Natural Acts: A Sidelong View of Science and Nature - David Quammen (1996)

ALL GOD’S VERMIN

The Widow Knows

HERE’S A CHEERFUL THOUGHT: Some knowledgeable people believe that black widow spiders, like locusts and jackrabbits, come in plagues.

No one, of course, has kept a precise running count on our total supply. Since the black widow spider is by nature shy, almost fanatically discreet, the intermittent explosions of black widow populations are gauged by extrapolation from the number of bites suffered by humans. At certain historical junctures of place and time, those widow bites have reached what are called “epidemic” levels. Spain had such an epidemic in 1830. Eastern Russia had another in 1838-39. France endured a peak forty years after Russia, Uruguay in 1910, and then, in 1926, black widows terrorized Yugoslavia. These outbreaks of spider bite and spider fear were all presumably caused by members of the genus Latrodectus, not precisely the same species as our American black widow but closely related. The last major episode of notoriety for Latrodectus mactans, our domestic version, occurred a half-century ago. In the autumn of 1934, with Huey Long gamboling in the Senate and John Dillinger freshly slain on the streets of Chicago, Americans were suddenly worried about black widow spiders.

The widow boom that year was no doubt a combined result of climatic conditions, ecological cycles, and publicity. Through the mild winter and dry summer of 1933, more black widows had been surviving, raising larger broods, biting more humans, and getting more journalistic attention for doing so. The Associated Press carried continuing reports on the condition of two unfortunate men, one in Alabama and one in Idaho, both dangerously ill from black widow bites. Local newspapers ran stories about barbarous mortal battles staged between black widows and scorpions, black widows and tarantulas, even black widows and snakes. With a peculiar repulsive fashionableness, black widows were in. Then Scientific American fanned the coals with an article announcing in careful detail why Latrodectus mactans should rightly be feared, and in November the sober journal Science published a note entitled “On the Great Abundance of the Black Widow Spider.”

The note’s author speculated about effects of the recent weather, commented that cousins and competitors of the widow seemed to be in eclipse, and concluded, “Possibly L. mactans is beginning to get the upper hand in the great struggle for existence.” But the struggle against whom? To some nervous observers, this upper-handhood seemed ominous in a creature with eight legs, not to mention a pair of poisonous chops, so within months that statement from Science had been translated—now in the less sober pages of Popular Mechanics—into this one: “Aided by favorable climatic conditions, the spider has multiplied so rapidly that it is becoming a real menace to man.” The headline in Popular Mechanics was “Wasps May End Black Widow Spider Menace.” Entomologists were groping about for some spider-eating savior, a natural predator, a nemesis, to turn back those menacing, swelling, surging hordes of Latrodectus mactans.

Two species of wasp were considered, a mud dauber that kills adult black widows as food for its young and another form that chews its way into the silken egg sac and deposits its own eggs where, after hatching, the wasp larvae can eat the unborn spiders. There was also talk that black widows might be subdued by a species of small parasitic fly, or by alligator lizards allowed to run rampant, or by a certain brave sort of toad, or perhaps by spider-eating spiders of the genus Mimetus. People were desperate.

Yet these scientists and other widow-watchers needn’t have been quite so concerned.

The Great Widow Scare of 1934 came and went, like the Kingfish and Dillinger, but no airplanes were ever called out to spray. No frantic eradication campaign or mass deployment of wasps ever took place. The crisis—more accurately, the perception of a crisis—passed away inconclusively, thanks to some form of natural equilibration. Latrodectus mactans continues to be widely distributed, present in every American state and common in many, abundant in most of the warmer ones. Chances are good that wherever you live, during the last year sometime you have sat down within ten feet of a black widow. (If you had occasion to use an old wooden outhouse, among the widow’s favorite habitats, you might even have been cheek-by-jowl with one.) Still, the species has never become a large-scale problem. Being bitten is nothing to take lightly, and if the victim is an old person or a small child, it can be fatal. But this is quite rare. The plague hasn’t happened. The widow hordes, with their eight upper hands in the great struggle for existence, have never come marching like driver ants over the hill into your town or mine. Why not? Partly because, in an ecological sense, the black widow knows its place.

More specifically, she knows her place. The most efficient natural predator controlling any population of black widow spiders is very likely the female black widow. In that delicate Malthusian balance between vast reproductive potential (many eggs per sac) and limited life-supporting resources (only so many good spots to build a web, and only so much food available to be netted), the female of the species serves a gyroscopic function.

Concerning the behavior of the female of Latrodectus mactans, one point should be clarified. A black widow spider is not like a mad dog or a vicious human: She doesn’t bite without reason, she doesn’t kill without purpose. She is not necrophilic or otherwise kinky. Yes, sometimes she does lasso her chosen male after copulation, as he makes for the exit, and suck him dry as a roasted chili. But that only happens if she’s hungry.

The female widow’s hunger, or lack of it, is the standard against which certain life-or-death decisions are made, and those decisions exert a geometrically multiplied impact on overall population levels of the species. When the concentration of widows in any area is high, competition is fierce for good web sites and ambient provender. As a result, each widow tends to be hungrier. The female has an amazing capacity to endure long stretches without food—three or four months, at least, and one widow in captivity went unfed for nine months, then was nursed back to health—which makes it unlikely that the species could ever die out entirely from starvation. But as competition grows keen and food grows scarce, the female widow takes some drastic population-control measures that tip the balance back toward circumstances of lonely affluence. In the process, she also slakes her own hunger. The first of these measures involves mating.

The male widow, much smaller than the female and more mobile across unfamiliar terrain, takes the romantic initiative. He appears at the edge of the female’s web, sets his feet on a few strands, and, by bobbing his abdomen, causes the whole web to vibrate. This is the mating offer. If the female is not in condition to breed or not in the mood, she will not respond. But if she happens also to be very hungry, she may wait cryptically for him to venture within reach, then grab him, swathe him in silk, and eat him. If she is ready to mate, there is an answering pattern of web vibrations from her. After about two hours of foreplay, during which he wraps her in a loose veil of silk, they consummate. That takes about five minutes, but the positions are exotic.

As they uncouple, the question again arises: Is she hungry? If so, he is snatched back and devoured. If not, he is allowed to leave peaceably, or he may linger in a corner of her web, safely ensconced as mate emeritus, until accelerated senescence kills him a few days later. Whichever of these outcomes occurs, the female has remained in character, neither sentimental nor sadistic but merely practical, all eight of her eyes fixed on the basics of survival. She is the incarnate future of her species, far more so than the male, and in a deeply instinctive way she acts upon that responsibility.

The same sort of chilly pragmatism governs her maternal behavior. She’s a solicitous mother, but only up to a certain point. Her eggs are laid—anywhere from 25 to 1,000 in a batch, on average about 200—and then wrapped by her in a silken cocoon. This egg sac is watertight and reflective; it shields the eggs from sunlight, rain, and predators and helps keep them warmly incubated. The mother has taken great care over it. She will fight to protect it. She will spend immense effort moving it from one part of the web to another, sunny exposure to shade, for the sake of maintaining its optimal temperature. Then, when the eggs hatch and her young crawl out, if the message of hunger has by now again reached her and the prospects seem meager for black widows, she will eat them.

Or at least some of them. After thirty or forty she may stop, and the rest of the spiderlings will go off to face all the other obstacles, including starvation, between them and adulthood.

The behavioral ecology of Latrodectus mactans has been studied that far, but not much farther. Even the scientists, even the arachnologists, are still puzzled; and their puzzlement is our ignorance. Does the maternal black widow perceive some conscious or unconscious signal about how many young spiders might be expected to survive? We don’t know. Is the male widow sexually capable of servicing more than one female? We don’t know. Does the possibility of his fathering further broods influence his first mate in her choice of whether or not to kill him? We don’t know. And the female, with her edgy perceptions of hunger, her hair-trigger readiness to cannibalize—has she been programmed by evolution to damper fluctuations in the population of her species? It seems plausible. But we don’t know.

There’s a welter of uncertainties. If the female black widow wore a red question mark on her abdomen instead of an hourglass, she could scarcely better exemplify the limitations of our understanding of this—and many other—familiar but neglected aspects of the natural world. Who knows what graceful intricacies lurk in the behavioral wiring of Latrodectus mactans? Not us. Maybe that’s why she seems so spooky, so necessary, and so beautiful.