The Shark and the Jellyfish: More Stories in Natural History

Puppeteers

158 Puppeteers Posed on the rock beside a dry wash, the fly is dead still, impassive, oblivious to its surroundings. Its wings are held at an exaggerated angle as though poised for flight, but even though the morning has warmed, the insect has not moved for long minutes. It is no longer a free being—its actions have been redirected to purposes not its own. Its body is now governed by a miasma spreading within. A second fly lands nearby. Flies are gregarious creatures, but before the newcomer can make contact with the first, the scene falls under the shadow of a more commanding presence—a hunting jay. The second fly escapes, but the first does not move. The bird ignores the potential meal— she recognizes the unusual pose and the dark flower of spores spreading away across the surface beneath it. She knows that the motionless fly is now unpalatable due to the infection that has killed it—this insect is a victim in the constant contest between the flies and their parasites. Earlier in the season, when the watercourse ran spring fed and the sun was at its highest, the story would have been different. The fly could have resisted the ruinous fungal spores it had wandered across—it could have shed them from its feet. The air would have been too dry for the spores to survive on the ground long enough for a second chance encounter. Those few spores that did manage to infect a fly would have been slowed by the insect’s immune system, which functions best when the sun of high summer heats its body. The insect could have mounted a behavioral fever in midsummer, killing the contagion simply by exposing its black back to continuous sunlight—raising its body temperature high enough to nearly kill itself. All summer long the flies manage to escape the pathogen. They spend hours rubbing their legs together in hand-washing motions, shedding the sticky spores they chance to pick up when they walk across infested ground. Upon close examination, those fiddling legs appear as fearsome as any spike-studded armor—they are sheathed in saw-blade rows of spines and bristles. The flies constantly curry themselves, and the spores they comb from 159 their bodies accumulate on their leg bristles. As the legs rub together, the spores are transferred back and forth between them. In the process the spores pick up dust and lose their stickiness, finally falling inert to the earth. Just as a cricket sings by rubbing files against scrapers, so the rhythmic cadence of the flies’ regular rows of leg bristles rasping back and forth produces a microscopic trilling chirp, audible only to flies. The theft of a host’s free will is a strategy that allows certain parasites to improve their own success. They cannot kill their host outright, as does a virulent pathogen, because that would break the infection chain. These parasites prosper by getting themselves transferred into their next host before their current host dies. To accomplish this, they manipulate the host’s behavior, so that the host works for them to ensure the transfer. This kind of behavioral parasitism is seen not only in flies, but also in ants, pill bugs, caterpillars, and many more—each victim driven to put itself at risk for the sake of its microbial tormentor. Many behavioral parasites pursue an alternate-host strategy. To complete their life cycle, they must pass between very different animals. One microbial parasite of ants must find its way into a grazing mammal before it can continue its development. This parasite will not kill either host outright, but it could sacrifice the insect if it could do so at the very instant the infection moves into its next host. The microscopic parasite accomplishes that—by altering the ant’s behavior. While uninfected ants forage in the morning, the infected ones cease their work and climb to the tops of the highest stems of grass. There they clamp their mandibles down on the blade tip and then hold that posture throughout midday. Once the afternoon passes, their internal master allows them to release their bite and return to foraging. The infected ants repeat this behavior every day—until they have been eaten. They are unintentionally consumed by grazing herbivores that ingest the ants and the parasitic microbes within them, and in so doing, become infected themselves. In an analogous scenario, a microscopic spiny-headed worm lives a parasitic life cycle...