We humans are masters of our lives. We no longer fear large animals that may prey on us, having long since dispatched most of them and their threat. We have conquered many human diseases, though more continue to emerge to challenge us. We have tamed animals and manipulated plants to provide a steady, more reliable food supply. We have made clothes and shelters to make life comfortable. We have made games and toys to entertain ourselves. Tarantula hawks have not mastered their lives as well as humans, although they are a close second. Of course, by “mastered” I do not suggest that tarantula hawks made conscious decisions to alter their lives as humans have (we have no evidence of consciousness in tarantula hawks); rather, nature, through natural selection, has made them masters of their lives. Tarantula hawks live long lives, they have no known predators of adult females, and they can be active any time of day and anyplace they choose. How was this good life achieved? Defense against predators is the most important factor in a long and free life. Without good defenses, animals must either live secretive and restrictive lives or have short lives and try to mate and reproduce before being eaten. No predators successfully prey on healthy female tarantula hawks,6 although I did once see a particularly small male wasp being eaten by a large praying mantis on a milkweed flower. Pinau Merlin, an Arizona naturalist, reported coming upon a roadrunner—that intrepid predator of many life forms, including rattlesnakes—stealing a paralyzed tarantula from a tarantula hawk and then feeding it to her young. The wasp was left alone. The obvious reason large predators such as roadrunners, other birds, lizards, toads, and mammals don’t prey on tarantula hawks is their sting. The sting alone would not be sufficient to protect the wasp from being smashed and eaten by a powerful bird beak or crushing lizard jaws. Here, the second defense, the same defense that protects against the tarantula—the hard, slippery, rounded body shell—provides the necessary time to deliver the sting. The wasp is too tough to be smashed fast enough by beaks and jaws to avoid a sting to the mouth or tongue, and mammalian teeth slip off the wasp body long enough to allow the sting to be engaged. The enormous body size of tarantula hawks relative to most insects and arachnids provides defense against arthropods. If size alone does not do the trick, the sting, hard-body integument, and powerful sharp jaws complete the defense against arthropods.
A universal law of life is that it is always better to avoid a fight with a predator than to actually fight the predator. For a tarantula hawk, why risk losing a leg or an antenna or having a wing crumpled by a bird or lizard if the attack can be avoided? The key to avoiding an attack is communication to the attacker that an attack is risky. Tarantula hawks are masters of communication, using many forms of aposematic warning signaling. Brilliant conspicuous color patterns of reds, yellows, oranges, or whites combined with black are classic examples of warning coloration. Strikingly shiny, reflective, or iridescent dark colors are another example. These patterns tell the predator “see me, I am bright, bold, and dangerous; if you attack me you will suffer.” Tarantula hawks with their strikingly reflective orange or shiny black wings and iridescent gunmetal blue-black or black bodies send the warning strongly. To supplement the visual color pattern, tarantula hawks engage in a distinctive jerky movement while they are on the ground, and they flick their wings while moving around, an action that ensures they are seen. Threatened tarantula hawks communicate acoustic warning sounds by buzzing their wings, much as threatened bees raise their buzzing to a high pitch. A final tarantula hawk warning signal is its powerful odor. Humans, as a species with poor olfactory abilities, only notice the odor when massive amounts are released by a threatened wasp. Small amounts of the odor likely are continuously released and operate as a long-distance, early-warning signal to olfactorily cued mammals, warning them not to approach. Given all these warning modalities, no potential predator is left unaware of a tarantula hawk.
Imagine for a moment what freedom from predators means. With no predators, there is less hurry in finding a mate and reproducing; no predator-based reason to have a short, efficient life; no reason to avoid open areas, flowers, or ground surfaces, where predators might take notice; and no reason to limit activity periods to times when risks from predators are minimal. For a tarantula hawk, such freedoms are essential. Tarantulas are not abundant, they are hard to find, they are widely dispersed in the environment, and they are available throughout much of the year. Tarantula hawks require much time and searching for both their own food and for tarantulas for their young. They could not easily pass on their genes to the next generation without a long life and few restrictions on their activities.
The sting has been accepted as an amazing given. Just what makes the sting so powerful? What is the chemistry that makes it so magical? The venom is nearly unique among insect venoms. Most wasp, ant, and bee venoms serve only one role: either offensive in prey capture or defensive against predators. For defense, pain, along with damage or killing power, is important. For offense, pain is irrelevant, perhaps even harmful if it causes unnecessary stress to the prey. Damage or killing power is not beneficial if the prey is to be kept alive and fresh for the young. For offense, the important venom feature is to paralyze, thereby inactivating the prey. Tarantula hawks bridge the gap between offense and defense. Their unusual venom both permanently paralyzes the prey and protects against predators. Pain is the hallmark effect against predators. Tarantula hawk venom damage to predators is trivial; at best, the lethality to mammals is only about 3 percent of that of honey bee venom. Why is tarantula hawk venom nontoxic and nonlethal? Perhaps because natural selection operated against a venom chemistry that is toxic or lethal to tarantulas. A venom toxic to mammals might well also be toxic to tarantulas. Dead tarantulas yield dead tarantula hawk larvae.8 In addition, tarantula hawks defend no nest and have little reason to damage or kill a predator: the goal is to get the predator to cease and desist, and to open its mouth quickly, allowing the wasp to escape. Only a momentary mouth opening is needed for the wasp to fly away, and the pain alone does that marvelously.
The chemistry responsible for tarantula hawk sting pain is not known. The venom contains the highest-known concentration of citrate, a small 6-carbon polyacidic molecule of any venom, but if, or how, that would cause pain is unclear.11 The venom also contains the neurotransmitter acetylcholine and kinins, both compounds that can cause pain.12 These compounds would not cause paralysis of tarantulas, something likely caused by one of a variety of proteins in the venom.13 Whatever the various active components of tarantula hawk venom, both humans and tarantulas survive a sting; an important difference between the two is that the tarantula succumbs to the tarantula hawk larva, and we do not.
THE BENIGN GIANT. CICADA KILLER. As ominous as the name sounds, cicada killer wasps are gentle giants among the wasp world. Rather than adopting the strategy of “speak softly and carry a big stick,” as Teddy Roosevelt said, cicada killers “speak loudly” and carry a tiny stick. True, they do have a big stick (stinger), and its relevance to cicadas is big, but its relevance to us is tiny. Phil Rau, that intrepid wasp naturalist of the early twentieth century, once wrote that cicada killers “expressed their indignation at being disturbed by the loudest noise that we have ever heard a wasp make.”1 Perhaps this is not surprising, as cicada killers are among the largest wasps in the world, rivaling tarantula hawks in size.
Cicada killers are sphecid wasps in the genus Sphecius of the family Crabronidae. The genus Sphecius contains five species in the Americas, with four in the United States. As the name implies, they hunt cicadas that are secured in underground cells as food for their larvae. In the purest sense, they are not “killers.” They are paralyzers, leaving the “killing” to the larvae as they feed on the paralyzed cicadas. Cicada killers are enormous 2.5- to 5-cm-long solitary wasps, sometimes called “ground hornets,” a particularly unappealing and misleading name. They are not hornets, which are best known for their frightening ability to deliver nasty and painful stings, and they do not like most ground, preferring pleasant sandy a
reas. They are busy wasps active during the warmest times of high summer and bring joy and amazement to anyone fortunate enough to be able to watch them.
The cicada killer life cycle begins in summer in synchrony with the emergence of the adult phase of annual cicadas, which emerge each summer after spending several years underground as immatures. Male cicada killers dig upward from their underground cells, feed on nectar or plant exudates, and establish territories near their emergence sites. About a week later, females begin emerging, again digging directly upward from their overwintering cells. Cicada killers are solitary, gregarious wasps; that is, each female wasp works alone to rear her young; yet, even though the females do not cooperate, they usually aggregate their nests in small areas. Individual nests are often only a meter or less apart. Aggregations range from under a dozen individuals to around a thousand nest burrows in one localized area. Life in the nesting area appears chaotic, with wasps flying every which way and having frequent interactions, none of which are cooperative, with one exception—mating. The brief cooperation between a male and a female to enable their next generation.
Once their single mating is over (females are efficient; why waste time with other males?), females set about feeding themselves with nectar and other sweet liquids, exploring the area, and preparing their nests. Nests are in the form of burrows about 30 to 50 centimeters long and 15 to 25 centimeters deep,2 dug in the sand with their front legs, likely aided with their mandibles to loosen tough obstacles, and pushed backward up and out of the burrow with the aid of strange enlarged spines, one on each hind leg, called calcaria. Males do no meaningful digging, and, not surprisingly, their calcaria are much smaller than those of females. When the female’s burrow is deep enough she shifts modes and focuses on catching cicadas. Our human intuition has led us to expect cicada killers preferentially to locate male cicadas by their loud, boisterous songs. Sound is important in our world, so it is natural we would think this way. In the cicada killer world, sound is of minor, if any, value, and we have no evidence cicada killers can even hear. Maybe hearing in cicada killers would be disadvantageous. Cicada alarm stress calls can reach sound levels as high as 105 decibels, 10 times the loudness of a jackhammer at 16 meters and well over the sustained level of human exposure for hearing loss. Loud cicada stress sounds are known to interfere with mammalian predators of cicadas3 and might, likewise, interfere with the ability of cicada killers that could hear to capture their noisy prey.
Instead of finding cicadas by sound, cicada killers locate cicadas visually, likely in conjunction with chemicals recognized on contact with the cicada. The female cicada killer visually finds cicadas by slowly scanning up, down, and across the branches of nearby trees where cicadas are located. When a cicada is detected, and for certain recognition, she darts in and out in front of the cicada to provide a better image, much as binocular vision provides better images for humans. She then pounces on the cicada, which often squawks shrilly (if it is a male) and quickly stings it.2 Paralysis is nearly instantaneous, with the cicada paralyzed within one to two seconds. She then flips the cicada over, holds it belly to her belly with her middle legs, and flies (or attempts to fly) off with the cicada toward her burrow. Cicadas are usually much larger than the cicada killer, making the flight an onerous and limiting task for all but the largest females. Small females often fail in transporting enough cicadas to their nest to successfully reproduce.4 It pays to be a big female in the cicada killer world. Many times paralyzed cicadas can be seen under trees where they were dropped by wasps unable to carry them.
Joe Coelho, a cicada killer expert at Quincy University in Illinois who also would make a fine helicopter engineer, has spent much of his academic career analyzing just how cicada killers and other wasps can fly with impossibly heavy-appearing loads. The question is somewhat like the famous calculations that bumble bees could not fly; yet they do. Joe found that a cicada killer could just barely fly with a cicada that weighed 1.42 times the wasp’s weight. In one population of cicada killers, he found that they solved the problem of carrying slightly overweight cicadas by flying with full lift power and tracking a gently forward and downward gliding path toward the nest. If she hit the ground before reaching the nest, she climbed a nearby tree or tall plant with the cicada and repeated the operation. By this stepwise method, she could eventually reach her nest with her “impossibly large” cargo.5 Is it any wonder that cicada killers only forage for cicadas within about 100 meters of their nest?
Jon Hastings and Chuck Holliday, respectively, at Northern Kentucky University and Lafayette College, discovered nature had another way of solving the heavy cicada problem. They studied two populations of eastern cicada killers in northern Florida that were about 100 kilometers apart. Both populations had the same four species of cicadas present in about the same ratios. The different cicadas formed a range from small to medium and large. In one location, cicada killers preyed mainly on medium and large cicadas; in the other location, they preyed nearly exclusively on small cicadas. The difference in the wasp populations was obvious: the wasps of the population that preyed on the larger cicadas were much larger in size than the wasps that preyed on small cicadas. The exact causes of this local stabilizing of size differences between the two populations is not clear, although the size of the cicadas fed to their larvae certainly has something to do with it.6 What is known is that the small wasps simply could not carry large cicadas and large wasps selectively chose large cicadas, even though small cicadas were abundant. Small wasps paid a large foraging penalty because of their small size. By being able to collect only small cicadas, they had to collect twice as many cicadas per young as the wasps in the population with large wasps. What selective force operated on this one population to produce small wasps despite the huge extra foraging cost is mysterious.
After successfully capturing and transporting a cicada to her nest, the female cicada killer places it in the prior excavated cell at the end of the burrow. At this point, she has to make a decision: make a male wasp or make a female wasp. If she chooses to make a son, she lays an unfertilized egg on the cicada, closes the cell, and prepares for the next cell. She can make this decision because she can choose to fertilize the egg or not: fertilized eggs become females, and unfertilized eggs become males. Because males are much smaller than females, that is, about half the size of females, usually one cicada is enough food to produce a male. If she decides to produce a daughter, the female leaves the burrow open (a dangerous proposition inviting parasites, intruders, and thieves into her nest) and seeks another cicada. The second cicada is added to the cell, a female egg is laid, and the cell is sealed. This is the general picture. Sometimes males get two cicadas and females more than two cicadas; in some populations such as in Florida, or in some species that use small cicadas, as many as four to eight cicadas might be required per cell.6 After a cell is completed, the female cicada killer uses dirt excavated from a nearby future cell to seal the cell and the tunnel between it and the new cell. She is now ready to hunt the next cicada for her new cell. During her lifetime of a month or so, she produces about 16 cells when conditions are good. Within the cells, the eggs hatch in a day or two, the larva feeds on the cicada(s) for 4 to 10 days, overwinters as a postfeeding larva, pupates the next spring for about 25 to 30 days, and emerges as an adult during the next summer’s cicada season.2
Sex in the cicada killer world: Most of the chaos in a cicada killer community, as in a human community, revolves around sex. It’s the males, not the females, that cause the commotion. Given a choice, females appear to simply want to get mated and then inconspicuously get on with the duties of producing their descendants. Males have only one way to produce descendants and that is to mate with females, a job they attempt with great energy. Males emerge before females and attempt to establish territories in prime areas within the previous year’s nesting area. Because about twice or more males are produced than females, the competition is fierce. If a male succeeds in establishing his small
territory around a perching location, the top of a plant, the end of a branch, a rock on the ground, or the bare ground, he must vigorously defend it from other males seeking to take over the territory. Intruders, such as an insect flying by, a small bird, a biologist, or, especially, another male, are investigated. If the intruder is not another male, the perch owner tends to quickly return to his perch. If it is another male, he will attempt to chase it off, frequently head-butting into it. If the intruder does not depart, the two engage in a spiraling dual in which both rapidly circle each other as they ascend into the sky. Serious challenges evolve into grappling bouts in which each male attacks the other, attempting to bite legs, wings, or whatever is available. These grapples can result in both wasps falling to the ground, continuing to grapple, sometimes with a loud buzzing sound heard. Some grapples might be the result of initial mistaken identity in which the male mistakes the intruding male as a female and attempts to secure her. In these territorial competitions, the larger male usually wins. Smaller males can set up territories in marginal areas around the nesting area or can attempt to patrol through the territories of other males in hopes of catching a female first. The smallest males can lurk in vegetation just outside the nesting area in hopes of intercepting a virgin female that somehow emerged and flew through the area to the nearby vegetation.
The Sting of the Wild Page 17