Velvet ant life history shares many similarities, and some differences, with other solitary wasps. Female velvet ants actively search for hosts to parasitize. In this search, they are both flexible and eclectic: flexible in accepting a number of different host species; eclectic in only accepting host postfeeding larvae or early pupae. Cells that contain host eggs, feeding larvae, maturing pupae, or simply host provisions are rejected. One other stringent requirement is that the host must be housed in some “package,” usually a cocoon or hard shell, as in a fly puparium or beetle pupation case. Most hosts are solitary wasps or bees, rarely other diverse hosts, including puparia of tsetse and other flies, pupae of moths within hard cocoons, beetle pupae within hard cases, and cockroach eggs in hard oothecal cases. Once a suitable host in the right stage is located, the female chews a small hole in the cocoon or package, inserts her sting apparently to sense the conditions within the cocoon, and lays an egg. In most situations, the female appears not to sting the larva or pupa, though she might sting some pupae to arrest their development.3 Once the egg is laid, she closes the hole in the cell with nearby nest materials cemented with saliva and resumes her search for more hosts. The egg hatches in two to three days, the larva feeds on the resting host, molting as it grows, finishes eating all of the host, spins a cocoon, defecates, molts into a pupa, and finally metamorphoses into an adult. During warm seasons, the cycle is uninterrupted. If winter is approaching, the young velvet ant overwinters as a postdefecating prepupa that then becomes a pupa in the spring and continues the cycle. Until recently, the rule of only one velvet ant young per host cell was apparently strictly followed. But nature, and in this case nature in Australia, tends to make exceptions to rules. Two Australian species of velvet ants produced four young per cell of their mud-nesting wasp hosts.4
One important part of the life cycle remains: courtship and mating. In many velvet ant species, sex resembles a task to dispense with as quickly as possible before getting on with life. Males fly over promising areas, searching primarily by odor for virgin females. Virgin females release a sex pheromone attractant that the male detects while flying overhead, inducing him to drop to the ground where he frantically searches for her. Vision plays little or no role, as he often runs right past her without noticing. Once he stumbles into her, he immediately recognizes her by contact chemicals and mounts her while singing with his abdominal stridulatory organs and buzz-honking with his wings. He probes her abdomen tip with his genitalia. If receptive, the female extrudes her sting a remarkably long distance and opens her terminal abdominal plates, allowing him to engage his genitalia. Now mating in the fast lane begins and lasts only about 15 seconds. The female runs off never to mate again. The abandoned male is on his own to search for more females.
This mating story is not universal among velvet ants. Velvet ants have the problem that females can’t fly, and their ability to disperse is limited by how far they can go on foot. In many velvet ant groups, this problem is reduced by the male grasping the female and flying with her as they mate. He often carries her for 2 hours, mating with her for about a minute each of five times, and then depositing her in a new location.5 The new location might be across a stream or other physical barrier she could not cross on her own. To fly while carrying a female, larger size is favored in the male. Denis Brothers, a talented South African entomologist with an irrepressible enthusiasm for velvet ants and other fascinating wasps, shows a picture of a mating pair of velvet ants in which the male is nearly three times as long and, by my calculations, 25 times the weight of the female.6 To put this in human perspective, this is like a 120-pound woman dating a 3,000-pound man. One suspects the male velvet ant would have no trouble flying with his lady.
Why are velvet ants insect tanks? Why do they need such armor, such firepower, such speed? Defense. Defense against what? Nearly everything: hosts resisting their attempts to parasitize; competitors, including ants, at sources of nectar or honeydew; and a nearly endless number of predators looking for lunch. In response, velvet ants evolved the best and strongest defenses known in insects. Most insects have only one or two defenses to augment behavior and lifestyle. Among these defenses are cryptic camouflage to prevent detection, strong jumping legs to bound away, powerful wings to rapidly escape, hard shells to resist puncture, internal poisons to make them unpalatable, chemical defenses to repel attacks, and stings. In addition to behavior and lifestyle, velvet ants have an amazing six defensive systems: their sting; their rock-hard body; their short, powerful legs for rapid escape and wresting free from grips; their aposematic warning colors; their warning, defensive sound; and their warning, defensive chemicals. Not all velvet ant species have all six defenses. For example, nocturnally active velvet ants lack, and have no need for, warning colors. One might reasonably ask why velvet ants need so many defenses when other insects get by with one or a few? Clues come from velvet ant life history. Hosts for velvet ants are frequently present in low numbers, are widely dispersed, and often live in open, exposed areas such as sandy patches or dunes, where avoidance of detection is nearly impossible. Velvet ants have generally low numbers of offspring, making survival of parent and offspring crucial. Velvet ant females cannot fly to escape. Finally, velvet ant females have very long life spans, often well over a year. Combined, these life history traits mean velvet ants live for a long time, nearly constantly exposed to a variety of predaceous spiders, beetles, ants and other insects, lizards, birds, mammals, and even toads. About the only predators they are not exposed to are fish. Velvet ants need effective defenses for each of these types of predators.
The most-studied model of velvet ant defenses is our old friend and poster child of velvet ants, the cow killer. The ultimate primary velvet ant defense is the sting. A cow killer’s sting sets not only the record for sting length relative to body length among aculeate Hymenoptera, it also is the most flexible and maneuverable of the stings, able to reach all parts of the body except very narrow portions of the thorax and abdomen. This length and maneuverability is achieved by having the stinger looped forward within the abdomen, then around, and finally back to the tip of the abdomen, much as a watch spring of an old watch is coiled within its case. At the exit point in the abdomen are muscles and plates that guide the stinger toward the right, left, or down and forward. As far as we know, the sting is only rarely used for stinging prey (which are already in a nearly immobile resting phase) and essentially solely for defense. The defensive value of the sting is especially important against the relatively enormous predaceous birds, lizards, mammals, and toads, but it also works well against spiders, praying mantises, and other smaller predators.
The sting alone would be poorly effective against a crushing blitzkrieg attack by a bird or a lizard. This is where the second primary defense comes in. Just as an army tank cannot be easily crushed, bird bills, lizard jaws, and mammal teeth cannot easily crush or pierce the shell of a velvet ant. The force to crush a cow killer was more than 11 times that required for a honey bee.7 This figure is only part of the story. The entire body of the velvet ant is rounded with tight-fitting body parts and no soft, membranous gaps where teeth or fangs can get through. The result is that bills, jaws, teeth, and fangs slip off like chopsticks sliding off a greased marble. No grip, no crush. In the meantime, while the hard shell is keeping the velvet ant intact, the sting is brought into play. The instantaneous result is the bills, jaws, mouth, and fangs are snapped opened allowing the velvet ant to escape unharmed while the predator rubs its mouth or plows it through the sand, trying to remove the pain.
A female velvet ant thorax is an amazing box of muscles. Since she doesn’t have wings and can’t fly, the space normally allotted to wing muscles is taken up by huge muscles to the legs. This gives her some of the strongest legs in the insect world, legs perfect for both wresting her body free from a predator’s grip, and for running rapidly once free. The powerful legs in conjunction with the hard body combine to make an ideal defense against ants, those omnipresent pes
ts of the insect world. Ants, even those most aggressive fire ants, cannot pierce or puncture any part of the body, and those ants that clamp onto legs are easily brushed off with other legs. Meanwhile, the velvet ant hotfoots it out of there.
When an animal is so well protected and able to punish would-be assailants, all kinds of opportunities to prevent attack are evolutionarily possible. Why risk the slobber or damage of being in somebody’s mouth if that can be avoided? Aposematic warning-signal systems go a long way toward achieving this. Once a predator learns an animal is unpleasant, it tends to avoid trying to eat that type of animal again. What better way to advertise your nastiness than color? Birds, lizards, amphibians, and most arthropods see color. Red and black are universal warning colors that signal to both experienced and naturally wary naive predators to leave them alone. The cow killer’s flaming red and black beacons from grass, soil, or sand the advertisement “I am here—think twice before making a mistake.” No matter whether the predator happens to be color blind, as many mammals are, the cow killer’s pure red appears stark white in a black-and-white visual environment. The message, like the black-and-white message of a skunk, gets through.
Not all predators are visually oriented and may respond to warnings only after initial contact with a velvet ant is made. Velvet ants emit a warning signal in the form of a rasping stridulation against these acoustically or tactilely oriented predators. This signal acts much as the rattling of a rattlesnake’s tail warns intruders. Both snake and velvet ant sounds are produced over an enormous range of sound frequencies, thus insuring the widest range of predators can hear them.7 Mammals and birds are particularly sensitive to sounds. Most spider and insect predators either lack or have poor hearing; nevertheless, velvet ant stridulatory sound can be highly effective defenses against these arthropods. Hunting spiders grab or pounce on prey, simultaneously trying to puncture them with their fangs. Fangs are hard and inflexible. Velvet ants are hard and inflexible. The result is like applying a small jackhammer to one’s teeth. The hard fangs, or mandibles, are released and the “vibrating rock” is rejected. Whether or not this vibratory defense works on its own with birds, lizards, or mammals is unclear.
Some predators, especially mammals, are highly attuned to odor as a clue to prey and edibility. Reptiles also possess acute senses of smell and taste, the contact analog of smell. When an insectivorous mammal or lizard grabs a velvet ant, the velvet ant releases its warning odor (along with stridulating and engaging the sting). After the predator experiences the sting, it associates the eau de velvet ant with the unpleasant sting and thereafter learns to associate the odor with the bad experience. The same probably applies to lizards, which usually lick prey before eating. A simple tongue flick to a velvet ant is usually enough for any lizard.
The warning odor may serve the dual roles of warning and chemical defense. Many species of velvet ants analyzed, plus those given the nose test in the field, produce the same blend of two major compounds, 4-methyl-3-heptanone and 4,6-dimethyl-3-nonanone, plus a few minor constituents.8 The first compound is a well-known alarm pheromone/chemical defense in a wide variety of ants and even in daddy longlegs arachnids, those long-legged creatures looking like tan pills on stilts that cluster in cool, dark, damp areas. This sets the stage for multiple mimicry in which a diversity of velvet ants and other creatures all use the same chemical signals to warn predators that they are inedible. The compounds likely are also direct chemical defenses that taste somewhat like turpentine when eaten.
Do these velvet ant defenses actually work? Naturalists recognized years ago that velvet ants were generally left alone and not attacked. In 1921, the British naturalist Geoffrey Hale Carpenter, working in Uganda, tested a wide assortment of insects for palatability to a gray vervet monkey. The monkey was presented with an insect on the ground or in a box and his behavior observed. Insects expected to be palatable were generally eaten by the monkey. When a velvet ant was offered, he “pounced on it and hurriedly rubbed it on the ground in the manner previously described, eventually seizing it and crushing it up very quickly. I think his lips and one hand got stung. Another, smaller Mutillid [velvet ant] was then put down, but M[onkey] would not have anything to do with it.” A month later he offered another velvet ant, and the monkey “with great eagerness seized it out of the box and bundled it into his mouth, getting his hands and lips stung. He shook his head and ran about, and the Mutillid fell out of his mouth, but he picked it up and ate it greedily, though for a few minutes afterwards he ran about shaking his head and wiping his mouth with his paws. M. must have been very hungry.”9 To test more species of predators, cow killers were extensively tested with many likely potential predators, including fire ants, harvester ants, a Chinese praying mantis, three species of wolf spiders, two species of tarantulas, four species of lizards, a bird, and gerbils (desert rodents from Asia).7 Most predators attacked the cow killer; those that did not attack carefully eyed it before making no attempt to attack. Only 1 of 13 tarantulas and 1 of 8 gerbils ate a cow killer. The gerbils provided an interesting example of behavioral personalities of individual predators. None of the gerbils had seen a velvet ant before. Four of the eight were frightened by the cow killer, two gerbils attacked once, were stung and did not attack again, and two attacked twice with one abandoning attacks thereafter, and one succeeding in eating the cow killer. The behavior of the successful gerbil was particularly telling. That gerbil grabbed the cow killer, quickly rotated it in its paws while making quick bites into the spinning cow killer. It finally punched through the hard shell, inactivating the wasp, and then consumed it. This behavior was specific to the cow killer; other insects, for example, mealworm larvae, were simply held like a sausage and eaten from head down.
Spinning velvet ants brings to mind the University of California at Davis entomologist Richard Bohart. Bohart, a sturdy, intrepid student of wasps of all types, was a mentor to many young entomologists. The traditional wisdom was that velvet ants could not be picked up without being stung. Instead, one had to scoop them into a vial or jar. Dick, either too lazy or simply too tough, would casually, albeit quickly, roll velvet ants between his fingers and drop them into his collecting jar. Apparently, he applied the wisdom of gerbils and other insectivorous small mammals to his own collecting. We don’t know if he was ever stung; perhaps if so, he kept stoic so as not to tarnish his reputation.
E. O. Wilson, the famous Harvard biologist, had a different experience with velvet ants than Richard Bohart. Ed was quite a bit younger than Dick at the time of his encounter with a velvet ant, an experience that may have helped fix him on a path of biology. “I could have been as young as three years old, and all I remember is that I was in this garden in somebody’s backyard and I recall vividly this velvet ant—this mutillid running along—and my grabbing it and, of course, they have a horrific sting and it was so painful that to this day I remember the appearance of the garden, I remember the wasp, I remember how I felt. I don’t remember anything else.”10
A final way to measure velvet ant defenses is to examine the diets of highly insectivorous predators in the wild. The broad-headed skink, Eumeces laticeps, is a large powerful lizard that easily crushes large insect prey. Cow killers are abundant in its habitat yet were never found in the stomachs of skinks, despite their readily feeding on other noxious prey, including blister beetles, woolly bear caterpillars, ants, and stinging paper wasps. Twenty-three skinks were presented with cow killers. Eight skinks never attacked, nine attacked one to three times, and six attacked more than three times. Ten skinks were clearly stung, eight of which thereafter would have nothing to do with cow killers. Only two skinks killed cow killers, and just one ate the entire wasp. That skink attacked 23 times over 9 minutes and finally ate the intact cow killer because it was unable to crush its abdomen.11 Meals of velvet ants are hard won.
The examples with cow killers and other velvet ants indicate something special about the stings and venom of velvet ants. What makes them so different fro
m solitary cicada killers, mud daubers, iridescent cockroach hunters, or water-walking wasps? As so often in biology, much remains to be explored. What is known is that velvet ant stings hurt a whole lot more than stings of those other solitary wasps. Velvet ant venom is not particularly toxic to mammals, with a lethality 25 times less than honey bee venom, and a whopping 200 times less than an average species of harvester ants.12 In its ability to destroy red blood cells, velvet ant venom is less active than the venoms of paper wasps and harvester ants by factors of 200 and 120. Clearly, cow killer venom is not special in ability to do damage. It also has low levels of the enzymes phospholipase and hyaluronidase but reasonably high levels of esterase.13 What is special is its ability to produce pain. How, we do not know. I learned firsthand a year or so ago just how painful a velvet ant sting can be. I was innocently asleep in my bed one night when something tickled my upper leg. Reflexively, reaching down, I felt something hard, then bang, it hit me. Lights went on and an investigation revealed the pebble to be a small nocturnal velvet ant female who had objected to my rubbing her. The pain was sharp but had a rashy flavor to it—something commanding an urge to rub, but when rubbed, hurt more. No obvious redness or swelling other than that caused by the rubbing was apparent. In less than 5 minutes, the pain receded, allowing the possibility of sleep. She yielded a big punch for her size: a 1.5 on the pain scale.
The Sting of the Wild Page 20