter s. The larvae would swim whenever I disturbed the lighting or tapped the container, but left to themselves they preferred to hang around, nearly motionless, at the water’s surface, eating and breathing. They dangled by their rear ends from the surface film at roughly a forty-five-degree angle, with only their respiratory tubes protruding to open air. Instead of patrolling for food, as they would have to do soon enough in adulthood, each larva beckoned food toward it with a pair of hairy, brushlike structures at either side of the head; these brushes, which were kept in constant, sweeping motion, created a current of food-bearing water that the larva examined with its mouth. A single larva, less than half an inch long, can filter more than a liter of water per day in this manner. Although primarily interested in such things as algae, bacteria, and detritus, it will apparently swallow any particle of the appropriate size. Its purpose is to eat and grow; as it grows, it sheds its skin, or molts, four times.
These are the general facts of larval life, but, as always, researchers have noted some spectacular exceptions. Some species supplement, or totally replace, their filter feeding by preying on other mosquito larvae. One such predator is the African Eretmapoditesferox, which inhabits the water accumulated in plant axils and is known to eat larvae of its own species; it was observed devouring another species by the mosquito investigator A. J. Haddow: “If an [Aedes] simpsoni larva is placed in a small dish with a large [Eretmapodites]ferox larva, the latter will in many cases cross the dish straight to its prey . . . and, seizing it immediately, will begin to shake and worry it much as a dog shakes a rat. The savage nature of the larva is shown by the fact that it almost always occurs alone in an axil.”
Some larvae are capable of survival outside of water, provided the environment is not too dry and they are not exposed too long. Anopheles maculipennis, a European species that sometimes breeds in the flooded hoofprints of large animals, has been observed crawling as far as seventy-five centimeters over moist ground, leaving one dried-up hoofprint for another, more hospitable one. Aedes kochi, of Australia and New Guinea, can crawl, caterpillarlike, from one leaf base to another, and larvae of the genus Sabethes, native to Central and South America, can evidently do something similar. All of these special cases, including the cannibalistic Eretmapodites ferox, make their homes in small accumulations of water that could disappear in a stiff wind or over a long dry spell. Such mosquitoes are clearly more likely to succeed in life if they can relocate when necessary, and if they can grow quickly to adulthood by eating whatever is handy, even if it happens to be a sibling.
THE RELATIONSHIP BETWEEN THE LARVA AND THE adult mosquito offers the stuff of intriguing speculation. For example, the two might be considered as entirely different organisms: one swims through the water and eats algae, the other flies through the air and sucks blood from large animals. How much more different could they be? Or, if one insists on considering them the same organism, one might ask which is the real mosquito? Is the larva the growth stage, leading to adulthood? Or is the winged insect the sexual stage, which helps the larva colonize new aquatic habitats?
Between the two, in any case, comes another stage of development—the pupa, which resembles neither. With its bulbous head and short tail, which it usually keeps gathered in close to the body, it looks like a comma or a tiny, dark, cooked shrimp. While the larva breathes through the tip of its abdomen—its rear end—the pupa breathes through tubes in its head. Though the larva is heavier than water, able to stay at the top only by the action of surface tension, the pupa is lighter than water, able to dive and generally to stay down only with a great expenditure of energy.
One thing the pupa shares with both its predecessor and its successor is an uncommon locomotive ability. Most insect pupae are fairly sedentary, but the mosquito pupa is a quick, agile swimmer whose comical dashes through water have no doubt amused mosquito watchers for centuries. By the time my larvae had begun molting into pupae, I was keeping them in coffee cans covered with plastic screens, because pupae generally become flying adults within two or three days. Often I tapped the sides of the cans just to see them scurrying for the depths, with a motion that reminded me of Pac-Man moving in three dimensions. The pupae swam by flexing their abdomens, extending their short tails and bringing them back again—Pac-Man’s constantly opening and closing mouth—and somersaulting head over tail through the water like a rolling coin. As soon as they stopped their exertions, which generally propelled them toward the bottom at an angle, the buoyancy produced by an air pocket in the thorax would send them floating straight to the top. Another tap and off they went again.
In reading about mosquitoes (I am especially indebted, by the way, to J. D. Gillett’s The Mosquito and Marston Bates’s The Natural History of Mosquitoes), I had been told that the emergence of the winged adult from the pupal skin was a sight not to be missed, so I kept the coffee cans on my desk and checked them frequently, hoping to catch a pupa in the act. On many nights, I gave up and went to bed having seen nothing, only to find a few adults hanging from the screen, or resting on the inner side of the can, the next morning. One afternoon, finally, I peered into a can and noticed a small, feathery protuberance at the top of a nearly motionless pupa; I put my work aside and took up a hand lens to observe the big event.
I’m afraid I cannot report on it with the same enthusiasm that other writers have shown. The mosquito did not climb or push its way from the pupal skin; it just oozed out. As minutes went by, the thing was obviously emerging—there was always more of it sticking out than there had been a while ago—but, aside from a few twitches of the pupal tail early on in the process, I could discern no movement whatsoever, just a straight line of tissue protruding from the pupal skin at an angle of about forty-five degrees to the water’s surface. Gradually the legs and mouthparts spread out from the body; about twelve minutes after I had first noticed it emerging, I was willing to concede that the thing looked like a mosquito. About three minutes later, it was free of the pupal skin and resting on the water, making tiny indentations in the surface film with its fragile legs. As it dried, its color darkened from greenish to the familiar gray. Now and then, it moved a leg or flapped a wing, but for the most part it just sat there, while I lost my patience. At 3:46 P.M., two hours and thirty seconds after I had begun watching, I turned my attention elsewhere (as I’d done several times previously), foolishly neglecting to cover the can. At 3:59, the mosquito was gone.
Having amused myself sufficiently with my colonies of larvae and pupae, I plucked a sample of specimens from the water with an eyedropper, executed them in vials of alcohol, and took them to Dr. Craig’s lab for identification. As I had suspected, the large, lazy larvae that I had found in the tree hole were Aedes triseriatus, the tree-hole species common in this part of the country and the vector, as I have already mentioned, of La Crosse encephalitis. Two of the species that I’d found in the fountain were Culex pipiens and Culex restuans, both of which have been implicated in the transmission of St. Louis encephalitis, and another was Culiseta melanura, a vector of eastern equine encephalitis. Culiseta melanura rarely bites humans or horses; it generally infects only birds, leaving to another species the job of transmitting the disease from birds to animals. Unfortunately, that other species is Coquillettidia perturbans, which I had also found on the hill, in adult form. In other words, of the four insect-borne encephalitides most commonly found in the U.S., three can be transmitted to humans by the mosquito population at 106 Parkside. (The fourth type is called western equine encephalitis.)* Since malaria was routed from the U.S. in the 1950s, these encephalitides—all of them viral diseases of the central nervous system—have been our only significant mosquito-borne diseases, and in most years they are not terribly significant: in 1982, the Center for Disease Control recorded 183 confirmed cases and four deaths. However, epidemics are possible. In 1975, an outbreak of St. Louis encephalitis, centered in Illinois, Indiana, and Ohio, struck more than 1,800 people and killed at least sixty-eight of them.
HAVE YOU EVER TRIED TO REMOVE THE RAINWATER from an old automobile tire? If not, think about it for a minute. It can be done, but it is not a job for the impetuous, or the hydrophobic.
Something else about tires: tires that are buried in landfills have an annoying habit of working their way back up to the surface. This tendency is evidently a result of the rubber’s plasticity and the tire’s configuration; rather than filling up with debris, the tire cavity traps air—as indeed it was designed to do—and the resulting buoyancy causes the tire to bend and ooze and wiggle upward, reaching daylight in months or years, depending on how deep and with what other materials it was buried. For this reason and others—tires also take up a lot of space and are nearly impervious to biodegradation—landfill operators are becoming disenchanted with tires. Many will accept them only if they are cut up or shredded, but in the day of the steel-belted radial, cutting and shredding (and, incidentally, retreading) are more difficult and expensive than they used to be. The tire and waste-disposal industries devote a good deal of time to the problem of tire disposal; they are especially hopeful that tires will one day be exploited for their considerable energy content, which, according to one estimate, makes each twenty-five-pound tire worth about two and a half gallons of oil. At present, however, we lack places and uses for the vast majority of the scrap tires generated in the U.S., and each year we add about 180 million to the pile.
George Craig is nearly fanatical on this subject. He will talk tires to anyone who will listen. The first day I met him, he showed me color slides not of mosquitoes but of used tires piled high in lots and trash dumps. Because of their ability to retain water, and perhaps also because of their black, shiny appearance—which, as we have seen, is favored by many egg-laying mosquitoes—tires are prime mosquito breeding grounds.
The mosquitoes with which Craig is most concerned, those of the genus Aedes, are remarkable opportunists, having evolved adaptations that enable them to inhabit the most unlikely of environmental niches. Mosquitoes are thought to have evolved in the tropics, some 100 million years ago or more, and about three fourths of the world’s species still live in the relative comfort of the tropics and subtropics. Aedes species have been found less than 1,000 kilometers from the North Pole; these species include the ones that breed and bite en masse, as of course they must, given the short warm season and the scarcity of animal hosts in their environment. Similar adaptations help to make Aedes mosquitoes the most troublesome kind in the northern U.S. Some species lay their eggs not inwater but in places that are likely to contain water in the future—on a moist ground depression that will flood with heavy rainfall, for example, or around the rim of a tree hole or man-made container. The eggs develop fully in such places, but just before hatching they lapse into a state called diapause, and they will not actually hatch until immersed in water. Eggs deposited in just the right place—say, two inches from the bottom of a coffee can that contains just one inch of water at the time of laying—will not hatch, therefore, until the amount of water in the container is sufficient to support the larvae through the week or two they will need to reach adulthood. Because the eggs are fully developed when they enter diapause, they can hatch immediately when conditions are right, so no time is lost while the precious water evaporates. The eggs of some Aedes species can survive for years in diapause, even in sub-freezing temperatures; some species, in fact, will not hatch unless the eggs are cooled and subsequently thawed. While Culex pipiens spends the winter as an adult—the inseminated female resting indoors—and Wyeomyia smithii spends it as a larva, frozen in the pitcher plant, many Aedes species spend the winter as eggs.
Aedes triseriatus—Dr. Craig’s pet mosquito—shares fully in this admirable record of evolutionary adaptation. For decades, this tree-hole species has been losing its natural habitat, as midwestern forests have fallen to subdivisions, shopping malls, and other forms of human development. But as man taketh away the tree hole, he giveth back the scrap tire, and Aedes triseriatus has made the transition neatly. According to studies done at Notre Dame, the species emerges sooner, grows larger, and lives longer if hatched in a tire than if hatched in a tree hole. The relationship between mosquito and tire has grown so close that the city of La Crosse, Wisconsin (where the La Crosse encephalitis virus was first isolated, in the early 1960s—from the brain tissue of a dead child) has recently been fighting its disease problem by attacking its tire-disposal problem. The results of the effort cannot yet be reported conclusively, but the cleanup seems to have reduced the city’s number of cases, formerly fifteen to twenty per year, by as much as 90 percent. La Crosse’s lesson is the one that George Craig wants to spread.
Having found Aedes triseriatus in abundance at 106 Parkside Trail, I devised a modest experiment to test their adaptive prowess. Because trees are far more plentiful in the area than tires, and because I had found a few Aedes triseriatus larvae in a tree hole on the property, I assumed that the specimens on the hill had grown up in tree holes, not tires. If a tire was offered to them, would they colonize it? I drove off from the house one morning in search of an old tire, and I found one within about three minutes—which may indicate the severity of the tire-disposal problem. I washed the tire out as best I could, to remove any eggs that might have been deposited there previously, poured tap water in, and set this home-made mosquito farm out on the hill. A few weeks later, sure enough, I had a colony of Aedes triseriatus larvae.
AT ONE TIME OR ANOTHER, MOST READERS HAVE probably wondered how they might best protect themselves from mosquitoes, or at least how they might relieve the discomfort that mosquitoes can cause. Before bringing this account to a close, therefore, I offer these few hints:
1. Accentuate the positive. If you live in a temperate climate, you might remind yourself that in other parts of the world your mosquito problems seem ridiculously trivial. Contrary to the myth that persists in comfortable lands such as ours, malaria and yellow fever were not banished from the planet upon the completion of the Panama Canal. Mosquito researchers are still able to pepper their papers and grant proposals with statements to the effect that mosquito-borne diseases kill more people annually than any other single cause. In 1979, Lewis T. Nielsen, a University of Utah biologist and a former president of the American Mosquito Control Association, reported in National Geographic that malaria alone strikes an estimated 120 million people each year, about one million of them fatally. Remember this the next time you spill your gin-and tonic trying to swat a biting mosquito.
While you’re at it, you might meditate for a moment on the benefits that mosquitoes have brought to humanity and to the natural community at large. This should be a fairly simple mental exercise, for, as far as I have been able to determine, the list contains only three items, and the third may or may not be seen as a benefit, depending on one’s point of view. First, mosquitoes provide a handy and plentiful source of protein for birds, fish, reptiles, and other insects. Second, in feeding on plant nectar, mosquitoes pollinate certain wild flowers. Third, mosquitoes liberated Haiti and drove the French from the LouisianaTerritory. In 1801, Napoleon sent an army to America to subdue revolution in Haiti and, according to the medical historian Edwin H. Ackerknecht, to conquer the MississippiValley. Ackerknecht reports that the force numbered 33,000, of whom 29,000 fell to yellow fever in 1802. By 1804, Haiti was independent, and the U.S. had picked up the LouisianaTerritory for less than three cents an acre.
2. Adopt a barnyard animal. Do not flatter yourself: given a choice, a great many mosquitoes would rather dine on some other animal’s blood. In 1937, J. B. Rice and M. A. Barber offered to a cage of Anopheles mosquitoes a man’s abdomen on one side, and the shaved flank of a cow on the other. After the mosquitoes had fed, the investigators analyzed the blood found in their guts and discovered that in two of the three species tested, the cow had won hands down; the third species had also preferred the cow, but only slightly. At least one strain of Anopheles maculipennis has shown a similar preference for oxen, and several species seem to be particularly fond of swine. “It has long been known in Italy,” J. D. Gillett reports, “that those who sleep with a pig in the room tend to remain free of malaria.”
3. Try a homemade larvicide. If your neighborhood is blighted by a water drum, a ditch, a standing pool, or even a particularly deep birdbath, you can poison the mosquito larvae that no doubt reside there with diesel fuel or heating oil. Detergent will produce the same result by reducing the surface tension of the water, making it impossible for the larvae to hang in their usual fashion. They will sink and drown.
4. Smear chemicals on your skin. Some people seem to be naturally immune to the attention of biting mosquitoes. This condition might result from any one of a number of factors—body temperature, skin color, personal odor or lack of it—but more often it is probably due to a happy combination of all these factors and more. In any event, those of us who are not so blessed can achieve a sort of artificial immunity with N, N-diethyl-meta-toluamide [DEET], the main active ingredient in most commercially available insect repellents. Mosquito experts seem to agree that this stuff is truly repellent to mosquitoes, and that the higher the concentration of it, the more effective the product.
5. Start your own mosquito farm. As noted earlier, some adult mosquitoes do not take blood, whether they are male or female, and some mosquito larvae eat other larvae. These winning characteristics come together in the genus Toxorhynchites, which has consequently attracted the attention of mosquito-control workers. As a larva, this mosquito is a wanton killer, able to consume up to 250 other mosquito larvae in the course of its four larval stages. Toward the end of its fourth stage, when, apparently, it has had its fill, it goes on a murdering binge in its tree-hole home, killing—but not eating—everything that moves therein, perhaps to protect the vulnerable pupal stage from predation. In any case, the mosquito seems to get all the protein it needs as a larva, for the female adult does not require blood for her eggs, and her mouthparts are incapable of obtaining it. Attempts to use Toxorhynchites species as mosquito-control agents have not succeeded so far, but work continues at the University of Notre Dame as well as in New Orleans and French Polynesia. Although chiefly a tropical genus, Toxorhynchites is found in the U.S. as far north as Illinois and as far west as Texas. As a rule, adults of this genus are extraordinarily large and colorful, a worthy addition to any backyard menagerie.
6. Think twice before buying an electronic bug-zapper. Electronic bug-killers, long used at such commercial establishments as golf ranges and drive-in restaurants, are now popular for backyard use, casting an eerie blue glow over suburban America and punctuating the night air with their incessant tzzzitt-tzzitts. Their “black” ultraviolet light attracts insects from more than 100 feet away, luring them to execution on an electrically charged grid. Roger Nasci, who, with two other researchers, tested a well-known national brand last summer, is convinced that, although the machines do kill many insects, they do not reduce mosquito biting in outdoor situations. A similar study done in Canada six years ago reached the same conclusion.
Nasci compared the mosquito biting activity in six adjacent back yards on six different summer nights; two of the yards, chosen at random each night, were equipped with electrocutors, two with conventional light traps, and two with nothing. Human workers in the yards collected the mosquitoes coming to bite, and they switched yards every fifteen minutes in case some workers were more attractive or more adept at collecting than others. Overall, those in the electrocutor-equipped yards suffered no fewer bites than the others; in fact, they suffered slightly more, though not to a statistically significant degree. Over five different periods of twenty-four hours each, the electrocutors killed an average of 3,200 insects per period; less than 7 percent of these insects were mosquitoes, and nearly half of the mosquitoes were harmless males. One of the two mosquito species common in the area was almost never killed by the machines, because it was not attracted to the light. In the electrocutor-equipped yards, the number of mosquitoes collected by the workers was invariably higher than the number killed by the machine over the same period. This last finding suggests (though Nasci does not suggest it) that the machines are actually capable of increasing mosquito biting in some cases, and it helps explain why the distributor recommends that the devices be placed twenty-five to fifty feet away from the area of human activity. A mosquito might be lured to your back yard, then decide upon arriving that you are more appealing than the glowing fluorescent tube.
7. Consider the odds. To give the bug-zappers their due, they do seem to be effective indoors against many kinds of flying insects, mosquitoes included. Outdoors, in Nasci’s study, they did kill about 100 mosquitoes per machine per day—probably more than you could exterminate with oil, garden sprays, or the palm of your hand. Their distributor recommends that they be kept running continuously, the implication being that they might reduce insect populations over time. Nasci did not test this possibility. He concluded that the machines produced no decrease in biting on any given night, but he did not compare the mosquito population on the first night with that on the last; indeed, because so many variables affect population size and collecting efficiency, even from one night to the next, he says, making such a comparison would be impossible.
It is possible, however, to make an educated guess about your chances of success in any sort of backyard mosquito-extermination effort, electronic or otherwise. First, figure out how many mosquitoes you’ve got. As you might imagine, this is a difficult task, but a couple of researchers named D. E. Eyles and W. W. Cox tried it once in Tennessee, by releasing a known number of marked mosquitoes into a natural habitat. After allowing their mosquitoes time to disperse, they returned to the area and collected whatever mosquitoes they could find, counted the number of marked and unmarked specimens, figured the proportion of marked mosquitoes to the total, and thereby calculated the natural population density. They were dealing with only females of one species, and they calculated that in the height of summer (July and August), the test area harbored from 8,450 to 14,750 of these females per acre.
Consider this number, as rough as it may be, along with the tendency of some mosquito species to fly up to forty miles, and along with the uncanny ability of the family as a whole to exploit the tiniest niche offered by nature, be it a cesspool, a few ounces of water in a leaf base, or a suburban back yard whose natural population has suffered a sudden decline. Consider also that the females of some species lay more than 400 eggs in a batch, that many are able to lay several batches in a lifetime, and that, in order for a mosquito population to maintain its numbers over time, only two of each female’s eggs must hatch and survive long enough to mate and produce offspring of their own. Finally, consider the expert testimony of Dr. George Craig, who says that killing a few hundred mosquitoes per day is like “trying to ladle out the ocean with a teaspoon.”
In other words, consider giving up and going inside.
*This article was published in the Atlantic Monthly in June 1983, before the identification of West Nile virus in the U.S.