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3 Insects in the Food Chain
The recipe for a successful insect life is simple: you just have to stay alive long enough to reproduce. And to stay alive, you need food. Much of an insect’s life consists of eating and trying not to be eaten.
Many insects eat one another. If there are fifty ways to leave your lover, I can assure you that there are an awful lot more ways of eating other creatures—including your lover. You can eat them from the inside or the outside. You can eat them as eggs, larvae, or adults. You can eat them using mandibles, sponges, or drinking straws. On the other hand, you can simply stop eating: quite a few insects eat only as larvae but don’t feed at all as adults.
Since the objective is to keep on the right side of life’s brutal but simple eat-or-be-eaten rule, insects go to extremes to avoid being gobbled up by other creatures. They may live in hiding, concealing themselves through camouflage or by pretending to be something else—preferably something dangerous or inedible. They can opt to survive by disappearing in the crowd or by collaborating with others in ingenious ways. Insects’ strategies for laying their hands on nutrition without themselves becoming food is an object lesson in jaw-dropping but often brutal adaptations, which would be criminal of me to keep from you.
Take parasites, for example. Many insects are what we call parasitoids—parasites that ultimately kill their host. The host is often devoured from the inside out: the parasitoid larvae hatch inside an animal, for example another insect, and slowly but surely eat their way through all its internal organs. The whole thing is elegantly done: the larvae leave the vital organs until last. Well, we all prefer fresh meat, after all! The host usually dies once the parasitoid larvae have eaten their fill and are ready for adult life.
The natural historians and theologians of the 1800s tore their hair out when they found out about this. It just didn’t fit in with their notion of a creation formed by a good and loving God. Darwin also struggled with it, writing to his American colleague Asa Grey in 1860, “I cannot persuade myself that a beneficent and omnipotent God would have designedly created the Ichneumonidae with the express intention of their feeding within the living bodies of caterpillars.”
If only he had known! There are much worse things than that.
The beautiful, green-eyed Dinocampus coccinellae is a parasite wasp. The female sticks her egg-laying tube, or ovipositor, into a ladybug and lays an egg. The egg hatches, and over the next twenty days the wasp larva chews its way through many of the ladybug’s inner organs. Then the larva somehow squeezes its way out of the ladybug’s abdomen while its unfortunate host is still alive. The wasp larva spins itself a little ball of silk between the ladybug’s legs, where it transforms into a pupa.
Something quite remarkable happens next: the ladybug’s behavior abruptly alters. It stops moving and just stands there, stock still, like a living shield. But every time a hungry foe of wasps approaches, the ladybug gives a jerk, thereby scaring off anything that might consider eating the now helpless monster that has just eaten it up. This lasts for a week until the wasp hatches and flies off, leaving the ladybug to its own devices.
The big question here is how the wasp mother can control the ladybug, transforming her into a zombie babysitter. After all, several weeks have passed since she laid her egg and vanished. The answer is that the wasp mother injects the ladybug not just with the egg but also with a virus. The virus accumulates in the brain and is controlled by a timing mechanism that paralyzes the ladybug at the precise moment when the larva is squeezing its way out. So the virus enables the wasp to take over the brain of the ladybug, making it serve not just as baby food but also as a babysitter. The only good thing we can say about all this is that, unbelievably enough, the ladybug sometimes survives the whole ordeal.
The cockroaches that fall prey to the soul-sucker wasp aren’t so lucky. Do you remember the dementors in Harry Potter, those flapping black monsters that suck out people’s souls? That’s what gave the Ampulex dementor wasp its name. It is one of several species in the cockroach wasp genus, a representative of which can even be found in Norway. During childhood, these wasps live inside cockroaches.
Here, too, the whole process starts with a mother who’s out and about wielding her egg-laying stinger. First she stings the cockroach in its chest to paralyze its legs for a few minutes—because the next stage involves high-level brain surgery, which requires “the patient” to lie completely still. Now the wasp stings the head. With extreme precision, she places a dose of nerve poison into two specific points of the cockroach’s brain. This blocks the signals that control its ability to start moving: the cockroach can still move but cannot itself take the initiative to set in motion. The cockroach is now at the mercy of the wasp’s will.
And the wasp’s will is to take the cockroach to a place where she can lay eggs on it. But since the cockroach is far too big for the wasp to carry, it’s handy that it has now lost whatever cockroach free will it might once have possessed but is still able to walk. This allows the soul-sucking wasp to simply bite into the cockroach antennae and lead her prey wherever she wants, like a dog on a leash—straight to death.
The cockroach is a biddable prey that allows itself to be led down into a hole in the ground; here the wasp lays an egg, which it glues to the cockroach’s leg. Then the soul-sucker wasp blocks the entrance to the hole with small stones and vanishes. Her little larva child spends the next month fattening itself up. First it sucks the bodily fluids out of the cockroach’s leg, and then it bores into the creature’s insides and gobbles up its intestines, before forming a pupa inside the cockroach, which eventually dies.
Ugh! Maybe Darwin was better off not knowing about this. It’s difficult to see any goodness in such ruthless behavior. Then again, evolution has never been driven by love and compassion.
Some insects live off the young of other insects. The bold blister beetle eats bee larvae yet is still able to hitch a ride with the parents all the way into the nursery.
One May day, as I was sitting out in the sunshine, a strange fat beetle came puttering across my garden table, shimmering and bluish black. It looked as if it had borrowed a tailcoat three sizes too small for it: its abdomen was so full of eggs that it bulged out beyond the hind edge of the wings. It was a blister beetle paying a morning visit. In Norwegian, it’s also known as a spring beetle, May beetle, or Easter beetle, and it is as rich in adaptations as it is in names.
The plump lady beetle is the source of what must be the spring’s strangest stowaway. In a little while, she’ll dig a hole in the earth and squeeze out a load of eggs, maybe as many as 40,000. The eggs hatch into larvae with hefty hooks on all six legs. They look a bit like long, narrow head lice or wingless stone flies and are filled with feverish energy. The triungulin larvae, as they are known, eventually gather in flowers, and there they wait for life’s great lottery draw.
The thing is, it is crucial for these larvae to end up in the right place if they are to have a shot at life. And they need a ride to get there. They hook onto the first insect that lands in their flower—but it’s game over for all the larvae that hitch a ride with the wrong kind of bee. And this is precisely why so many eggs are needed in the first place: the only ones whose future is ensured are the fortunate few who get lucky and stow away on a wild bee that’s heading in the right direction.
Blister beetle triungulins gather in a flower in such a way that they form a shape that resembles a bee. The larvae also emit scent signals that imitate the smell of a lonely female bee. A male bee soon comes a-wooing. As he tries to mate with what he thinks is a female bee, she disintegrates and the triungulin larvae climb up onto the male bee. When the bewildered bee then flies on and, with luck, meets a real lady bee, the larvae hop onto her like rats leaving a sinking ship. That way, they are ensured a lift home to her hive.
The triungulins repay their driver by shape-shifting into legless larvae. They lie still in the hive, slurping up all their driver’s pollen. And for dessert, they generally gobble up the wild bee larvae that are the rightful inhabitants of the hive. Once the blister beetle larvae are good and full, they pupate and wait for spring. That way, the cycle can start all over again.
Blister beetles owe their name to the fact that they secrete a blistering agent called cantharidin—one of the more poisonous substances known. An amount the weight of a grain of rice is enough to kill a human being.
For some reason or another, somebody got the (mistaken) idea that cantharidin was an aphrodisiac. Dried blister beetles of the “Spanish fly” species (Lytta vesicatoria), which are found further south in Europe as well as in the east, were once used as a sexual stimulant for men. It is said that Livia, the scheming wife of Emperor Augustus (of Nativity story fame), sprinkled her male guests’ food with crushed Spanish fly in the hope that it would make them cast aside all discretion and self-control and do things she could later use against them.
In reality, the substance causes blisters and festering sores if it comes into contact with your skin, as well as painful irritation and swelling of the urethra if you eat it. What’s more, there’s an incredibly thin line between nonlethal and lethal effect. You don’t want to mess with this stuff.
Blister beetles are adapted to emerge at the same time as the first flight of the solitary wild bees they parasitize. That’s why you see them only in early spring. You’re best off leaving a blister beetle to live out its peculiar life in peace if you happen to spy one.
I’m not much good at cooking Sunday dinner. We’ve often been on a hike, and nobody feels much like cooking when we eventually get home. And we don’t know what to make, either; we were in no state to think two whole days ahead when we did the shopping on Friday afternoon, exhausted after a hectic week.
Oh, to be an insect at a time like this! Or, more precisely, a spotted predatory katydid, a big, bright green Australian bush cricket. It would sort things out in a jiffy and make sure the food was delivered to the door, good and fresh. So fresh, in fact, that it would deliver itself.
All katydids need to do is sing for their supper. It comes running, straight into the jaws of a poor soul starving for Sunday dinner. What do they sing? Well, put it this way—it’s along the lines of Romeo’s serenade beneath the balcony. The katydid has learned to imitate the mating signal of a totally different species, a classy cicada cutie, and this brings unsuspecting male cicadas strolling over. They head toward the sound, but instead of finding a sweet fellow cicada, they encounter a hungry and much larger enemy. Sunday dinner has just served itself.
In scientific language, this is called “aggressive mimicry”—a process whereby a predator or a parasite imitates the signal of another species in order to exploit the recipient of the signal. There are several examples: the Photuris versicolor firefly, for instance, can imitate a total of eleven relatives and pass itself off as a hot-to-trot female of all those species. Thus it is able to sit around idly, flashing away like a short-circuiting Christmas tree, and make its food come to it.
Even stranger is the bolas spiders’ home delivery system. These spiders spin a thread with a sticky lump at one end, which they swing round and round until it hits a passing moth. The moth is then hauled in like a fish on a hook and packed up neatly in silk to be digested in peace and quiet when the night is over. This hunting weapon is reminiscent of a bolas, the implement consisting of two heavy balls linked by a cord that is used by gauchos, Argentina’s answer to North American cowboys.
It’s one thing to be a gaucho on horseback throwing your bolas at an animal you’re chasing down; it’s quite another to be a spider, sitting stock still. What are the actual chances of an owlet moth passing the spot where you are sitting still, seemingly innocent, swinging your bolas? Roughly equal to zero.
That’s why this spider has also found a way to sing for its supper. It sings with scent. It has learned to imitate the complex scent signals of various species of moths. Sensing that love is in the air, Mr. Moth flies closer and closer to the source of this natural version of a femme fatale’s scent—until it finds itself stuck fast in the spider’s trapping gear.
There is a day for everything: we have the World Migratory Bird Day, the World Day of Happiness, we even have Waffle Day and International Tea Day. But perhaps you didn’t know that the last day of April every year is World Robber Fly Day. The creator of the #WorldRobberFlyDay hashtag, Erica McAlister, is an insect specialist at the Natural History Museum in London. She thinks that we ought to celebrate insects a lot more than we do. And why not start with robber flies?
Robber flies (Asilidae family) are pretty hefty predators. The family includes species that are up to 2.5 inches in length, which is nothing short of gigantic by fly standards. They are sun-loving, dark, often slender flies with powerful legs, enormous eyes, and a bushy mustache on their upper lip. They have full command of the air and can switch direction as they hover around waiting for unsuspecting prey to fly peacefully by.
In the blink of an eye the prey is trapped by six powerful, hairy robber fly legs. Without bothering to land, the robber fly thrusts a solid proboscis into its prey, which may well be an insect larger than itself—in warmer areas, it may even be a hummingbird. The robber fly injects its victim with a cocktail of spit, poison, and digestive fluids, instantly transforming the innards of the trapped creature into a kind of insect smoothie served up in a handy beaker. A couple of swift slurps—generally at top speed—and the robber fly casts aside the empty shell. Not for nothing are these roughnecks also known as “assassin flies.”
Many robber fly species are rare, and we know little about how they spend their larval phase. As we know, they are important in controlling other insect populations and keeping them down, so it might be wise to learn more about these hefty flying predators and their role in the food web.
Imagine a red-eyed army of insects crawling up out of the earth, slow and silent. Every insect is the size of your thumb, and they emerge in such vast quantities that they call to mind a bad horror film about the end of the world. We’re talking about a density of around 3 million insects on an area the size of a soccer pitch—but this is neither science fiction nor a doomsday prophecy. It is simply Swarmageddon, as some wits have dubbed the cyclical emergence of North America’s seventeen-year cicadas.
These sap-sucking insects are quite content to forgo the outdoor life for as many as sixteen years in a row. They hide deep down in the dark alleyways and alcoves beneath the earth’s surface, where they lie silently waiting. Now and then they’ll take a sip of a root sap cocktail through the built-in drinking straw that serves as their mouth. Then, in year seventeen, the troops gather, preparing for some serious gate-crashing.
They emerge from the earth in hordes: pale brown, quiet, and wingless. The silent assembly climbs up into the trees and embarks on its final molting, which transforms the cicadas into adult individuals equipped for procreation. Voilà—out of the old exoskeleton steps a winged being, all dressed up and ready to party. The pickup sessions get under way, love is in the air, and silence is a thing of the past. If you’ve spent seventeen years lying quietly in the earth, you’ve got plenty to say for yourself. We humans hear the cicadas’ song as an intense, high-frequency, grating racket. Multiply that by millions of singing male cicadas, and it’s no wonder people can suffer hearing loss if they spend too much time out and about when the seventeen-year cicadas strike. The sound level can be as high as 100 decibels. Although the seventeen-year cicada doesn’t sting or bite, people have to cancel garden parties and open-air wedding ceremonies when Swarmageddon strikes, because it simply isn’t possible for people to talk together outdoors while this is going on.
Still, the party is short lived. After spending 99 percent of their lives underground for seventeen years, the cicadas’ adult life is over in three to four weeks. Their song leads to mating, and mating creates new cicada eggs. The eggs hatch over the course of a few weeks, and small cicada nymphs crawl and crawl along the branch they were born on until it runs out and—boom! The newly hatched, wingless nymphs fall to earth and dig their way down—down into seventeen years of darkness.
Long before the nymphs hatch, their mom and dad are dead, having fulfilled their role. Now the only thing left to do is for people to fetch their snow shovels, clear pounds of lifeless insect carcasses off their drives and verandas, and await the next appearance in seventeen years’ time—with anticipation or dread.
The seventeen-year cicadas are, in fact, the longest-lived insects we know of, along with their cousins the thirteen-year cicadas. There are several species, each of which may have several broods with different timing in different parts of the United States. No wonder the genus name of these peculiar insects is Magicicada.
So what is the point of the seventeen-year cicada’s astonishing life story? And how in the world do the insects manage to count?
It appears that this behavior evolved because it reduces the chances of being eaten. Since cicadas are large and rich in protein, they are much-sought-after food for birds, small mammals, and lizards. This dramatic flooding of the food market ensures that a larger proportion of the cicadas are able to survive, mate, and lay eggs. It is quite simply a question of surviving by disappearing in the crowd. Since the time interval is so long, it is hardly likely that any predator will be able to adapt to it. And it is far from random that 13 and 17 are both prime numbers (numbers that can be divided only by themselves and 1). This means that a predator with a shorter cycle will never be “in sync” with the cicada boom cycle. Having a cycle that involves a pretty large prime number therefore reduces the chances of being eaten. This really is a fairly impressive math trick from an insect with the arithmetical ability of a toaster.
But how does the seventeen-year cicada know when it’s time to put down its long drink and prepare to join the party on the surface? The trigger for their synchronized appearance is soil temperature. When the soil at depths of 8 to 12 inches remains above 65 degrees Fahrenheit for four days for the seventeenth time, the cicadas’ internal alarm clocks all go off simultaneously. But we don’t know quite how the countdown to year seventeen happens. Part of the explanation appears to be a biological clock in which chemical compounds alter over time. Perhaps external signals from the tree also play a role, with the cicadas “counting” the number of times it blossoms. Scientists who manipulated trees to blossom twice in one year did, in fact, find that seventeen-year cicadas emerged a year early.
There are singing cicadas in Europe, too, but they are not cyclical. Many people confuse cicadas (which are true bugs, order Hemiptera) with crickets and other grasshopperlike insects (which belong to the order Orthoptera; see page 27). Many of these also make a noise, but in different ways and at different times. The surging, intense insect sound you hear in the middle of a hot sunny day in southern Europe is typically the cicada.
Have you ever noticed little “spitballs” in the grass in summer? In many places, those spots of foam in the grass are known as “cuckoo spit,” although they have nothing to do with birds. Inside the protective foam lies a meadow spittlebug, a distant cousin of the fat seventeen-year cicadas. European spittlebugs, which do not sing, spend the whole of their childhood at a foam party. The foam is created when the cicada nymph blows air through some slime it excretes from its rectum. This protects it against both predators and dehydration.
We can blame or praise insects for many things, and zebras’ stripes may be among them—because in the same way insects have evolved to deceive predators and trick victims, larger animals have evolved in response to irritating insects. Actually, the mystery of these stripes has plagued biologists since Darwin’s time. Why in the world are these particular animals striped when the same types of animal elsewhere are not? Scores of creative theories have been proposed over the years. Might the stripes provide camouflage for the animals when they are standing among small, scattered trees that cast a shadow? Maybe the pattern confuses predators, so they can’t quite see where one zebra ends and the next one begins? Might the stripes have a cooling effect because the air warms up more rapidly over the black than the white sections, thereby creating tiny eddies of air? Or might the stripes serve a similar purpose as a conference name badge, letting the zebras know who’s who?
This stripy debate hasn’t yet been resolved, but some recent research rejects all these suggestions in favor of a fifth theory: the stripes repel insects.
Many infection-bearing insects live in the zebra’s habitat, including tsetse flies and other kinds of biting flies that transmit diseases to large mammals. But if you are stripy, you get off lightly. Infection bearers don’t like landing on striped surfaces. Why? Because stripes apparently confuse the insects’ visual orientation, especially when the zebras are moving. The stripes create a kind of optical illusion, like the way we humans perceive the rotation of a spoked wheel or propeller as different from its actual rotation. So the new theory is that evolution has promoted the zebra’s stripes because they lead to less insect trouble and consequently improved survival rates.
By the way, have you ever wondered what color the zebra is beneath its stripes? Well, its skin is not striped; it is black. In other words, the zebra is black with white stripes and not vice versa. There’s a handy piece of trivia for your next cocktail party.
Insects are staple food for birds, fish, and many mammals. At the same time, we know that insects often also eat one another, and this is absolutely crucial for keeping down the populations of what we think of as troublesome pests.
We know that an agricultural landscape where the fields are interspersed with varied flora provides a habitat for many of the pests’ natural enemies. Similarly, woodland consisting of natural forest contains more predatory insects and parasites that keep spruce bark beetles and other pests in check than do managed forests. Predatory insects and parasites control the number of other small creatures in the forest. Swedish studies have found that the great spruce bark beetle, a species that can cause major damage to timber, has a great many more enemies in a natural forest with a variety of dead wood than in intensively managed forests.
In the garden, too, insects help keep things in order. Take the wasp, for example. A growing wasp’s nest requires a lot of nourishment. It is said that a wasp can eat and eliminate two pounds of other insects from a garden measuring a couple thousand square feet—although the source of this claim is uncertain.
When it comes to spiders, however, we have fresh estimates of how much insect meat the massed spiders of the world actually devour in a year. And it is far from trifling: the planet’s eight-legged insect population gobbles down between 400 billion and 800 billion tons of insects a year. That’s more than the entire human population manages to polish off even if we combine meat and fish consumption.
To put it another way, the spiders of the planet could eat up every human being on Earth in a single year and still have room for more. Fortunately for us, they prefer to feast on the earth’s many insects instead.