When I ask Sara Weinstein about her favourite parasite, she laughs, and has a quick answer. “I think the flies that parasitize frogs are really interesting,” she says.
There are many species of them, including twoknown as toadflies. The females lay their eggs on frogs and toads—sometimes on their skins and occasionally in their nostrils. When the maggots hatch, they eat their way inward, and you can sometimes see them through the wounds that they open in their host’s flanks. Their appetites are usually fatal. Once their hosts have ribbited their last, the larvae fall away and turn into adults.
Many insects also get devoured alive by the larvae of parasitic wasps, flies, and more. But the idea of a back-boned animal—a vertebrate like ourselves—succumbing to a similar end is what really creeps Weinstein out. “We don’t think about vertebrates being affected by parasites that can take over the whole organism,” she says. “It’s like the movie Alien.”
Parasites are those creatures that, at some point in their lives, survive by feeding on another individual. It’s easy—hopeful, perhaps—to think of them as oddities of nature, as grisly outliers that we would only encounter through extreme bad luck. But as I noted in my TED talk, parasitism is the rule rather than the exception. It’s estimated that around 40 percent of animal species are parasites. Forget elephants, hummingbirds, whales, and tortoises—pick a random animal, and it’s far more likely to be a blood-sucker, disease-carrier, host-castrator, or flesh-devourer.