by Matt Simon
Here’s a figure for you: one-fourth. That’s not the fraction of Earth’s species that are invertebrates. That’s not the fraction of Earth’s invertebrates that are insects. That’s the fraction of all animal species on Earth that are beetles. One. Quarter. So perhaps it’s not surprising that Wallace stumbled upon such a bounty of the bugs, for beetles are unparalleled in their success in the animal kingdom. Some are built like tanks. Some specialize in rolling dung into balls and devouring them because, hell, someone has to. Still others are nimble, turbocharged killers, and king among these are the comically long-legged tiger beetles, insects that chase their prey with such speed that they go blind.
Now, it wouldn’t be quite fair for me to just tell you that among the 2,700 species of tiger beetle, there’s a particularly speedy Australian variety that’s been clocked running at 5.6 miles per hour. That seems like nothing. Until you consider that according to the British Heart Foundation, when walking for exercise, four miles per hour is recommended “for a person with excellent fitness,” while three miles per hour is more average. So the next time you get up and take a few steps, know that a beetle three-quarters of an inch long could have burned you. If it were running alongside you for one second, the tiger beetle would have covered 125 body lengths while you covered less than one of your own. Even at full gait, the fastest humans can cover only six body lengths a second, while cheetahs manage sixteen. For its size, the tiger beetle is an incomprehensibly fast animal, hitting speeds equivalent to a human being sprinting at 480 miles per hour.
WINGING IT
So what gives with all the success for the beetles? A lot of it comes down to their characteristic wing covers, known as elytra, which are modified wings that shield their flight wings. You know the spotted coverings on ladybugs that snap open before they take flight? Those are the elytra. For species of beetles that have lost their ability to fly, the elytra function as extra armor. For beetles with a penchant for water, the coverings trap air bubbles, so the creature can breathe while submerged, a lot like the diving bell spider. And desert-dwelling species rely on the elytra to help retain moisture. So think of the elytra as a superhero’s cape of sorts, only instead of being made of fabric they’re made of “highly sclerotized dorsal and less sclerotized ventral cuticles.” Which sounds much more impressive, to be honest.
The beetle is so fast, in fact, that even though it has some of the sharpest eyes among insects, its giant peepers can’t collect enough light when the creature is in hot pursuit of prey. So every once in a while the tiger beetle has to stop to again lock onto its quarry. In an average pursuit, this will happen three or four times, but the beetle is so fast, the setbacks don’t matter a bit. It catches up with the prey, snagging it with enormous mandibles and ripping it to pieces.
But it’s not only the prey that the tiger beetle needs to keep its eyes on during these pursuits. There’s also the matter of avoiding rocks and sticks and such, for a good face-plant can really set a predator back. It turns out that it isn’t all about vision here: The beetles are feeling their way around by holding their antennae forward, with the nocturnal species among them tending to kind of twirl them around in circles (cockroaches, which are also largely nocturnal, do the same to sense their world) and diurnal (a five-dollar word meaning daytime) species holding them straight out and pointed slightly downward. That’s interesting because while we’d expect nocturnal insects to rely on their antennae and not their eyes to make sense of their world, the diurnal varieties of tiger beetle are so fast they have to supplement their vision with mechanosensation—that is, by picking up mechanical cues from the environment.
You might be wondering how scientists figured this out in such a fleet-footed creature. It was pretty simple, really. Researchers got three groups of tiger beetles: one normal, one whose eyes they painted over, and one whose antennae they cut off. They then put together an obstacle course and set up a camera shooting four hundred frames per second, and with a few encouraging bumps with a paintbrush, sent the beetles scurrying. Beetles with their antennae amputated did a whole lot of face-planting, but the normal and blinded beetles performed equally well. The video showed that the antennae are pivotal for obstacle avoidance: Once the feelers made contact with the barrier, they would briefly snag and bend before snapping back into place as the beetle detected the impediment and bounded over. So out in the wild, even if it’s a diurnal hunter, the tiger beetle is behaving more like a nocturnal one, its vision snatched away not by darkness, but by raw speed.
A GENERALLY ELONGATED CREATURE
Arachnids don’t have antennae, but a particularly creepy group among them sure think they do: the whip spiders. (Not technically spiders, and don’t confuse them with the similar-looking whip scorpions, which aren’t technically scorpions, though whip spiders are also sometimes called tailless whip scorpions. Make sense?) Like the tiger beetles, they feel their way around their environment, only they do so with their absurdly elongated front legs, which can be four times as long as their other already absurdly elongated legs. When one of those front legs hits a victim, the whip spider snags it with spiny graspers called pedipalps and gnaws on it alive, adding digestive juices and slurping down the resulting smoothie. (Not technically a smoothie.)
Even tiger beetle larvae, which are largely sedentary and don’t yet have the lanky legs of their adult form, are in their own way as blindingly fast and brutal as their elders. They hang out in cylindrical burrows in the ground, with their armored head and giant mandibles plugging up the hole, while their elongated body—which has two hooks on the back to anchor the larvae so they don’t get dislodged when they’ve got hold of larger prey and it’s trying to wiggle free—extends below. If an unfortunate insect like an ant happens to stumble by, the tiger beetle larva will fire part of its body out of the burrow, snag it, and yank it down into what you can imagine is a terrible death. Should it overwhelm enough prey, the larva will grow big and strong, leaving behind its cozy burrow for the life of the world’s fiercest, most remarkable sprinter.
CHAPTER 7
You Can’t Let Them Get Away That Easily, Can You?
In Which Predatory Snails Weaponize Insulin and Bugs Have the Nerve to Assault Charles Darwin
Being able to run down prey is one thing, but keeping a grasp on them is a whole different challenge. For that, evolution has come up with some pretty inventive solutions. I’m talking poison arrows. Slime cannons. And a particularly nasty mouth that had the gall to attack Charles Darwin himself.
Bolas Spider
PROBLEM: Moths can escape typical spiderwebs.
SOLUTION: The bolas spider mimics a female moth’s sex pheromones, luring in the males. Then it swings a specialized web, just a drop of goo on a line, to snare its prey.
In his South American travels, Darwin developed quite an affection for the gauchos—the masterful horsemen and southern cowboys—who stunned him with their skills. Among their tools, none was more effective than the bolas, two or three stones or iron balls wrapped in ropes of leather to create twirling mayhem. When gauchos threw the bolas at prey, the whole mess would tangle around limbs, oftentimes with enough force that it’d snap bones.
Ever the good sport, Darwin figured he’d give it a whirl. Galloping about on his horse and spinning the bolas above his head, he let loose on . . . himself. The bolas hit a bush and dropped to the ground, entangling in the steed’s limbs. “Luckily he was an old practiced animal,” Darwin writes in The Voyage of the Beagle, “and knew what it meant; otherwise he would probably have kicked till he had thrown himself down. The Gauchos roared with laughter; they cried out that they had seen every sort of animal caught, but had never before seen a man caught by himself.”
Appropriately enough, sitting in the very bush that Darwin assaulted may well have been an arachnid that takes its name from the gauchos’ mighty weapon: the bolas spider. This kind of orb weaver isn’t spinning complex webs or throwing leather
-wrapped balls of iron at its prey, but it is hunting them in a similar, though far more complex way. During the day, bolas spiders hunker down in plain sight, with some species conveniently resembling bird crap, thanks to a splotchy black-and-white pattern that makes them look unpalatable. But at night, when the spiders emerge to hunt, the males and females take up very different strategies.
The female bolas forgoes all the work it takes to construct an intricate web—instead, she extends a single line of silk between two leaves or twigs. To that she attaches one more line. As she’s spinning the second line, she uses her hind legs to comb a sort of glue from her spinnerets onto the silk, forming a viscous glob at the end. When she cuts the line off from her spinneret, the weight of the glob pulls the line down, so it hangs vertically, perpendicular to the supporting silk. Her weapon at the ready, she holds it with a leg and waits.
Depending on the species, the spider uses her bolas in different ways to hunt moths, her prey of choice. On one end of the spectrum, she’ll wait until she’s detected a target before she flings the bolas with great speed and precision, snagging the moth with the glue. (It’s so fast, in fact, that one species, Mastophora dizzydeani, is named after the baseball pitcher Jerome “Dizzy” Dean, who was so called because he was an eccentric, not because he had balance problems. As for the name “bolas spider” in general, one researcher advised that the name not be changed even though it doesn’t work exactly like a bolas, but he did suggest that, no joke, a more accurate moniker would be “sticky yo-yo spider.”) Still others take a more spastic approach, rapidly whirling the bolas when a moth is nearby until the thing runs into the trap. And still others don’t even bother waiting for the prey to show up, flailing about as soon as the line is ready for as long as fifteen minutes. That glue will start evaporating away, so if she’s unsuccessful in her hunting after about a half hour, the spider will gobble up the line.
THE MERITS OF LOOKING LIKE BIRD TURDS
Lots of animals disguise themselves as bird crap because, let’s face it, no one wants to mess with bird crap. Perhaps the most spectacular among them are some of the bolas spider’s relatives in the orb weaver family. Unlike the bolas, these do indeed spin intricate webs, but that leaves them out in the open and vulnerable to predatory wasps. So orb weavers spin thicker white silk around the center of their web that looks like a splat, then hang tight there, with their splotchy body coloration completing the ruse.
She’s going through all the trouble because moths can escape your typical spiderweb. They’ll stick, all right, but they’re covered in scales that rip off and allow the moth to escape. Accordingly, the bolas spider’s trap is no simple dab of glue. The goop is made up of two liquids of differing viscosity. The outer layer is less viscous, while the inner layer is nice and thick. This inner layer also holds a mass of extra coiled silk, allowing the glob to elongate as the spider swings the line around, increasing the strike distance. When she does snag her prey—and she most certainly will, an average of twice a night—the less viscous outer layer flows past the moth’s scales and sticks to the cuticle below. And that extra silk in the glob acts as a shock absorber, both for the initial impact and as the moth struggles to free itself. Prey secured, the bolas spider climbs down the line, gives the moth a bite, and starts wrapping it up, sometimes leaving it right there on the thread as she builds out new bolas.
Strangely, though, the bolas spider hunts only the males of specific moth species. How could this be? How could the bolas snag only particular kinds of moths when the forest is swarming with insects, and for that matter, why would she want to limit herself to a tiny menu? Well, it turns out the bolas spider is releasing scents that mimic the sex pheromones of female moths of a given species. Male moths swooping in thinking they’re going to get laid instead find themselves tangled in the bolas spider’s trap. The spider can even mimic the scents of two different kinds of moths with two different pheromones by putting off a smell that’s a mixture of the two. The bolas spider Mastophora hutchinsoni, for instance, targets a moth that appears early in the evening and one that shows up later. By modifying her pheromone as the night wears on, lowering its attractiveness to the first moth species, she can better attract the latter.
THE MERITS OF HAVING A GLOWING BUM
Even though it’s unrelated to the bolas spider, there’s a glowworm in the caves of New Zealand that’s hit on a similar manner of hunting its prey. It’s the larval form of the fungus gnat, and like the spider it begins by stringing a horizontal support line, in this case a tube of silk filled with mucus. From that it hangs up to seventy fishing lines, each loaded with several droplets of glue. But instead of giving off pheromones to attract its prey, the larva glows with bioluminescence. When an unfortunate insect homes in on the light and slams into the line, the hunter squirms through the mucous tube, reels up the catch, and devours it. So, unlike the bolas spider, it spares itself the weirdness of having another species sexually attracted to it, which is nice.
Such specialization poses a problem. While other spiders build their webs and catch insects indiscriminately, the bolas spider can find herself in a habitat without any of her target prey. So she has to test the environment. When night falls, before she even begins building her bolas, the female emits the scent that drives male moths nuts. If none show up, fine: Instead of wasting the energy and resources and time building a bolas, she moves on to another part of the forest. But if the moths do show, only when they get close enough for her to feel the vibrations of their wing beats does she start setting up shop. (If you have a mind to prank a bolas spider, just hum near her and she’ll think you’re a moth and start building a trap. Seriously.)
As for the males, they’re comparatively tiny—remember the size rule with spiders. Being so small, they can’t take on moths, so they don’t even bother building bolas. Instead, they wait at the edges of leaves and use their hairy arms to snag flies. That’s a whole different niche, keeping the male and female bolas spiders from competing with one another for food. The male doesn’t get to bolas him some moths, sure, but then again, unlike Darwin he’ll never know the emotional pain of screwing it up and having all his friends laugh at him.
Velvet Worm
PROBLEM: Worms aren’t celebrated for their speed.
SOLUTION: But the velvet worm is celebrated for its weaponry. It fires jets of glue out of two modified legs—yes, legs—to entrap its prey. Then it takes its time gnawing through their exoskeletons.
The bolas spider enlists a complex system of chemical seduction and sticky entrapment, but you might say it’s living in the dark ages. After all, having to be so close to your victim to launch an attack is like a knight risking injury to draw in close and stab his victim (chemical seduction optional). So if the bolas spider is the foot soldier of yore, surely the velvet worm is a deadeye gunslinger. Like the bolas, it deploys its own immobilizing goo, only from a distance. The worm then leisurely closes in on its prey—no hand-to-hand combat required.
The first thing you need to know about the one hundred or so species of velvet worm, which grow up to six inches long and keep to tropical and temperate forests, is that they’re pretty much all legs, even the bits that don’t seem to be legs. Depending on the variety, there can be as few as a dozen pairs of the stumpy, squishy little things or as many as forty, all supported with pressurized fluid. But the legs don’t stop there. In a clever evolutionary maneuver, the velvet worm has adapted a pair of legs into antennae, another into slime cannons, and yet another into jaws. That last one shouldn’t make a lick of sense, but it turns out that each leg is tipped with a claw that helps the creature get a grip in the undergrowth—indeed, the worms are known collectively as the onychophorans, or “claw bearers”—a great material to turn into a pair of jaws, each of which is a pair of fanglike blades.
Those leg-antennae go a long way in helping the velvet worm sniff out its prey’s chemical cues, but the hunter’s most improbable sensory
organ is its entire body. Covering the worm are minute bumps that calculate shifts in air currents from the movement of potential prey, including spiders and termites, on up to larger quarry like beetles and crickets. When the worm does lock on to something, it takes its sweet time to sneak up on the soon-to-be victim—partly because, hey, it’s a worm, and it can’t be rushed—and partly because you don’t need speed when you’ve got cannons on your face.
LEGGING IT
The many-legged velvet worm may resemble the centipede or millipede, but it’s only distantly related to these armored arthropods. The venomous centipedes, the name meaning “hundred feet,” usually have far fewer than one hundred legs, and the harmless millipedes, with their supposed one thousand legs, top out at 750.
If you come across something you suspect is a centipede or millipede and feel like picking it up, a good way to tell the two creatures apart is if it assaults you. Or, better yet, don’t let it come to that. The more cylindrical millipede is a sluggish and harmless detritivore, meaning it feeds on decaying plant matter. The centipede, however, is a streamlined and frenetic hunter. Like the velvet worm, it has modified two of its legs into weapons, in its case claws that deliver venom. Those toxins may work magic on smaller foes, but for us humans the effects are typically no worse than from a bee sting (though—as with bee stings—some folks may experience a more severe allergic reaction), and that’s even if the claws can manage to penetrate the skin. But don’t say I didn’t warn you.
