The Sting of the Wild Read online

  I am aware of few reports of young children being stung by a cow killer. Why would a child seeing a beautiful red velvety cow killer running across the backyard not simply pick it up? Perhaps some do and the screaming child cannot describe the source of the sting to the parent. But surely the parent would look for the source and should easily spot the culprit. The more likely reason cow killer stings to young children are so infrequently reported is that they rarely occur. Just as we instinctually notice and avoid snakes and spiders,2,3 we instinctually notice bees, wasps, and other potentially dangerous stinging insects, including cow killers. The bright red and black coloration both attracts notice and signals pause and caution: look before you leap; watch before you touch. Contrasting patterns of red and black are classic aposematic warnings that signal would-be predators to “back off, leave me alone … if you do not, you will regret the consequences.” Aposematic, derived from the Greek words apo = away, and sematic = signal, perfectly describes the cow killer. Its nasty sting backs up the warning, and in the case of the cow killer, additional warnings also come in the form of sound, a squeak of broad frequency range that resembles a miniature rattle of a Lilliputian rattlesnake, and an odorous chemical warning signal. The cow killer releases these warning chemicals from glands at the base of the mandibles (the insect’s jaws), a blend of volatile ketone molecules that smells like fingernail polish remover. For nocturnal predators, or those with poor vision, one or both of the sound or smell warnings are memorably received.

  Cow killer defenses do not stop here. In case of an actual attack, two powerful backup defenses come into play. The first is the immensely hard integument, or shell, of the cow killer, rather like a biological tank with a hard, impenetrable body armor. Cow killers are so hard that stainless steel insect pins sometimes bend without penetrating the body. Equally impressive and more biologically relevant, adult tarantula spiders are unable to penetrate cow killers with their impressive fangs, and, on feeling the vibratory squeaking, a feeling akin to a mini jackhammer against one’s teeth, quickly release the cow killer. Immense leg strength is a final defense. The cow killer’s box-shaped thorax, the middle of the three insect body parts, houses not powerful muscles for flight, as in most insects, but instead enormous muscles that power the legs. These powerful legs combined with the rounded, slippery body enable the insect to wrest itself free from a predatory grip and then rapidly run away and escape. Does a child or adult consciously know of these defenses? Not likely. But the signals are clear: be cautious and avoid me or you’ll be sorry. The sting message is conveyed; truth is communicated.

  2

  THE STINGER

  Petruchio: Come, come, you wasp, i’faith you are too angry.

  Katherine: If I be waspish, best beware my sting.

  —William Shakespeare, The Taming of the Shrew, ca. 1590

  IF A STINGING INSECT COULD SPEAK, the first words it might shout are, “Who is at my door?” This simple thought can determine life and death. Life requires growth, reproduction, and survival. Without any one of these, a species would not survive to the next generation and would not have arisen in the first place. Survival is the lack of death. Animal survival is simple in theory: fill one’s stomach with nutritious food, and don’t end up in anyone else’s stomach. Both of these are challenges because most animal tummies are filled with plants, and plants are the world’s best chemists at synthesizing a dazzling array of chemical compounds. These compounds are synthesized expressly to prevent being eaten or to outcompete other plants for light or nutrients. Predatory animals face other problems with their food, mainly finding, capturing, subduing, and consuming their prey. For the stinging insect, how not to be eaten assumes crucial importance, and herein lies the value of the sting.

  The stinging insect is focused on not ending up in the stomach of the visitor at the entrance to the nest. Is that activity near the nest caused by an animal, or is it incidental to wind, to weather, or to nearby plants? If the latter, no predatory threat is present. If the former, the insect must determine whether the animal is a threat or is harmless. The visitor at the entrance could simply be a bumbling cow or a wayfaring rhinoceros. The main threat, then, is being accidentally stepped on or having the insect’s home, the nest, destroyed. If little risk of being eaten exists, then usually, no defensive action is needed. An interesting exception was described by Fritz Vollrath and Iain Douglas-Hamilton involving elephants and honey bees.1 Elephants are not known to eat honey bees, but they eat trees, including large trees and branches housing honey bee colonies. Destruction of the bees’ nest by an elephant knocking down or destroying the tree would be a serious problem. Bees mount stinging attacks against intruding pachyderms to eliminate the threat of nest destruction, by targeting the elephants’ vulnerable eyes and nose, thereby driving the elephant herd away from the nest.

  Not all intruders at a bee, wasp, or ant nest are vegetarians. Some are specifically seeking the nutritious inhabitants, their helpless young larvae and pupae, or any food stores, including honey, pollen, or dead/paralyzed prey within the nest. The stinging insect must now determine the least dangerous way to stop the intruding predator from continuing its mission. An ideal defense is to threaten the predator from a distance. Chris Starr, a former classmate, who is now in Trinidad, explored carefully the range of threats signaled to potential predators by paper wasps in the genus Polistes. Polistes are reluctant to fly off the nest and attempt to sting birds, mammals, or Chris. Instead, they engage in a series of increasing threats to avoid death by being bitten, smashed, or eaten: face the intruder with body raised high on legs; raise wings above the body and spread apart; flip wings up and down in a fast motion; buzz wings momentarily while remaining on the nest; flutter wings while still remaining on the nest; wave raised front legs toward intruder; curve the gaster (the part of the abdomen after the narrow waist); and fly off the nest but not toward the intruder. These threats often deter the intruder and at little cost to the wasp.2 Wasps sting only after these threats fail.

  Warning threats can take many forms, including sounds and smells that do not depend on predator vision. A variety of ants that include Pogonomyrmex harvester ants, leafcutter ants (Atta), Australian bull ants (Myrmecia), and bullet ants (Paraponera) stridulate to produce a broad-frequency squeak. All investigated velvet ants (Mutillidae) also readily sound a stridulatory squeak threat when they sense possible danger. Hornets in the genus Vespa have perfected mandible snapping or clicking as another effective acoustic threat. When I was in Japan in 1980, some generous students of social wasps and their professor assisted me in collecting an entire colony of the giant mandarin hornet, Vespa mandarinia. This enormous wasp with its brilliant orange blocky head vies for the title of the most intimidating insect on Earth. Their preferred food is the young of other hornets, social wasps, and honey bees, the adults of which they dispatch quickly and simply by crushing them with their enormous mandibles, having no need to waste precious venom in this operation. My Japanese colleagues and I presented a different threat to the mandarin hornets than their usual stinging prey. We were their predators, not their prey! After climbing into my armored bee suit, I grabbed a handheld insect net with a 6-inch handle and approached the hornet nest swinging. The students, perhaps wiser than I, ingeniously attached insect nets to long straight tree shoots and caught any hornets attacking me from the rear. The most memorable part of the ordeal was seeing and experiencing enormous wasps hovering eyeball-to-eyeball in front of me, loudly snapping their jaws. The best bee suit in the world could not allay the fear and awe generated by these threats. And these are not idle threats—a single sting can kill a rat. We were luckier than a rat and collected all the hornets and their nest with nobody getting stung.

  Warning threats can be odorous chemicals. Ants are masters of chemical warfare with the added benefit that these odorous compounds serve as warning—“stay away or suffer the consequences of stings and bites.” Pogonomyrmex harvester ants release volatile ketone compounds that sm
ell like nail polish, and velvet ants use a nearly identical blend of ketones for the same purpose. Bullet ant chemical warnings include odors that resemble burnt garlic. Tarantula hawks (Pepsis) perhaps produce a most distinctive warning odor: a pungent and unaesthetic odor emanating from glands in the head. All of these odors can be released to warn intruders not to attack, thereby reducing the risk of actual attack. If predators attack, these odors enhance predator learning that attacking this stinging insect is punishing and a bad idea.

  The insect world is a blizzard of odors. Chemical odors are not just aposematic warning signals, a minor role in insect life. Odors run most of life, ranging from sex pheromones to help males and females find one another at the right time and other pheromones communicating alarm, aggregation, and individual recognition, to a near limitless variety of chemicals that convey information about food. From early in my youth, odors that convey information about dangers, especially the presence of large, dangerous predators, have been particularly interesting to me. To sting and drive off a predator, stinging insects must first detect and recognize a predator. I worked for many years with honey bees, asking, among other questions, how honey bees detect a predator. My research revealed that odor—in this case, mammalian breath—is the strongest cue to honey bees of the presence of a mammalian predator. Breath is hot, humid, and contains carbon dioxide plus a variety of small volatile aldehydes, ketones, alcohols, esters, and other compounds. To bees, breath is a smelly pool of airborne chemicals, an immediately recognizable stench. When Africanized (“killer”) honey bees arrived in Arizona in 1993, a colleague, perhaps naively, hived a few reproductive swarms of killer bees and kept them on the research location in the middle of Tucson. They would periodically go on a rampage stinging nearby people. Fortunately, none of the people were innocent public bystanders. The unfortunate victims were high school and college students, employed part time by the director of the Tucson Bee Lab, who bore the brunt of the attacks. I, as a behaviorist, needed to observe activities directly at the hive entrance and took advantage of these colonies hived by my colleague. The solution to close-range observation was simple: be “invisible” to the bees. To achieve invisibility in the presence of bees, stop breathing (granted, it is hard to stop breathing entirely for long) and move slowly. Hold your breath as you stand inches to the side of the landing board and then turn your head to exhale gently a few feet behind the hive between breaths. One day my good friend John Lewis in the maintenance department was walking 25 feet away from the colony I was watching and got stung. “Hey, Schmidt, how come I get stung and you poke your nose six inches from the entrance and don’t get stung?” No, it was not bad living, just his “bad” breath.

  Suppose a bee recognizes a potential predator but threats do not work? As a last resort, it may engage in the riskiest defense, planting the sting (sometimes interchangeably called the stinger) in the flesh of the intruder. The outcome depends on how well the sting is inserted, whether the stung animal (or the stinging insect) can remove the sting, the composition of the venom, and whether the target is susceptible to venom. The insect stinger is the original biological syringe, complete with a needle and a chamber that holds the liquid that’s injected through the needle. Unlike a solid, tubular medical syringe needle, the insect sting shaft is composed of three parts, two of which slide in channels along the third immobile part. The sliding design overcomes the problem of the insect’s small size. Imagine a mouse-sized doctor attempting to inject an antibiotic through a syringe into a patient. Could the doctor be large enough to grasp the syringe barrel and strong enough to push the needle into the flesh and depress the plunger? The insect’s self-penetrating stinger solves these problems. The stinger works its way in through muscles that first slide one mobile side of the stinger deeper into the flesh and then the other side. Backward-facing barbs on the sliding stinger components help keep the embedded part from sliding back out while the moving side is inserted deeper. Instead of a thumb on the syringe plunger, insects solve the delivery problem in several different ways. For some stinging insects, a gizzard of muscles surrounding a sac of venom forcefully expels the venom. For other insects, a valve system inside the stinger assists in pumping venomous fluid through the hollow stinger shaft, usually aided by fluid pressure caused by muscle contractions, which telescope abdominal segments inward, generating the necessary pressure to squeeze the venom through the stinger and into the body of the target.

  This marvelously functional device, the stinger, came from evolutionarily humble origins. Deep ancestors of stinging insects were sawflies, vegetarians that, despite the name “fly,” are primitive wasps that use a stiff, hollow ovipositor to bore through plant tissues and stems to lay eggs in protected places. This tube was the key to the evolution of the stinger. The stinger is a hollow drilling tube that delivers venom instead of eggs into targets. A large series of developmental steps occurred between the ancestral sawfly ovipositor and today’s ant, wasp, or bee stinger. A notable intermediate step is exhibited by parasitic wasps, which continue to use the ovipositor/stinger for depositing eggs but add venomous fluid that paralyzes or otherwise aids in preparing the host as suitable food for future offspring. Stings by parasitic wasps generally cause little or no pain in humans, an indication that parasitic wasp stings have not yet evolved a meaningful defensive role. The significant evolutionary change in the stinger, which dramatically altered its role, was the addition of a cocktail of venom components, concurrent with the elimination of the role as an ovipositor. Eggs were now delivered through an opening at the base of the stinger, freeing the stinger to function solely as a venom-delivery device.3 Liberated of its egg-laying role, the now pure stinging apparatus was free to evolve venom that was active not only on hosts but also for defense against predators. Dual paralytic and defensive venom components continue to be seen today in many primitive ants and some solitary wasps, but this dual role is absent in all bees, an enormous group of 20,000 species that have lost the use of live animals for food, mainly replacing them with a vegetarian diet of pollen and flower nectar. In bees and social wasps, the role of the stinger and venom is strictly for defense against predators, with occasional use against other competing individuals, as witnessed during death fights among newly emerged honey bee queens and usurpations of established colonies by invading queen yellowjackets. Most advanced ant species primarily use venom for defense; however, on occasion, venom is used to capture prey.

  “Careful, don’t let him sting you” is an all-too-familiar phrase to warn against stinging insects. But male stinging insects do not sting. You read right. Males do not sting. Why not? The answer could not be simpler—they do not have a stinger! Even if a male bee (or ant or wasp, for that matter) attempted to sting, it lacks the equipment. The stinger is a highly derived, egg-laying tube, and males cannot lay eggs. They simply cannot evolve a stinger similar to a female’s stinger. Consequently, males are harmless, have no ability to hurt large predators, and do not even aid their sisters to defend against predators. Threaten a male bee or a wasp, and it flees or hides. One of the more enjoyable aspects of teaching children about stinging insects is to reach into a jar and grab a robust buzzing honey bee. Invariably, the surrounding group gasps, in wide-eyed awe. How did I do that? Was it magic? Did I have a special mental power to control the bee? The bee was, of course, a male, often derogatively called a drone, and harmless. If only all such teaching were so didactic. In Arizona, the difference between male and female stinging insects can be vividly illustrated. Huge black carpenter bees, many times the size of a honey bee, abound in the spring. An audience’s initial surprise at seeing me pick up one of these giant bees turns to shock when I gently place it between my lips. I don’t mention that these wood-chewing bees have powerful jaws and can bite. No matter, the lesson is conveyed, although few are willing to volunteer to repeat what they have seen.

  The previous example should not imply that males have no defensive tricks of their own. Nature has a never-ending store of
surprises, illustrated magnificently by male bees and wasps. In place of a stinger, a male bee or wasp has hardened genitalia (the term for insect genitals) used for grasping the female during mating and transferring sperm. Male genitalia are a showcase of structural plasticity. Each species has minor to major structural differences from related species, which reduce the probability of mating between different species. This plasticity is also a preadaptation for evolving useful defensive structures, in this case, sharp, pointed sting-like projections from the end of the genitalia. When grabbed, males exhibit remarkably realistic stinging motions, and jab these hardened pseudo-stings into the skin of the captor. The predator’s automatic response to being stung is to let go of the stinging creature. A pseudo-sting is usually sufficient to secure release, even from an experienced entomologist who intellectually knows better but who is overcome by natural instinct. To my chagrin, I have been tricked by a male wasp and lost a coveted specimen.

  The sting is effective for only one reason: the venom. Venom is a liquid blend of materials that can be injected through the stinger. Most venoms consists of small water-soluble proteins, peptides, biogenic amines that also act as neurotransmitters within animal bodies, amino acids, fatty acids, sugars, salts, and a few miscellaneous compounds. Some insect venoms, notably from fire ants and their relatives, consist of alkaloids in chemical classes similar to coniine, the compound from water hemlock that Socrates was forced to drink. Other ant venoms are terpenes that smell like pine. All of these venoms act when they are injected below the protective epidermal barrier and into the body. Many are inactive if applied on the skin, because they cannot penetrate the skin to target vulnerable tissues and the bloodstream. The lack of penetrability of proteins, peptides, and biogenic amines, in particular, limits their use in traditional chemical defenses that are sprayed, dabbed, or oozed onto adversaries’ skin. By enabling delivery of active components beneath the skin, the sting and venom opened a bonanza of opportunities for the evolution of highly specific and active components, particularly proteinaceous materials.