How tiny little insects get us to do exactly as they wish

By Richard Conniff|Sunday, June 01, 2003

One morning not long ago, an American entomologist named Justin Schmidt was making his way up the winding road to the Monteverde cloud forest in Costa Rica when he spotted Parachartergus fraternus, social wasps known both for the sculptured architecture of their hives and the ferocity with which they defend them. This hive was 10 feet up a tree, and the tree angled out from an eroded bank over a gorge. Schmidt, who specializes in the study of stinging insects, got out a plastic garbage bag and shinnied up to bag the hive.
   "There's always a few that get out," he says, so he took the precaution of putting on his beekeeper's veil. Undeterred, the angry wasps charged his face, scootched their hind ends underneath their bodies in midair, and, from a range of four inches, squirted venom through the veil straight into his eyes. "There I was, 10 feet up a tree, holding a bag of live wasps in one hand, basically blinded with pain."
    He slid down the tree like Wile E. Coyote after a tête-à-tête with Road Runner—but he held onto the nest. The one thing wrong with the story is that he only got sprayed by the wasps, not stung. He has been more successful on other outings. Without ever deliberately trying to get stung (which he admits would be unnatural), he has managed to sample the stings of about 150 different insect species on six continents. (Antarctica, with no stinging insects, is not worth the trip.)
    Schmidt, who works with the Southwestern Biological Institute and the University of Arizona, is the world's leading authority on the nature of stinging. The subject obsesses him, not just in the field but also at his Tucson home, where one room is largely occupied with terrariums full of venomous animals behaving badly. He is coauthor of the standard text in the sting field, Insect Defenses: Adaptive Mechanisms and Strategies of Prey and Predators. He is also the creator of the Justin O. Schmidt Sting Pain Index, a guide to just how bad the ouch is, on a scale of one ("a tiny spark") to four ("You might as well just lie down and scream").

Biochemical Hackers

    Insect stinging fascinates Schmidt because it constitutes a kind of high-tech warfare. Think of it this way: Your body is the computer, and stinging animals are the hackers—the dweebs and misfits that have managed to crack the biochemical code. With a little venom, they can penetrate cell membranes, manipulate neurons, convert systems of self-defense into instruments of self-destruction, alter the function of the heart, and even, in some cases, take over the central nervous system and subvert a victim's entire behavior pattern to their own needs. The toxins are so subtly attuned to biochemical nuances that pharmaceutical companies regularly study them in search of new drugs. Human tests are currently under way, for instance, using components of scorpion venom to treat brain tumors. A toxin from marine cone snails is the basis for a promising new spinal pain remedy.
    Stinging animals can also kill. In this country, fatal attacks by jellyfish and scorpions are rare. But according to Schmidt, about 40 people die each year from insect stings. Yellow jackets, honeybees, and fire ants—all members of the order Hymenoptera—are the usual culprits.

The Mother of All Stings

Stinger Anatomy 101
Click on the image to enlarge (92k)
Graphic by Don Foley

    Only the females of these insect species sting, and sex—or rather reproduction—is the reason they first learned how. It started back in the Jurassic Period, in an unknown species of parasitic wasp. Such wasps commonly use their ovipositor, a pointy extension of the abdomen, to lay their eggs on living caterpillars, beetle grubs, and other hapless victims, usually at a rate of one egg per victim. Some species actually have a serrated edge on the ovipositor to saw through flesh and deposit the egg inside the body. The wasp egg hatches, and the larval wasp then feeds on its living host until it sucks it dry, or in the case of a larva inside the victim, until it is big enough to burst forth, Alien-fashion, and fly away.
    The intended host understandably does not like big Mama Wasp buzzing around, and it typically throws up a frenzied resistance. But at some point in the primordial struggle, the saw-blade lubricants or other fluids in the ovipositor of some wasp species became paralyzing to victims. This made life infinitely easier for the wasp, and from this eureka moment, venoms evolved to suit an immense variety of circumstances, and ovipositors adapted to function as stingers. Bees and ants eventually evolved from Mama Wasp, and at least 60,000 different species in the order Hymenoptera now possess some form of stinger. Impression fossils of a wasp from Russia show that this evolutionary flowering was already well under way more than 120 million years ago.
    Even now, the vast majority of stinging insects use their venom primarily to parasitize tomato hornworms, cabbage loopers, and the like. Insect stinging is thus more a blessing on humanity than a curse: If female parasitic wasps were not out there busily killing agricultural pests, we would starve.
    But this is all too easy to forget in a moment of pain. For us, stinging mostly means nasty encounters with bees and other social insects that have retained no trace of the parasitic lifestyle. They now sting purely to defend the hive, and they are dismayingly good at what they do. On Justin Schmidt's sting pain index, honeybees rate only a two ("like a match head that flips off and burns on your skin"). But no instrument of biological terror is quite so thoroughly understood by science.

Sting Like a Bee?

    When a honeybee stings you, the barbs at the end of the stinger catch in the flesh, and the hind end of the bee rips off. This kills the bee. What's left embedded in the victim's skin is a hypodermic syringe capable of injecting venom for about 10 minutes. The package includes an alarm pheromone, which calls in other bees and incites them to sting too.
    The sharp end of the hypodermic syringe consists of a grooved tube, or stylet, flanked by two sharp knives called lancets. Each lancet is serrated with seven or more barbs. The ruptured bulb on top of the needle contains a neural ganglion that causes the lancets to slice up and down in alternation, so the barbs saw their way deeper into the skin. The top part of the syringe also contains the venom sac, as well as a valve-and-piston arrangement to pump venom down the stylet and into the wound.
    A single honeybee contains relatively little venom—about 600 micrograms of clear, colorless liquid—and usually injects even less. According to Schmidt, you'd need about 47,000 bees—more than two hives' worth—to get just one ounce of liquid venom. Yet even the minuscule dose injected by a single bee can be shockingly effective at driving a predator away from the hive. The venom contains at least 40 different ingredients geared for painful cellular warfare.

Hitting the Target

    Although they employ strikingly different weapons to get there, all venoms have the same basic target: The cell membrane, a two-layer wrapping around all biological cells. It consists largely of proteins and fatty phospholipids. The phospholipids are ingenious molecules with a bulbous head that's attracted to water and a fatty-acid tail that can't stand the stuff. So the phospholipids of the outer layer naturally line up side by side, their heads all pointed outward to bask in the great liquid sea of life. The phospholipids of the inner layer line up the other way, with their tails pointing toward the tails of the outer layer and their heads facing the calm inner sea of the cell. This makes for a very stable two-layer membrane, with a water-resistant fatty-acid middle that keeps the cell as snug as a house that's wrapped in Tyvek and R-19 insulation.
    Then the honeybee's venom bursts in, bent on havoc. A peptide called mellitin strikes the opening blow, shouldering in among the closely packed phospholipids of the cell membrane. This throws open the door to a powerful enzyme in the venom, phospholipase A, which rushes in and severs the connection between the head and tail of the phospholipid. The membrane begins to break apart. If the victim of this attack is a red blood cell, hemoglobin spills out in a widening stream until the entire cell dissolves. If the victim is a neuron, damage to the membrane upsets the delicate relationship between the ions inside and outside the cell, causing the neuron to fire little jolts of pain, called action potentials, over and over.
    Norepinephrine and other substances in the venom shut off the flow of blood, turning the skin white and keeping the venom concentrated around the sting. Thus the repeated, stabbing pain may persist for five minutes or longer, until the mellitin is gradually diluted and carried away from the area.
    At the same time, other substances in the venom are working to spread the pain. A substance called hyaluronidase liquefies the mucuslike glue of the connective tissue, enabling the mellitin and phospholipase A to scramble onto new targets. This so-called spreading factor is common in snake and spider venoms as well as in mammalian sperm, where it helps clear the path to the egg.
    Swelling and redness start to appear because of the mayhem induced by a variety of chemical irritants, including a peptide called MCDP (mast cell degranulating peptide), which is laser targeted on the mast cells in the skin. These cells are our frontline security system, specialized defensive cells present in virtually all body tissues. MCDP triggers them to release histamine and other substances. This dilates the blood vessels—a good thing in theory, because it brings macrophages and other immune-system tools to the scene.
    But it becomes a problem for people who are allergic to bee stings, because their mast cells are studded with antibodies specifically attuned to components in the venom. The tiniest dose unleashes a hypersensitive flood of histamine, which can cause swelling, bronchial spasms, and plummeting blood pressure. Without proper treatment, allergic (or anaphylactic) shock can lead to death in less than an hour, particularly if the victim is elderly and suffers other medical problems.

The Pain Index

    Most insect stings, however, do no damage at all. They just scare the wits out of us. And this is why they fascinate Schmidt. We typically outweigh any insect tormentor by a million times or more. We can usually outthink it too. "And yet it wins," says Schmidt, "and the evidence that it has won is that people flap their arms, run around screaming, and do all kinds of carrying on." Above all, we heed the insect's message, which is: Leave my nest alone.
    Stinging, says Schmidt, is a far more complex and paradoxical business than we might think. For instance, harvester ants, found from California to Florida, possess painful venom. In fact, one North American species has what Schmidt calls "the world's most lethal arthropod venom." Ounce for ounce, the poison rivals that of the Australian taipan, said to be the most dangerous snake on Earth. And yet harvester ants are what American parents give the kids to play with almost every time they buy an ant farm. These ants happen to be ideally suited for life in a plastic box, and they are so unaggressive that there's little chance a child will suffer even a single sting, much less the hundreds needed to cause death.
    Insects generally inject far too little venom to do serious harm to humans even when they attack en masse. So-called killer bees make headlines because they attack so quickly and persistently that they will sometimes chase a victim and inflict hundreds of stings. But since these hybrid bees started arriving in the southwestern states in 1990, Schmidt has documented only 11 deaths from envenomization. Dog bites kill more Americans in a single year.
    In any case, pain matters more than killing power for the insect's survival. "How does the insect win?" asks Schmidt. "By making us hurt far more than any animal that size ought to be able to do. It deceives us into thinking serious damage is being done." A trapped or threatened insect is a bit like Goldie Hawn in some Hollywood caper, cornered in the bad guy's corporate headquarters. She can't do any real harm. But she can hold a smoldering book of matches up to a fire detector. Much as the fire detector thinks "massive conflagration" and screams its head off, sting pain fools us into thinking "serious trouble." We panic, and harvester ant Hawn makes her getaway—or better yet, stings again. Schmidt rates harvester ants a three on his pain index (like "turning a screw" in the flesh or like "pulling out tendons and muscles").
    A reputation for inflicting pain liberates stinging insects from their predators and enables them to open up whole new ecological niches. Honeybees can visit flowers by day and not get eaten by birds. Pepsis wasps, including species that are the most painful stinging insects in the United States, freely roam the deserts of the Southwest, and all potential predators busily look the other way. Many insects adopt bright yellows, whites, reds, and blacks to advertise how painful they can be. Some cheaters even mimic the bright colors or threatening behaviors, although they in fact possess no venom. Some male wasps, for instance, often curl up as if to sting, fooling even experienced entomologists into momentary panic.
    Pain alone (much less the mere appearance of being painful) isn't enough to discourage all potential threats. Animals aren't dumb, and if they keep hearing a fire alarm where there's no fire—no real damage—they eventually calm down and figure it out. Bears learn to put up with bee stings as a cost of getting honey from a hive. Capuchin monkeys will gobble down a wasp's nest full of juicy larvae as if it were a ham sandwich, the stings seemingly no worse than a little hot mustard on the side. Harvester ant stings have evolved a whammy wicked enough to scare off birds, amphibians, and almost everyone else. But the horned lizard has called the ant's bluff by becoming resistant: It licks the ants up with impunity.
    Schmidt's belief in the defensive importance of pain is what inspired him to come up with his sting pain index as a more or less scientific way of quantifying the whammy. But he also believes the tendency of stinging insects over time is to supplement mere pain with "truth in advertising"—that is, to make the pain more persuasive by also causing serious damage or a prolonged debilitating effect.
Entomologist Justin Schmidt marvels at a honeybee hive draped in a mesquite tree in the Sonoran Desert outside Tucson. "Usually colonies can't exist outside of a cavity such as a hollow tree," he says.
Photograph by James Smolka

    Once, in Brazil, Schmidt was digging up bullet ants when he got stung. These large ants get their name because the sting "feels like a bullet went into you," he says. "I knew right away. I said, 'WOOH!'" When he sat down to savor the experience, the hand that had been stung started shaking uncontrollably. After a few minutes, he went back to his digging and got stung three more times. A few hours later, "quivering and still screaming from these peristaltic waves" of pain, he retreated to a local tavern to put ice on the stings and apply several large cervezas "to kind of deaden the central nervous system." But the waves of pain were still coming after 12 hours. "They're big ants, they've got a lot of venom, and it's quite toxic. A small thing like a bird or lizard could get some serious physiological damage if one of these things really nails them. A small monkey might weigh 10 pounds. If it starts messing around with these things, maybe the venom's not going to kill the monkey, but the hand trembling—that's not good for moving through the trees or avoiding predators. And that's how natural selection works."
    Schmidt speculates that the honeybee's ability to exploit the human immune system may represent another form of truth in advertising. About 2 percent of the human population is allergic to bee or wasp stings, a high incidence given that most people get stung only four or five times in their lives. Might bees and their cousins have evolved to target the mast cells and the immune system, Schmidt wonders, because the potential damage is more persuasive than the sting itself? Teaching people to stay away from the nest is an expensive process, if you have to do it one person at a time, and especially if each sting means the death of a bee. "But if you have a social organism like us," says Schmidt, "and if the family group sees Jill go into anaphylactic shock after a sting, everybody instantly gets the idea. You don't want to mess with this thing."
    It may not be terribly comforting to realize, next time you suffer a painful sting, that you are a show-and-tell exhibit in some insect's continuing education program. But it could be worse. In one horrific case in southern Africa, a man was attacked so relentlessly by honeybees that he had to jump into a river and hide beneath the surface. The bees continued to sting him every time he came up to breathe. The swarm was so dense he had to suck bees into his mouth and chew them to get any air. The attack went on for four hours, producing diarrhea, among other systemic effects, so that he was passing bees out one end while still ingesting them at the other. Finally, nightfall drew the bees back to their hive, and the victim dragged himself ashore. His face was literally black with embedded stings, and his hair was matted with dead bees. The doctors who treated him over the next few days counted 2,243 stings.
    But the victim lived to deliver the insects' lesson plan, which was, as always: Leave the hive alone.

A Sting Sampler

Pain Index: least painful most painful

Honeybee - Apis mellifera
Contrary to myth, pinching out the stinger doesn't cause it to inject more venom. So don't waste time looking for a credit card to scrape it out; just get it out quickly. The amount of venom injected depends entirely on how long the stinger is stuck in your skin. Pouring salt water on the wound and taking ibuprofen or other anti-inflammatory drugs will help dull the pain.
Africanized ("killer") bee - Apis mellifera scutellata
Swarms of these notoriously aggressive hybrid honeybees, originally imported to Brazil in 1956 to increase honey production, have spread in recent years from Mexico into Texas, New Mexico, Arizona, Nevada, and California. Africanized honeybees can pursue a victim up to one mile and often stay agitated for several days after an attack. But such attacks have proven to be rare.
Yellow jacket - Vespula germanica
This is the wasp that crawls into your soda can seeking sugar, with nasty results next time you take a sip. Unlike a honeybee, it can sting repeatedly, and it causes about 20 deaths a year in this country, usually from allergic reactions. If you're not allergic, you might want a nest in the yard, since North American species feed on pests like mosquitoes and houseflies.
Pepsis wasp - Vespula germanica
Several Pepsis species in the desert Southwest are known as tarantula hawks because they attack hand-size tarantulas, paralyze them, and drag them off to a burrow to serve as food for wasp larvae. You're unlikely to get stung, unless you happen to be unusually short, hairy, and eight-legged. The debilitating agony from a Pepsis wasp sting lasts about three minutes in humans.
Paper wasp - Polistes annularis
This narrow-waisted wasp often takes up residence along the eaves of the roof or in the attic. You can recognize paper-wasp nests by the open cells left for the larvae. As with all stinging insects, avoid stirring them up by waving or other sudden movements. Paper wasps are aggressive stingers, and their venom kills one or two people a year as a result of allergic reactions.
Hornet - Vespa crabro
Hornets, close relatives of yellow jackets, live in nests that are sheathed in a papery covering and have a football shape. Never attempt to remove a nest without professional help, because hornets can sting repeatedly and their venom contains acetylcholine, a powerful pain stimulant. Applying meat tenderizer to a sting breaks down venom components, reducing pain.
Harvester ant - Pogonomyrmex maricopa
Why put ants with a painful sting into toy ant farms? Because these ants lack sticky substances on their feet and can't climb out. They also dig like crazy and are easy to see. In the wild, they're found from Florida to California. It would take 800 or more stings to kill an adult human, but no one who's been stung once stands on these ants' nests long enough to get stung twice.
Imported fire ant - Solenopsis invicta
Native to South America, these ants landed in Mobile, Alabama, around the 1930s, probably in soil that was used for ships' ballast. They are common throughout the Southeast and have spread as far west as California. Fire ants typically attack en masse. The sting of one ant causes the victim to twitch. Then other ants in the swarm get agitated and sting as well.
Scorpion - Centruroides exilicauda
More than 50 scorpion species live in the United States, mostly in the Southwest. The stinger at the end of the upraised tail looks fierce, but the effect is usually no worse than a honeybee sting. The bark scorpion, Centruroides exilicauda, is the one U.S. species with dangerous venom. Scorpion stings have caused only a few deaths in this country over the past decade.
Jellyfish - Physalia physalis
In this country, the Portuguese man-of-war and one or two other species can kill, but that rarely happens. Don't rub the sting area, because it will only cause irritation; nor should you apply ice or fresh water, which will only increase the pain. You should remain still to slow the spread of venom. Remove any tentacles with a pair of tweezers, and apply vinegar or isopropyl alcohol. has plenty of physician-provided information on what to do if you get stung by various poisonous animals. Topics include bees, wasps, and ants:; jellyfish:; scorpions:; and sea urchins:
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