The sting and venom notwithstanding, life can be tough for a stinging insect. No defense is automatic. Just because an insect can sting does not mean that its sting is successful. Predators are not without defenses against stings. Notable first defenses against stings are dense, thick hairs covering most mammals; tightly overlapping layers of feathers on birds; hard, tough scales on reptiles; and rubbery, slippery skin on amphibians. These barriers can be difficult to overcome, especially for a single or a few individual stinging insects that must also navigate through the snapping, slapping, and scratching defenses of an aware and moving adversary. Often only tiny areas, typically around the eyes, nose, and lips and perhaps the underbelly of the attacker, are penetrable by the insect and its sting. The insect must recognize and then succeed in reaching these areas to achieve success.
Once the defensive barriers of an attacker are breached, other problems arise, such as delivering sufficient venom to inflict meaningful pain or damage that conveys the message to cease the attack. One advantage warm-blooded mammals and birds share is enhanced quickness relative to cold-blooded animals. Avian and mammalian quickness often means the stinging insect, with its recently planted stinger, is brushed off before much venom is delivered. Stinging insects possess two potential tricks in this ongoing evolutionary war game to help overcome the problem of slow or limited venom delivery. The first is to enhance the speed of venom delivery by enabling almost instantaneous delivery. This is achieved with powerful muscles surrounding the venom reservoir. The strength of these muscles can be seen in the stream of venom sometimes sprayed a distance of up to a foot through the air by social wasps. This delivery system ensures that a fair dose of venom is delivered before the insect can be removed. A second means to overcome the venom-delivery problem is called “sting autotomy.” As the word suggests, in these species, including honey bees, several social wasps, and some Pogonomyrmex harvester ants, the stinger acts as a separate, semiautonomous unit from the rest of the insect, a unit retained in the skin of the victim by back-facing barbs and pulled out of the body of the insect as it retreats or is brushed off. The remaining small sting apparatus, unnoticed by the target animal, continues to pump venom from the reservoir via muscular action coordinated by a ganglion in the autotomized stinger. This system of autotomy ensures that complete venom delivery is achieved, thus maximizing the effectiveness of the sting.
Stinging insects and their venoms sometimes face two further defensive hurdles during the thick of battle. The specific species of predator encountered might not be affected by the venom, which might have evolved effectiveness mainly for another type of predator. Harvester ants are an example of this. The primary predators of harvester ants for which the venom evolved are vertebrates. In mice, harvester ant venom is the most lethally potent insect venom known; in contrast, it is less than one hundredth as lethal to insects. The difference in activities is related to the chemical composition of the venom and how it affects different animal physiologies. Another problem, equally problematic for the insect, is the predator’s evolved resistance to the toxic effects of the venom. In this situation, the target physiology was originally vulnerable, but the animal evolved mechanisms to block the toxic action. Our old friend the Pogonomyrmex harvester ant is again a good example. Its main predators are horned lizards, which eat them with impunity. Why don’t the stings, which would easily kill a mouse, affect the lizard? The answer lies in a venom-neutralizing factor in the blood of horned lizards. This factor renders horned lizards 1,300 times more resistant than mice.4 How commonly stinging insects encounter this problem is scientific terra incognita.
Let’s return to our hypothetical stinging insect. If she could speak, the first words she might shout to visitors are, “Who is at my door?” Her second words might be, “I sting.”
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THE FIRST STINGING INSECTS
One of the characteristics that sets man apart from all other animals
is a need for knowledge for its own sake. … All knowledge, however
small, however irrelevant to progress and well-being, is a part
of the whole. —Vincent Dethier, To Know a Fly, 1962
BIOLOGY IS THE ECONOMICS and energy of life. With only a limited amount of energy and raw materials to go around, life forms scrabble to get their piece. Essentially all energy for life comes from the sun’s radiation. Only plants and other photosynthesizing life forms can capture this energy and turn it into useful molecules. (Oh, there are exceptions, including deep-sea thermophilic bacteria that live on chemicals released by deep-sea hydrothermal vents, but let’s ignore them for this discussion.) Sunlight is a limiting resource that plants must compete for by growing taller, adapting to places where other plants cannot easily survive, or battling nearby plants with chemical warfare and other tricks. Materials in the form of carbon, nitrogen, oxygen, water, phosphorus, sulfur, potassium, magnesium, and a mind-numbing array of other elements needed for life are also in limited supply or in limited availability. Plants can make energy-rich and structurally necessary molecules from light and raw elemental materials, but they cannot make the raw materials themselves. No plant can make magnesium, for example, and thus plants must compete for these basic raw materials in addition to competing for light.
Life for animals also boils down to energy and materials. Animals, unable to photosynthesize with light, must obtain all their energy from either plants or other organisms that ultimately get their energy from plants. A small energy contribution from basking in sunlight to warm the body also occurs, a contribution dwarfed by the importance of photosynthesis. Animals are thereby forced to be herbivores (predators of plants), scavengers of former life, or predators of other animals, fungi, or microorganisms. Like plants, animals cannot make basic elements, including magnesium, and usually, but not always, need to obtain the basic materials of life from their food sources (macaws and elephants obtain some minerals by eating clay). Animals also cannot synthesize many essential molecules, including amino acids, vitamins, and some fats, and need to obtain these from their food sources. Overall, an animal’s life is a continuous struggle for energy and materials in a world with millions of other species struggling for a similar set of limiting materials and energy.
Human societies use money as the basic economic unit, a resource approximating energy and materials. Money, though important, is not the total force that drives a human economy. Food, shelter, reproduction, and safety are the true drivers of societal life. Money is the currency for achieving these. The same drivers—food, shelter, reproduction, and safety—apply to animal life. Energy and materials are the “money” for achieving these in animals. Without energy, an animal cannot find food, cannot find or make shelter, cannot reproduce, cannot maintain a safety net, and cannot obtain materials. Animals are entrapped in a hamster-wheel world where energy is obtained from food, and food is obtained by using energy. A necessary requirement in this loosely circular world, as Gene Odum, my former ecology professor at the University of Georgia called it, is for the animal to obtain more energy and essential nutrients from its food than are required to locate, capture (if necessary), process, and digest the food. This requirement of obtaining more energy from food than required to obtain it was a key factor in the evolution of the insect sting.
Insects, though small and often dispersed in the environment, are nutrient-rich, dense packets of food, perfect for attracting the attention of hungry predators. Plants generally possess a much lower density of nutrients, contain much indigestible material, are harder to digest than insect or other animal materials, and usually contain nasty toxic compounds. Compared to plants, insects constitute an ideal source of food, but they are small compared with vertebrate animals. Size counts. In life’s economy, a small bit of food is less valuable to a large predator that might expend more energy obtaining that food energy than it receives from consuming the food. Strange as it seems at first this cost-benefit relationship spares insects from having to defend vigorously against many
huge predators. One simple, frequently used but sufficient defense consists of hiding in “plain sight” by cryptically resembling the resting background, a strategy that limits detection and increases the searching costs for the predator. Another commonly effective insect strategy is to flee rapidly and evasively, sometimes with a startling effect similar to humans’ reaction to encountering an explosion of escaping quails. Flight gives a time advantage to the prey, and the confusion increases the cost to the predator attempting to capture the prey. Aposematic warnings and mimicry of aposematic insects are also often sufficient defenses. A disadvantage of advertising oneself is the possibility that detection might result in an attack; counterbalancing this attack risk is the inherent hesitancy of most predators to attack a potentially nasty prey and waste energy and time in the process of the failed attempt. In the classic example of Lincoln Brower’s blue jay, the bird vomits after eating a monarch butterfly. Everybody knows the inordinate unpleasantness of stomachaches and vomiting. This misery is genetically hard-wired as nature’s way of protecting animals against repeating the behavior that caused the vomiting. After its unpleasant encounter with a monarch butterfly, the blue jay refused future monarchs.1 Not only did the bird endure the misery of discomfort and the loss of expended energy, it also suffered the additional insult of losing the hard-earned energy from the previously captured prey already in the stomach.
One small insect may be safe from large powerful predators because it is not worth the predators’ effort. But what about a collection of small insects? Most people would not cross a room for one small blueberry, but if a bowl of blueberries is present, the story changes. Blueberries are now worth the effort. The same principle applies to a collection of insects. An aardvark is not likely to pursue a single termite, but it lives on collections of termites in termite colonies. Collections pose a serious problem for insects. A single insect’s usual defenses are no longer as relevant. Better defenses are needed. Most termites nest underground where the soil barrier increases the difficulty and cost to the predator. In addition, termites might produce specialized soldier castes whose sole role is to defend against predators, large and small, with powerful, sometimes razor-sharp mandibles or by spraying sticky or turpentine-like compounds on their adversaries. These compounds are sprayed from the heads of soldiers, making these soldiers the original “nozzle-heads.” Aggregations or collections of insects can thwart predators by confusion, as illustrated by simultaneous quail escape flights or whirligig beetle groups on a water surface. The predator becomes confused and cannot readily focus on a specific individual. A different but common defense by collections of insects is toxicity. In conjunction with their bright colors, ladybird beetles, commonly called ladybugs, contain toxic coccinelline and other compounds that taste bad and can sicken predators. Blister beetles, the source of the famed “Spanish fly,” produce cantharidin, which is stored in their blood. Cantharidin is a general tissue irritant as well as potentially lethal toxin to humans, which achieves its reputation as an aphrodisiac by irritating the genital tract, thereby drawing attention to that area.
Ancestors of stinging insects likely lacked most of the above defenses of aggregations or collections of insects. As solitary insects, they experienced relaxed selection pressure from vertebrate predators. If any of these ancestral sawflies did aggregate, as is seen in some sawfly species today, they probably also had nasty chemical defenses, as do today’s representatives. Sawfly dietary life was tough. They eat mainly pine needles and leaves from living trees or bore into fibrous plant stems. Nutrient levels in these materials are low, and toxins are usually high, but sawfly life gave them the preadaptation of a sawing, penetrating egg-laying ovipositor for boring into wood. Insect larvae in wood presented a much richer new food source for sawfly ancestors of stinging insects than the wood itself. Thus, a shift occurred from herbivore to predator, technically from herbivore to parasitoid. A parasitoid is an animal that, during its immature stage or stages of development, lives in or on the body of a single host individual, eventually killing that individual. Common examples are ichneumonid wasps that sting, sometimes paralyzing, caterpillars or other prey and lay eggs within the body of the prey. The eggs hatch into larvae that consume and ultimately kill the prey. Other examples of parasitoids are tachinid flies that lack stingers but accomplish the same goal by laying eggs on the host prey. The eggs hatch and burrow into the prey where they feed and develop. Ichneumon wasps, along with other parasitoid wasps, are examples of lineages that evolved from this sawfly-parasitoid ancestor. All of these solitary parasitoid wasps that use their sting-ovipositors for stinging prey and depositing eggs are subjected to little predatory pressure from large predators. They rarely sting entomologists who remove them with fingers from insect nets, a procedure not recommended for removal of honey bees or yellowjackets. On the rare occasions when a very large ichneumonid wasp actually manages to sting a person, the sting pain is typically trivial, confirming that defensive stinging against vertebrates is not a behavior that was developed, in part, because the sting and venom are ineffective and nearly useless defenses.
A major milestone in the evolution of stinging wasps, ants, and bees was a functional shift of the parasitoid wasps’ sting-ovipositor to a dedicated stinger. This group with dedicated stingers is called the Aculeata. The name comes from the Latin word aculeus, meaning a stinger, and is a good description of the group. The significance of this modification is that eggs no longer needed to pass through the stinger, and the glandular secretions associated with the egg and its passage through the narrow stinger tube were free to evolve new functions, as painful and toxic defensive venoms. Like parasitoids, the original aculeate Hymenoptera were solitary, a lifestyle the majority of aculeates still maintain. These individuals offered little nutritional quantity to large predators. Consequently, they were not strongly targeted by vertebrate predators. Even today, most solitary aculeate Hymenoptera rarely attempt to sting in defense, and when they do sting people, their stings rarely hurt. Nevertheless, the wasp ancestors of modern wasps, ants, and bees occupied the pivotal position at the cusp of one of nature’s greatest evolutionary achievements: the generation of the aculeate Hymenoptera, a group of some 100,000-strong species that changed and dominated the world.
With the innocent-appearing change of an ovipositor into a stinger, a major radiation of species was set and ready. All that was needed were behavioral and venom composition changes that enabled the nascent aculeate to expand its diet breadth by exploiting many available potential new hosts. Concomitant with expanded opportunities for new host food sources were problems of encountering new predators. Without some means to blunt the new suite of hungry predators, the ability to expand into new biological niches could not be realized. Here is where the stinger became crucial. With no role in laying eggs, and no important role in any of the insect’s bodily functions, the sting was liberated to be molded into radically new roles. An important new role was the production of secretions that contained pain-inducing or toxic components. As populations, species numbers, and time and activity of stinging insects increase, notice by predators and attacks would increase. All that was needed in the evolution of a new defensive role for sting venom was for insects to sting the predator and for that sting to result in the venom bearer’s escape. A sting that by chance mutation or genetic recombination caused even some pain would be more effective than a painless sting. The genes of even these few escapees would be passed along to future generations, initiating a cascading series of changes in venom chemistry, each more effective than the last.
As long as individual stinging insects remained solitary, the individuals presented too small a meal to attract attention of large predators. Therefore, little pressure for the evolution of powerfully painful venomous components occurred. Aggregations of individuals often have advantages not available to solitary individuals. All individuals benefit if one individual finds a large food source and attracts others of the aggregation to share in that rich o
pportunity. This is biological tit-for-tat. You help me. I help you. Nobody loses, and we both benefit. Of course, this is not a conscious decision by participants who are unaware of the process. It is a process that favors individuals who, for whatever reason, happen to act in this fashion. Aggregations also benefit locating members of the opposite sex for mating. A cost tagging along with the benefits of aggregation is the greater nutritional reward available to large predators, who now find predatory efforts worthy. In this aggregated situation, a painful sting is beneficial and favored. The stage is set for an arms race: The prey evolves more painful and effective venom and stings, and the predator evolves new means of overcoming the defensive stings and venoms.
The ultimate form of aggregation is sociality. When a species becomes social, many individuals live together, usually in a protected nest, share in rearing offspring with overlap in generation with the adults (parents and adult offspring present in the nest at the same time), and different individuals specialize in performing tasks, such as egg laying, foraging, or defense. A serious disadvantage of social life is the burden of protecting the immobile, immature members from predators. Eggs, larvae, and pupae are highly sought for their excellent nutrition and digestibility, and they cannot flee. Instead of escaping, tending adults need to guard the home fort if they are to protect their young. Selection pressure from predators would disfavor social aggregations or nests of ill-defended prey, leading to the expectation that social insects would be rare. This is far from the case, given that social insects comprise an enormous portion of the animal biomass in most ecosystems.2 What explains this paradox? The answer: the sting. The painful and toxic effectiveness of the sting was honed in parallel with the increased predatory pressures of vertebrates. Genetics and selection pressures at the group and population levels were the ultimate causes of the evolution of sociality. The venomous sting was a major, perhaps the most important, proximate cause enabling the evolution of higher sociality to proceed.
The Sting of the Wild Page 3