How is the nebulous and murky nature of pain explained?
In The Sting of the Wild, (Johns Hopkins University Press, 2016) the colorful Dr. Justin O. Schmidt takes us on a journey inside the lives of stinging insects, seeing the world through their eyes as well as his own. He explains how and why they attack and reveals the powerful punch they can deliver with a small venom gland and a "sting," the name for the apparatus that delivers the venom. We learn which insects are the worst to encounter and why some are barely worth considering. With colorful descriptions of each venom’s sensation and a story that leaves you tingling with awe, Dr. Schmidt’s one-of-a-kind style will fire your imagination.
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Everybody knows what pain is. It is the sensation felt when a knee is scraped in a fall, the skin is overexposed to the sun, or a bare foot steps on a bee. Pain is familiar, yet mysterious. We know pain when we sense it. Pain is clearly recognizable. Warmth is not pain, though it can become pain if too much heat is experienced. Likewise, chills caused by cold temperatures are clearly unpleasant, but they are not pain. Cold, like warmth, can produce elements of pain but is not classic pain. As the tobogganing kid knows, toes get wet, cold, and unpleasant but not usually truly painful like a stubbed toe. But, oh wait until the true pain comes as the toes warm before a toasty fireplace. Though we might call stomach upset and nausea painful, is it really pain? We call nausea “pain” for lack of a proper descriptive word, but everybody knows nausea pain is different and, I would argue, far more unpleasant than the pain of a runner’s stitch caused by the spleen contracting to squeeze more red blood cells into the bloodstream to provide acutely needed oxygen to muscles.
We know pain when we feel it. Do we know physiologically and medically what pain really is? The response gets murky. Describing actions that cause pain — slamming one’s finger in a door, for example — is simple. Distinguishing pain from nonpain is generally easy. Hunger, though often called pain, is certainly different from the pain of a stomach ulcer caused as acids eat into and destroy stomach tissues. Again, we lack a common, distinguishing word for hunger pain, though perhaps “hunger pangs” is a proper phrase that when spoken sounds the same as “hunger pains.”
There may be no universal consensus on what is and what is not pain. We generally recognize pain as a distinct experience, one that presents in a variety of flavors. Common pain is the sensation felt when skin is damaged, a tooth injured, a bone broken, a muscle pulled, the spleen makes a stitch, or a variety of other mostly dermal or skeletal-muscular problems. Another broad category, visceral pain, is experienced when visceral organs signal damage or potential damage. The visceral pain resultant from tonsillectomy in adults, hemorrhoid surgery, childbirth (I am told), or other sources one hopes to rarely experience is distinctly different from common pain and a considerably less pleasant form of pain. Headache pain is another category of pain. The point of this discussion is not to define pain concretely, to make a pain phylogenetic “family” tree, or even to claim that all the separations above are clear-cut (they are not); the point is to illustrate just how complicated and murky pain is.
How is the nebulous and murky nature of pain explained? What is the cause of this lack of tidiness, both in descriptive language and in real-life sensation? Several poorly defined explanations can be offered. A medical explanation might focus on the separate pathways of nerves and distal structures between the motor and autonomic nervous systems and the sensory nervous system. Action potentials sent from the brain to muscles travel down different nerve axon pathways than pain signals generated when the tongue is bitten. Signals to the brain originate from receptors located throughout the body. Many of these receptors are sensory receptors that detect temperature, pressure, stretch, chemicals, itch, or a variety of other sensations, including pain. The signals from these receptors are transmitted to higher nervous centers in the spinal cord and brain through fine nerves of the separate sensory nervous system. Matters become more subtle. Pain and itch, for example, are separate sensations.1 Are they related, that is, is one just a smaller degree than the other? No, they are not simply degrees of difference, and, unfortunately, how they relate is unclear and an active topic of research. Is the tickle sensation related to the pain and/or itch sensation? Again no tidy answer is forthcoming. Complicating the issue further is the feature that tickling can be a pleasant sensation, especially in social situations, or it can be an excruciatingly unpleasant experience. How are the two tickle responses related? In degrees of stimulus? The difference is unclear.
Is pain always unpleasant or, as pointed out by my grade 7 science teacher, can it also be pleasurable? A love-hate situation occurs when baby teeth are about to fall out and to be replaced by permanent teeth. The loose tooth hurts, but the urge to wiggle it is irrepressible. We can wiggle the tooth just enough to cause a little pain, an enjoyable sort of pain, but not too much. And we can control this dynamic precisely and entirely ourselves. What is the difference between the two tooth pains? Is it simply strength of the nervous signal emanating from the tooth receptors? Probably not. Here, other important players in the pain system come into play, the higher processing centers of the spinal cord and brain. These centers filter and process the signals to determine the importance of the signals and then send them to our conscious centers of the brain. If the signal indicates a dire situation, as in a hand placed on a hot stove burner, the processing centers convey outside the conscious pathways to the action centers to signal reflexive removal of the hand. The conscious centers are involved in the process of learning for directing future behavior to avoid placing the hand on a hot object.
Pain serves a higher purpose in the biology of life than revealed by analyses of nerve pathways and processing centers, however interesting these might be. Why should pain, a most universal sensation in living animals, exist at all in nature? Certainly not for pleasure or torture. Only adaptations and sensations of value for promoting the life, survival, and reproduction of an organism stand the test of time. Pain is a basic sensation of life, experienced by all animals. Even the simple single-celled paramecium moves away when it encounters the high acidity from the drop of vinegar placed in its watery bath, just as we jerk our fingers from a hot stove. Does the paramecium experience pain? Certainly not in the way humans do, as it has no brain or self-awareness, but it responds the same as we do to the negative situation, so in practical terms, we can call it a pain response. In biological terms, pain is simply the body’s warning system that damage has occurred, is occurring, or is about to occur. Nothing more. Pain is not damage. It is merely a harbinger of damage. Is pain truth? Perhaps. If damage occurs concurrent with pain, then pain is truthful in sending the honest signal that the body is at risk and has been compromised. A bruised shinbone sends truthful pain.
What if pain is intense and no meaningful damage occurred? Is that pain truthful? This paradox of the veracity of pain’s role, damage is about to occur, is just what stinging insects exploit. Returning to the bee and the foot, the sting to the sole elicits pain and lifting the foot is a response that benefits the bee (well, maybe no longer to that bee, but to her nestmates). Has meaningful physical damage to the person been done by this sting? Often, the answer is no. Stinging insects are masters at exploiting this weakness to their benefit in the honesty of the pain signal. To stinging insects, we might simply be fools who fall for the trick. To us, it is better to be safe than sure; thus, we believe the signal is true. If the damage were real, the downside cost could far out-weigh any benefit obtained by ignoring the pain. Why take a risk? In life’s risk-benefit equation, the risk often dwarfs any potential benefit. Herein lies the psychology of pain. Unless the animal or human can know that a rainbow of benefit is awaiting on the far side of the pain, natural psychology dictates not to chase the rainbow.
Pain can be a lie. The insect sting exploits a weakness of the pain signal system to propagate a masterful deception. That deception, the lie, benefits the stinging insect by cheating its adversary out of a meal, perhaps cheating it out of the use of space near the insect or its nest, or even cheating it out of some other resource, such as a feeding site. In most painful circumstances, it is adaptive for the stung individual to accept the lie and ensure its safety. One can lose many small meals and survive. One serious poisoning and the individual might not survive. The math is on the side of caution.
For every liar and cheater, someone out there is not fooled. For the lying pain of stinging insects, some animals and people have broken the trick: they ignore the pain and reap the rewards offered by the stinging insect. In much of North America, skunks are common denizens of the rural countryside. Beautifully adorned in tuxedo black with brilliant white stripes or spots, skunks are known mainly for their aromatic properties, but they are also efficient predators of insects and other small game. Skunks have a fondness for stinging insects and avidly dig out and consume the contents of yellowjacket wasp nests. They also enjoy honey bees, another spicy dietary staple, and have learned to discount the pain. Bears are another trick-breaker, famously known for their love of honey. Bears tear apart beehives in hollow trees or beekeepers’ boxes, relishing the sweet honey and rich brood, all with apparent impunity to the bee stings. Common wisdom dictates that the dense fur of bears protects them from stings, but this half-truth wisdom mainly protects our empathy for the bear and its potential pain. In real life, the bear suffers many stings, especially around its sensitive eyes, nose, ears, tongue, lips, and mouth. It has learned that a certain number of bee stings can be endured without injury and that the reward is worth the pain. Likewise, for the fabled African ratel, or honey badger. Ratels, relatives of wolverines, are medium-sized black-and-white, tough-skinned intrepid animals that routinely feed on all sorts of prey, including poisonous snakes (reputed to be unable to bite through the tough skin), chase lions and other carnivores from their prey to claim the kill, and are best known for their love of honey and bee brood. Ratels, like bears, learned that a certain number of stings cause no meaningful damage, and thereby they have learned to overcome the pain. This is a tricky game for ratels. Bee stings are truthful as well as painful. Enough stings, about 4 for a mouse, or an estimated 140 stings for a ratel, can kill. Until around a hundred stings, the ratel is safe. No one knows how well ratels can count in the ratel–bee brinkmanship game, but they likely can sense when a dangerous level of envenomation is near. The game can be tricky, however, for some ratels have misjudged and paid the ultimate price of being stung to death.
Truth, like beauty, can be in the eye of the beholder. Pain truth comes in two flavors, imagined and realized. With stings, our imagination is vivid and strong, even if the sting pain is not realized. The paper wasp, Polistes instabilis, provides a real-life example. Perhaps the name instabilis tells us something. No matter the actual origin of the name, their behavior appears unstable to people walking through the scrubby brush of their tropical habitat. Their presence is usually painfully detected as a nasty sting to the back of the neck or bare arm subsequent to brushing past a leafy tangle with a nest attached to a small branch. This truthful pain was realized by our cowboy guides, as they led a group of biologists on an expedition through the thickets to the most northerly location of the magnificently beautiful military macaws. After the second in line was stung on his wrist and yelled, we all stopped to allow the wasps to return to their nest and become calm, albeit alert. Up to this point, our guides had considered us a bunch of inept, cowardly biologists. We needed to both move forward and change the guides’ perceptions of us. As the only entomologist in the group, I obviously needed to take charge. Here’s where knowledge of stinging insects is crucial. For wasps and bees, the two greatest factors that stimulate attack are human breath and rapid movement. To continue our trek, both factors had to be minimized. Theory is fine, but reality was calling. I carry a 2-liter, wide-mouthed plastic jar for rare opportunities, and this was the perfect occasion. With eyes glued on every movement and hint from the wasps, I held my breath and slowly advanced, jar in my left hand and lid in my right. During this eternity of 30 seconds, the jar was snuggled underneath the nest and the lid just above. Snap. The lid was on the jar and all wasps inside. Except the branch prevented the closing of the lid. A yell for assistance brought a cowboy with a machete to cut off the twig, securing all wasps inside. This reverse showmanship worked: rather than showmanship on the part of the wasps, it was showmanship by the predator, which fooled the wasps and earned biologists some status.
The Australian bull ants, sometimes called bulldog ants, are inch-long, lithe creatures with enormous eyes, long mandibles, and lightning speed. And they jump. Their uncanny behavior of turning their heads to follow observers adds to their mystique. In Australia, they are highly respected, if not outright feared, for their fabled stinging ability. Among all of Australia’s native insects, bull ants head the list of painful stingers. This is partly because Australia has no native honey bees, no hornets, no yellowjacket wasps, and their social wasps are mostly in the generally placid genus Ropalidia, a group similar to many Polistes paper wasps in Europe and the Americas, though generally milder in disposition. Hence, Australians lack comparisons between their bull ants and other painful stinging insects around the world. Given the background of stories about bull ants, I approached collecting them with some anxiety and caution. However, I didn’t know about their athletic abilities, something mentioned but frequently glossed over by writers of articles on these ants. As I collected some individuals from a nest, an alarm was sent and a boiling mass of ants issued from the colony. My athleticism didn’t match theirs, and the feared stings were realized. I was stunned, not by the pain, but by the low level of pain. The balloon of anticipation had been deflated. Why did the stings not hurt so much? The pain was less than the sting of a honey bee. Flare and swelling were also minimal, and the pain was short-lived. Had I been stung too many times and simply could no longer detect pain? This was a valid concern, so how could I address it?
As fortune would have it, the greatest congress of social insect scientists was meeting about that time in South Australia. Midway through the meeting we took a break, climbed on some buses, and visited Kangaroo Island. On the return trip, the driver spotted a large bull ant colony along the side of the road and asked whether we would like to stop. A resounding yes echoed throughout the bus. Ah, opportunity. My reputation with insect stings was well established. That set the stage for quiet showmanship and trickery to demonstrate my ability to evaluate pain. Normally, it is unfair to expose people to insect stings, but this group of experienced social insect colleagues was fair game. I went to a colony, picked up individual ants, and dropped them in a jar. Others saw this and realized my approach was much faster and easier than trying to pick up hypermotile ants with clumsy forceps. Sure enough, five colleagues got stung. I ask them casually, “Does it hurt much? How does it compare to a honey bee sting (all had been stung by honey bees)?” In all five cases, the reply was that the sting was surprisingly less painful than expected and hurt less than a honey bee sting. Apparently, my sting pain detection system operates well.
Truth, lying, and cheating are not solely in the domain of female stinging insects — the ones that can sting — and in humans who exploit or study them. Males of some stinging insects can lie about stings and pain also. Although they have no stinger, no venom, and cannot harm a large predator, males can put on a good show. Because imagined sting pain is real in people and in other animals, male bees and even mimicking flies buzz ferociously when they are captured, as do female bees. The higher-pitched buzzing of a captured insect is an aposematic warning signal that conveys danger. This male auto-mimicry of stinging females is energetically costly and would be very unlikely to have evolved if it were not effective. The sharp spines on male reproductive genitalia, particularly in wasps, have a dual role of matching the structure of the female reproductive system and for providing a modicum of protection against large predators. Like many evolutionary questions, which was a more important selection factor — mate matching or defense — is unclear. Probably both factors were important. In addition to possessing hard, sharp spines, these males exhibit uncanny stinging movements in their similarity to the movements of females. When grabbed, these males curve their abdomens and jab the sharp spines into the fingers or mouth of the offender. Many an experienced entomologist, including me, have been tricked by this maneuver, and our instinct caused the release and escape of the male, to our chagrin. Score one point for the male wasp, zero for the entomologist.