National Geographic : 2014 Nov
42 national geographic • month 2011 From the wasp’s point of view, this is a very positive development. A growing D. coccinellae wasp nestled in its cocoon is intensely vulnerable. Lacewing larvae and other insects will happily devour it. But if one of these predators approaches, the ladybug will thrash its limbs, scaring off the attacker. In effect it has become the parasite’s bodyguard. And it will continue to loyally play this role for a week, until an adult wasp cuts a hole through the cocoon with its mandibles, crawls out, and flies away. Only then do most of the ladybug zombies die, their service to their parasite overlord complete. This sinister scene was not conceived by a scriptwriter. Across much of North America, wasps are converting ladybugs into zom- bie bodyguards in backyards and empty lots, in farm fields and wildflower meadows. Nor is the spotted lady beetle unique. Scientists are finding the same is true for a vast number of host species, ranging from insects to fish to mammals. They serve their parasite even if they must literally hurl themselves to their own death to do so. Across the natural world the same question arises again and again: Why would an organism do all it can to ensure its tormentor’s survival rather than fight for its own? Serving as bodyguard is only one of the protective services provided to parasites by their hosts. A fly that infects bumblebees causes them to burrow into the ground in autumn, right before the fly emerges to form a pupa. In the ground the fly is protected not only from predators but also from the cold of winter. In Costa Rica, the orb-weaving spider Leucauge argyra will go to ex- travagant lengths to accommodate the needs of Hymenoepimecis argyra- phaga, another freeloading wasp. The female glues its egg to the host’s body. After the larva emerges, it pokes a few holes in the spider’s abdomen and sucks its blood. When the larva has grown to full size, in a couple of weeks, the spider takes it upon itself to rip down its own web and build a new one of a radically different shape. Instead of a multistranded net designed for catching flying insects, the new web is merely a few thick cables converging at a central point. Having sucked its host dry, the larva spins its cocoon on a thread hanging from the intersection of the cables. Suspended in the air, the cocoon is nearly impossible for would-be predators to reach. Parasites can also coax a host to guard them while they’re still living inside it. Before infecting a human host, Plasmodium, the protozoan that causes malaria, spends the first stages of its life cycle in a mosquito. The mosquito needs to drink blood to survive. But this behavior poses a risk 42 national geographic • November 2014 Carl Zimmer last wrote on new ways of exploring the brain in the February 2014 issue. Biologist and photographer Anand Varma communicates science through images. Matt Twombly is a graphics editor at the magazine.