The Sting of the Wild

by Justin O. Schmidt

  Solitary life for wasps is a trade-off between the advantages of sociality versus the advantages of solitariness. One advantage of solitariness is timing. If food is present in abundance only during a brief time, for example, when young katydids are present, then a solitary wasp species needs to be present only during that time. The wasp has no need to be active or present during the rest of the year. This saves energy and reduces risk from predators and the abiotic factors of weather and climate. Solitary species also can become super niche specialists that do one thing better than anyone else. For example, the colorful striped wasp, Cerceris fumipennis, specializes in finding, capturing, and paralyzing rock-hard metallic wood borer beetles and in finding them efficiently.2 In contrast, entomological experts studying these same beetles have great difficulty locating them and, in several cases, have turned to mining nests of Cerceris to find beetle species new to science. Another example is the cicada killer wasp, which readily finds and captures cicadas, something social wasps and people cannot frequently achieve.

  As parents know well, preschool children are prone to catching and transmitting contagious diseases. Contagious diseases of many varieties also plague social insects. Solitary species, thanks to their low contact with one another, avoid many problems of infectious diseases. Another major advantage of the solitary lifestyle is the ability to be less obvious to predators. The bustling activity of a colony and its often large nest are more apparent to hungry predators than a solitary wasp nest. Solitary wasps with their infrequent activity are far less likely to be noticed or to draw unwanted attention. Importantly, an individual solitary wasp is simply not worth the effort for many large predators to capture and eat or to dig up and destroy its nest to consume provisions and young. A large predator is more likely to view numerous individuals in a nest as worth the potential pain and effort. A human equivalent is the enticement of a person to cross a room for a single peanut versus the lure of a bowl of peanuts.

  An associated defensive option more available to a solitary species than social species is the advantage of fleeing from predators. When workers of social species are near their nest, they tend to defend and hold the fort against predators. The option to flee is reduced for the worker. If the worker doesn’t prevent the destruction of the queen(s), all, or most, is lost. If the nest of a solitary wasp is attacked, the wasp simply flees, saving herself. Even if the nest is destroyed, an unlikely event, considering most predators might not recognize the nest presence or decide it is of too low value, the wasp can readily build another and continue.

  The greatest risk of life is living. The expressions “live and let live” and “eat, but don’t be eaten” capture the essence of the problem. Life is all about the family, the individual’s family, and ability to continue the family into the future. Life is required for reproducing the family into the future, but the process of living is a major risk. At any time, life can terminate by ending up in someone’s stomach. The longer one lives, the greater the risk of being eaten. One solution to life and family is to reproduce quickly and then, the job over, to die. Mayflies exemplify this extreme. Some live as adults less than an hour, mate, and die on the water’s surface with eggs pouring out as they die.3 Nature selects for maximal efficiency of reproduction with the minimum time devoted to activities that do not directly enhance the reproductive effort. For many solitary wasps, short life is an adaptation to avoid risky living and non-reproductive time exposure to predators. If the wasp is a prey specialist, as most are, and the prey is present each year for only a few weeks or so, then the best life strategy is to emerge as an adult wasp just as the prey presence is beginning, to work feverishly to catch prey while it is available, to reproduce, and to die after the prey season ends. No point in staying around beyond that. Many social species do not have the option of short adult life span. They must maintain the colony throughout the year, including long seasons of total, or near-total, unproductivity, all the while exposed to predators.

  STUNG BY A TARANTULA HAWK? The advice I give in speaking engagements is to lie down and scream. The pain is so debilitating and excruciating that the victim is at risk of further injury by tripping in a hole or over an object in the path and then falling onto a cactus or into a barbed-wire fence. Such is the sting pain that almost nobody can maintain normal coordination or cognitive control to prevent accidental injury. Screaming is satisfying and helps reduce attention to the pain of the sting. Few, if any, people would be stung willingly by a tarantula hawk. I know of no examples of such bravery in the name of knowledge, for the reputation of spider wasps—specifically tarantula hawks—is well known within the biological community. All stings experienced occurred during a collector’s enthusiasm in obtaining specimens and typically resulted in the stung person uttering an expletive, tossing the insect net into the air, and screaming. The pain is instantaneous, electrifying, excruciating, and totally debilitating.

  Howard Evans, the great naturalist and author of the eminently enjoyable books Life on a Little Known Planet, Wasp Farm, and The Pleasures of Entomology, was an expert on solitary wasps. Howard, a slight, reserved man with a shock of white hair and a sparkle in his eyes, was especially fond of tarantula hawks. Once, in his dedication to the investigation of these wasps, Howard netted perhaps 10 female tarantula hawks from a flower. He enthusiastically reached into the insect net to retrieve them and, undeterred after the first sting, continued, receiving several more stings, until the pain was so great he lost all of them and crawled into a ditch and just sobbed. Later, he remarked that he was too greedy.4

  I know of only two people who were “voluntarily” stung by tarantula hawks. I say “voluntarily” as both were film actors performing their duties, which, among other things, “encouraged” being stung. One was a young, handsome athletic entomologist who knew of the wasps. He deftly reached into the large cylindrical battery jar and grabbed a wasp by the wings. He had her in such a position that her sting harmlessly slid off his thumbnail. We prattled for a minute or so about tarantula hawks while the camera scanned close up to the long sting as it slid harmlessly, missing its mark. Then with a great heave the wasp pulled its abdomen back and thrust the sting under the nail. Yeee…ow (we can’t recall if any expressions unsuitable for general audiences were uttered), the wasp was hurled into the air and flew off unharmed. One point for wasp, zero for human.

  The other actor was a solidly built fellow who could easily have been a football linebacker, and who was a master of performing pain-defying acts of bravery. I, however, was charged with catching the wasp and delivering it to the scene. Five or six tarantula hawks were easily netted from flowers of an acacia tree; unfortunately, the net snagged on some thorns, and all but one wasp escaped. The remaining wasp was a male, so I summoned the cameraman to demonstrate how males cannot sting and are harmless. I reached in and casually grabbed “him.” The him was a her. Yeee…ow, except this time it was me. I managed to toss her back in the net, while attempting to explain my blunder and pain on camera. Unfortunately, I was not the actor, so the footage was relegated to some obscure studio archive, perhaps someday to be resurrected on YouTube. That episode over, the tarantula hawk was delivered to the rightful actor. He grabbed her, was stung, and showed no reaction beyond a begrudging “ouch, that did hurt a bit.” I figured the guy had no nerves. But his director handed him a habanero pepper, the tarantula hawk of chili peppers, which he enthusiastically bit into. He became instantly speechless, convinced fire was blasting from his mouth, nose, and ears. Apparently, he did have some nerves—sensitive at least to chili peppers.

  Tarantula hawks have never been recorded as a part of human warfare, but they might be candidates in some future altercations, and surely they come a close second in personal battles. Howard Evans, in a moment of exasperation, wrote of an experience in Mexico: “[Tarantula hawks] are spectacular creatures, on a number of occasions I collected these wasps in the Southwest and in Mexico, followed by a group of urchins who asked questions and tried to help
. My trick to be rid of them was to pick a tarantula hawk off the flowers with my fingers and show it to them. Of course I always picked up a male, which cannot sting. But my curious followers would pick up a big one, usually female, and quickly decided they wanted no more of that.”5

  How could such a small animal as a tarantula hawk embed itself so strongly in the human psyche and win? Several years ago I attempted to address this question in a paper entitled “Venom and the Good Life in Tarantula Hawks: How to Eat, Not Be Eaten, and Live Long.”6 The natural history of tarantula hawks provides some insights. Tarantula hawks are the largest members of the spider wasp family Pompilidae, a family of some 5,000-species strong that prey solely on spiders.7 The feature of tarantula hawks that makes them so special is their choice of the largest of all spiders, the fierce and intimidating tarantulas, as their target prey. The old saying “you are what you eat” rings true for tarantula hawks: if you eat the largest spiders, you become the largest spider wasps. As with other spider wasps, the female wasp provides each young with only one spider that serves as breakfast, lunch, and dinner for its entire growing life. The law of supply and demand applies: large spiders produce large wasps; small spiders produce small wasps. The story doesn’t end here. Momma wasp is not entirely at the whim of fate and fortune in the size of spiders she encounters, with her young randomly enduring the consequences. She has the special ability to choose the sex of her babies. Hymenoptera are oddballs in the genetic world. Females are produced from fertilized eggs, and males are produced from unfertilized eggs. This not only means males have half the genetic information of females (but that does not translate into males being half as intelligent as females, a thought that might enter the human female mind), it also means mom can choose to produce a son or a daughter by selectively allowing stored sperm to fertilize the egg. In the tarantula hawk world, females are valuable. They do all the work, take all the risks of capturing the spiders, and have to drag a spider sometimes eight times their weight to their burrow. Thus, females need to be big and strong to do the job efficiently and to produce the most young. Males, however, mainly sip nectar from flowers, chase other males, and mate with females. A small male can mate with a female, so size is not so crucial, though a bigger male is usually more successful in winning more females. Mother tarantula hawks choose to give the valuable resources of large tarantulas to female young and small tarantulas to male young.

  Tarantula hawk life history is similar to that of many other solitary wasps. Female adults emerge from their underground cells to seek nectar for food and to mate. Males emerge to seek flowers and to begin mating behavior. Male tarantula hawks in the genus Hemipepsis are famous for their hilltopping behavior. They go to hilltops, ridge lines, or other prominent high points and establish leks, or mating territories. In these territorial leks, males battle other males to defend their territories, with larger males usually winning the best territories, often near the center of the lek. Virgin females visit the lek to seek mates. They mate briefly once in their lifetime and get on with life. In Arizona, mating systems for tarantula hawks in the genus Pepsis appear to be centered around preferred flowers, especially some milkweeds, western soapberry trees, or mesquite trees. The males actively patrol these resources; otherwise, mating is likely equally ephemeral as in Hemipepsis. Mated females then embark on finding tarantulas. They tend not to be fussy and take tarantulas of several species, both male and female tarantulas and adults and large immatures. Large, plump, juicy female tarantulas are mostly destined to become food for baby female tarantula hawks. Male tarantulas are scrawny, long-legged creatures that usually weigh much less than females. They mostly are destined as food for the next-generation male tarantula hawks; hence male tarantula hawks are often tiny compared with their sisters.

  Tarantula hawks sting their tarantula prey between a leg base and the sternum, the plate between all the legs. The sting, directed at the large nerve ganglion that controls the legs and fangs, completely inactivates and permanently paralyzes the spider within one-and-a-half to two-and-a-half seconds. The now limp spider is dragged to a nest burrow, constructed by the female wasp, or to the tarantula’s own nest burrow. Anyone fortunate to witness at dusk the unfolding of one of nature’s great dramas, as a tarantula hawk drags her enormous spider long distances over the ground, is treated to an adventure remembered for a lifetime. The spider is placed in a cell at the bottom of the nest tunnel, an egg is laid on the spider, and the tunnel is filled with dirt and sealed. The mother’s duties now done, she is off for another prey. The egg hatches in a few days into a first instar larva that imbibes blood from the live, paralyzed spider. Over the next 20 to 25 days, the larva grows, molting its skin four times, and finally becomes a fifth instar larva. The spider is still alive at this point, even though the larva has eaten blood, muscle, fat, digestive system, and reproductive system, leaving the heart and nervous system. The fifth instar now rapidly consumes the rest of the spider before it can spoil. With the food exhausted, the larva spins a silken cocoon and pupates. In early season, the pupal stage may be only several weeks, after which the adult emerges. Later in the season, the wasp overwinters in the cocoon phase to emerge the next spring. Adult males live a few weeks or so, whereas females can live four to five months.8

  One of the great mysteries is why tarantulas do not fight back when a tarantula hawk attacks. Why a huge spider whose massive, powerful fangs easily crush an enormous cockroach, usually with a satisfying crunch, or a hard beetle, fails to defend itself against a wasp is unfathomable to the human mind. How can the tarantula passively submit to its killer usually without the tiniest defense? We cannot journey into the mind of a spider to answer this question. This question also pertains to the spider prey of most, if not all, of the thousands of other spider wasp family members. Perhaps escape and freezing is a better defense in the long run than fighting. How the tarantula distinguishes a tarantula hawk from a cockroach or beetle is also not known, although some ideas seem reasonable. Unlike humans who “see” the world through our eyes and, secondarily, through our ears, with touch, taste, and smell minor modalities, spiders, insects, and most other invertebrate animals sense the world primarily through smell and secondarily with touch and some vision or hearing. In spiders and insects, smell includes contact receptors on antennae, pedipalps, legs, and other body parts. Many of these receptors detect surface chemicals on their prey. The surface chemical blends provide a signature of their source that the insect or spider can recognize. To us, a wasp smells pretty much the same as a beetle, a moth, or a fly—that is, it has no odor—but to a spider or an insect, they are distinct. The tarantula, a nearly blind animal, likely recognizes the tarantula hawk primarily by its odor, perhaps aided by the wasp’s “feel” and the vibrations it sends through the soil surface or air pressure waves. The wasp might also be recognized by the distinct airborne odor it releases. This distinctive odor is easily detected by humans, especially when the wasp is captured or threatened. The odor is pungent but not harsh, acrid, or repulsive (as in a dead animal or overflowing sewer). The odor is distinctive and somehow seems to get into the human psyche, engendering a strong repellency.

  Naturalists frequently commented on the odor. Alexander Petrunkevitch, an eminent early arachnologist at Yale University, noted one tarantula hawk, when contacting the jaws of a tarantula, “raised her wings and suddenly produced a rather pungent odor,” which he concluded “the production of this odor must be a sign of anger, perhaps of warning.”9 F. X. Williams, the man who probably studied tarantula hawk behavior more than any other person, described the odor as “the Pepsis odor,” a familiar and universal odor among Pepsis species.8 Howard Evans noted that both males and females of Pepsis “have this characteristic odor, and this odor may well be repellant to predators.”5 Sadly, although we know the odor is produced in the mandibular glands, so named because they are at the base of the mandibles, we have not identified its chemical nature. This is not from lack of trying. I have worked with five or
six excellent chemists over more than three decades, and we have failed to solve this mystery. Also mysterious is the role, or roles, of the odor. The most evident role is chemical defense against predators, including entomologists, that try to catch them. This defense is not direct, as with carpenter ants that spray corrosive formic acid on assailants or blister beetles (the group containing “Spanish fly”) that cause painful, vesicating skin rashes; rather, it appears indirect in the form of an aposematic odorous warning to stay away, or else. The honesty of this warning defense is obvious to anyone who has grabbed a female tarantula hawk. The odor also might be an aggregation pheromone that attracts both males and females to rich floral sources, to resting places to congregate, or to lekking areas for mating. Finally, the odor might serve to flush a tarantula from its burrow and/or prevent the spider’s natural defensive behavior.10 As often occurs in biology, the odor might have first evolved for one role and later been selected for several additional roles.

  Let’s return to the question of why the tarantula does not fight back. Could it be that the wasp somehow inactivates the spider’s defenses or frightens it with its motion, wing buzzing, or odor that the spider is paralyzed with fear? Such a concept surely seems too wild-eyed to be real, but who knows. We have little understanding of fear and how it changes behavior. One thing we do know is that the battle is highly lopsided in favor of the wasp. Even when the tarantula does fight back, its fangs are mostly useless, simply sliding off the wasp. Tarantula hawk bodies are hard, smooth, and slippery, have no rough areas, indentations, or ridges, and have rounded bodies. The tarantula has the same problem with its sharp fangs that a person would have attempting to hold a glass beer bottle in one hand while drilling through its side with an electric drill in the other hand: the fangs and drill bit simply slide off sideways. Several observers have reported that, when tarantulas actually attempt to bite and crush a tarantula hawk, loud snapping sounds can be heard as the fangs under immense force abruptly and repeatedly slip off the wasp’s body. In the end, the wasp emerges unharmed.9 Perhaps, rather than fighting, the best spider strategy may be to run and then freeze in hopes the wasp loses interest. I am glad our own species rarely encounters similar situations.