To a parasite, a host is a living island. Bigger hosts tend to have more species of parasites in them than small ones, just as Madagascar has more species than the Seychelles. But as islands go, hosts have some quirks. Parasites can find in them a vast number of ecological niches, because a body has so many different places to which they can adapt. On the gills of a single fish, a hundred different species of parasites may each find their own niche. An intestine may look like a simple cylinder, but to a parasite, each stretch has a unique combination of acidity, of oxygen levels, of food. A parasite may be designed for living on the surface of the intestines, inside the film that coats it, or deep among its fingerlike projections. In the bowels of a duck, fourteen species of parasitic worms may live (their combined population is on average twenty-two thousand), and each species takes as its home a particular stretch of intestine, sometimes overlapping with its neighbors, often not. Parasites can even find a way to parcel out the human eye: one species of worm in the retina, one in the chamber, one in the white of the eye, one in the orbit.
In hosts where parasites can find enough niches, they don’t compete over their island of flesh. But when they all want the same niche, ugliness usually breaks out. A dozen species of flukes may be able to infect a single snail, for example, but they all need to live in its digestive gland to survive. When parasitologists crack open the shells of snails, they typically don’t find those dozen species of flukes inside, but several individuals from one species. The flukes may devour their competition or release chemicals that make it harder for newcomers to invade. Other parasites living inside other animals can also compete with one another. When thorny-headed worms arrive in a rat’s intestines, they drive tapeworms out of the most fertile region, exiling them down into a stretch of the bowels where it’s much harder to find food.
The most vicious and unneighborly behavior of all, though, can be found among some of the parasitic wasps that so impressed Darwin. This shouldn’t come as too much of a surprise, given the gruesome way the wasps treat their hosts. The mother wasp roams over the countryside, sniffing the air for the scent of the plants its host—often a caterpillar but sometimes another insect such as an aphid or an ant—feeds on. Once it gets closer, it sniffs for the scent of the caterpillar itself or its droppings. Parasitic wasps alight on their host and jam their stinger into the soft section between the plates of the caterpillar’s exoskeleton. Their stinger isn’t actually a stinger at all, though; it is actually called an ovipositor, and it delivers eggs—in some cases just a handful, in others hundreds. Some wasps also inject venom that paralyzes their hosts, while others let them go back to feeding on leaves and stems. In either case, the wasp eggs hatch, and larvae emerge into the caterpillar’s body cavity. Some species only drink the caterpillar’s blood; others also dine on its flesh. The wasps keep their host alive for as long as they need to develop, sparing the vital organs. After a few days or weeks, the wasp larvae emerge from the caterpillar, plugging up their exit holes behind them and weaving themselves cocoons that stud the dying host. They mature into adult wasps and fly away, and only then does the caterpillar give up the entomological ghost.
When different species of wasps compete for the same caterpillar, it can become a brutal struggle. A clutch of wasp larvae may end up stunted and starved if they face too much competition, and the danger is worse for wasps that need a long time to mature in caterpillars. The wasp Copidosoma floridanum takes an entire month to mature inside the cabbage looper moth. As a result, it is a staggeringly unfriendly parasite.
Typically, Copidosoma lays only two eggs in its host, one male and one female. As with any egg, each begins as a single cell and divides, but then it veers away from the normal path of development most animals follow. The cluster of wasp cells divides itself up into hundreds of smaller clusters, each of which then develops into separate wasps. Suddenly, a single egg gives rise to twelve hundred clones. Some of the clusters develop much faster than the rest, becoming fully formed larvae only four days after their original egg was laid. These two hundred larvae, known as soldiers, are long and slender females, with tapered tails and sharp mandibles. They roam through the caterpillar, seeking out one of the tubes the caterpillar uses to breathe. They wrap their tails around a breathing tube, and like sea horses anchored to a coral reef, they rock in the flow of caterpillar blood.
The task for these soldiers is simple: they live only to kill other wasps. Any wasp larva that passes by, whether other Copidosoma floridanum or another species, prompts a soldier to lash out from its tube, snagging the larva in its mandibles, sucking out its guts, and letting the emptied corpse float away. As this slaughter goes on, the rest of the Copidosoma embryos slowly develop and finally grow into a thousand more wasp larvae. These larvae, called reproductives, look very different from the soldiers. They have only a siphon for a mouth, and they’re so tubby and sluggish that they can move only by being carried by the flow of the caterpillar’s blood. Reproductives would be helpless against any attack, but thanks to the soldiers, they can just drink the caterpillar’s juices as the shriveled corpses of their rivals float past.
After a while, the soldiers turn on their siblings—more specifically, on their brothers. A mother Copidosoma lays one male egg and one female egg; after they’ve both multiplied, they produce a fifty-fifty split betwen the sexes. But the soldiers selectively kill the males so that the vast majority of survivors are females. Entomologists once documented two thousand sisters and a single brother Copidosoma emerging from a caterpillar.
The soldiers turn on their own brothers for sensible evolutionary reasons. Males do nothing for their future offspring beyond providing sperm. Copidosoma’s hosts are hard to find—they are spread out like islands separated by miles of ocean, so males that emerge from a caterpillar will probably mate successfully close to home with their sisters. In such a situation, only a few males are necessary, and any more would mean fewer females for them to mate with, and fewer offspring. By killing the male reproductives, the female soldiers ensure that the host will be able to support the most females possible and help carry on the genes they share with their sisters.
As ruthless as soldiers may be, they’re also selfless. They are born without the equipment for escaping the caterpillar themselves. While their reproductive siblings drill out of the host and build themselves cocoons, the soldiers are trapped inside. When their host dies, they die with it.
Making that final journey—leaving the host—is the most important step in a parasite’s existence. It takes particular care to be ready to get out when the time is right, because otherwise it will be doomed to die with its host. That’s why people who need to be tested for elephantiasis, as Michael Sukhdeo was as a child, have to be tested at night. The adult filarial worms live in the lymph channels, and the baby worms they produce move into the bloodstream, spending most of their time in the capillaries in tissues deep within the body. But the only way for a baby worm to grow to adulthood is to be taken up in the bite of mosquitoes that come out at night. Somehow, deep inside our bodies, the worms can figure out what time of the day it is—perhaps by sensing the rise and fall of their host’s body temperature—and move out into the blood vessels just under the skin, where they’re likely to get sucked up by a mosquito. By two in the morning, the worms that haven’t been picked up in a bite start moving back to their host’s core to wait for the next dusk.