Even flowers can become zombies to their parasites. A fungus called Puccinia monoica lives inside mustard plants that grow on the slopes of Colorado mountains. The fungus sends its tendrils throughout the stem of the mustard plant, feeding on the nutrients the flower draws from the sky and the soil. In order to reproduce, it needs to have sex with the Puccinia inside another mustard plant. To do so, the fungus stops the plant from sending up its own delicate little flowers and forces it to turn clusters of its leaves into brilliant yellow imitations of flowers. These fakes look exactly like other flowers found on the mountains, not just in visible light but in ultraviolet light as well. They lure bees, which can feed on a sweet, sticky substance that the fungus forces the plant to produce on the imitation flowers. The fungus crams its sperm and its female sex organs into them, so that the bees can fertilize the fungus as they travel from mustard plant to mustard plant. But the plant itself remains sterile.
No matter how comfortable a parasite may make itself by altering its host, it has to leave sooner or later. Some parasites head on to the next host in their life cycle, others go to a free-living adulthood, and in many cases the parasites stage-manage a careful exit. Simply letting the host go on with its normal life would mean death for most parasites. The tobacco hornworm normally moults five times and then wanders down from its plant to the ground. It digs a few inches into soil and forms its cocoon, where it stays until it emerges as a moth. When hornworms are parasitized by the wasp Cotesia congregata, however, they take a different path. They moult only twice, and they never get the call to wander off their plant. Instead, they go on chewing leaves, nurturing their parasites until the wasps are ready to emerge. The hornworm then slows down and stops eating, losing its appetite. The wasps seem to be responsible for the anorexia, because a healthy hornworm will happily devour dozens of wasp cocoons.
Another species of wasp goes even further, turning its host—the cabbage worm caterpillar—into a bodyguard. When the wasp’s larvae have matured, they paralyze the cabbage worm and push their way out of its abdomen. They then spin their cocoons on the underlying leaf. Yet, even after the wasps have devoured the guts of the caterpillar and riddled it with escape hatches, the cabbage worm recovers. It doesn’t limp away; instead, it weaves a mesh over the wasps to shield them from other parasites and coils itself on top. If anything should disturb the caterpillar as it stands guard, it lashes out, biting and spitting up noxious liquids—in other words, protecting the cocoons. Only when the wasps emerge from their cocoons does the cabbage worm end its duty to them and lie down to die.
While wasps can live on dry land once they’ve left their hosts, many other parasites need to get to water. There are parasitic nematodes, for instance, that live as free-living adults in streams, where they mate and lay their eggs. When their offspring hatch, they attack the mayfly larvae that live alongside them. The nematodes pierce through the mayfly’s exoskeleton and curl up inside its body cavity. There they grow as the mayfly grows, siphoning off its food. The mayflies go through a long, lingering insect adolescence in the water before they transform into delicate, long-winged forms. The males rise from the water and form great clouds that attract the females. The nematodes rise invisibly into the cloud within their hosts.
Male and female mayflies find each other in the swarm. Embracing, they fall to the grasses and reeds along the stream, and mate. You can tell the difference between the sexes not only by their genitals (the males have little claspers to help them mate) but by other parts of their bodies such as their eyes: the female has small eyes pointing out to either side, while those of the male bulge out so much that they touch over the top of its head. Once they’ve mated, the males have finished their life’s work. They fly lazily away from the stream to find a place to die. The females, meanwhile, make their way upstream to find a protruding rock. They crawl under it and bob their abdomens up and down as they lay their eggs. If the female is carrying a nematode, the full-grown parasite breaks out of the mayfly’s abdomen and burrows away into the gravel to find a mate of its own, leaving its host dead.
The nematode’s strategy has one big, obvious flaw: if it happens to climb inside a male mayfly, it will end up in a patch of grass. Instead of getting back to the water, it will die with its host. The nematode has a solution, one that’s reminiscent of Sacculina: it turns the male into a quasi-female. When an infected male mayfly matures, he never forms his claspered genitals or even his high-domed eyes. The nematode makes him not only look like a female but act like one, too. Instead of flying away, he drops down to the stream, even going so far as to try to lay imaginary eggs as the parasite bursts out of his body.
The nematode needs to get back to the stream for two reasons—to move on to the next stage of its life, and to be in a place where its offspring will be able to find a mayfly of their own to invade. Getting to the next host is a consuming passion among parasites, because there is no alternative: “Live free and die” is their motto. A fungus that lives inside house flies provides a spectacular example of this. When the spores of the fungus make contact with a fly, they stick to its body and dig tendrils into the fly’s body. The fungus spreads throughout the fly’s body with Sacculina-like roots and sucks up the nutrients of its blood, making the fly’s abdomen swell as it grows. For a few days the fly lives on normally, flying from spilled soda to cow turd, using its proboscis to sponge up food. But sooner or later it gets an uncontrollable urge to find a high place, be it a blade of grass or the top of a screen door. It sticks out its proboscis but uses it as a clamp this time, gluing itself to its high perch.
The fly lowers its front legs, tilting its abdomen away from the surface. It flaps its wings for a few minutes before locking them upright. The fungus has meanwhile pushed its tendrils out of the fly’s legs and belly. On the tips of the tendrils are little spring-loaded packages of spores. In this bizarre position, the fly dies, and the fungus catapults out of its corpse. Every detail of this death pose—the height, the angles of the wings and the abdomen—all put the fungus in a good position for firing its spores into the wind, to shower down on flies below.
As if this were not enough of an accomplishment for a speck of fungus, infected flies always die in this dramatic way just before sunset. If the fungus matures to the point where it can make spores in the middle of the night, it doesn’t: it holds off the process, waiting through the dawn and the day. It is the fungus, not the fly, that decides not only how it will die but when—just before sundown. Only then is the air cool and dewy enough for the spores to develop quickly on another fly, and only then are healthy flies leaving the air for the night and moving down toward the ground, where they make easy targets.
Parasites such as this fungus use their hosts to get to other hosts of the same species. But for many other parasites, the game is more complicated: they have to make their way though a whole series of different animals. Sometimes they force their current host to get into the vicinity of their next one. Along the coasts of Delaware lives a fluke that uses mud snails as its first host and fiddler crabs as its second. The only problem is that the snails live in the water and the crabs live on shore. But when the snails are infected by the fluke, they change their behavior. They grow restless; they wander onshore or onto sandbars during low tides and linger there while healthy snails keep to the water. They shed their flukes on the sand, putting the parasites so close to the fiddler crabs that they can easily burrow into them. It’s as simple as getting a taxi to a bus station.