Ever build a play fortress and hide inside? A tiny parasite called Strepsiptera does the same thing--but it's not all fun and games, at least not for the insect that it lives in. The parasite larvae dig into their hosts and wrap themselves in a sack created by the host's tissue. The sack protects the larva from attack by the host's immune system, and it even helps supply the growing larva with nutrients.
Insects' immune systems lack adaptable weapons such as antibodies, but they can still be strikingly effective. In response to an invading parasite, mobile cells from an insect's blood surround and kill the intruder. For parasites that want to evade these defenders, there's a “continuous arms race,” says Jeyaraney Kathirithamby, a zoologist at the University of Oxford, U.K. Parasites have evolved various ways to avoid scrutiny from the host's immune system, and most have adapted exquisitely to one host. The order Strepsiptera stands out because these bugs can make a home inside everything from ants to bees to grasshoppers. Kathirithamby wanted to know how Strepsiptera manages to be so versatile.
Using a microscope, Kathirithamby videotaped Strepsiptera larvae burrowing into katydid hosts. As reported online on 3 June in the Proceedings of the National Academy of Sciences, the larva jabs its head into the host's cuticle, a hard protective covering. The larva wriggles into the soft tissue underneath and surrounds itself with a bag that eventually pinches off. The larva can then move within the host's body, receiving nutrients through the layer of abducted cells. The host cell layer provides a tailor-made cloak that keeps the larva effectively invisible to the host's immune system. “Essentially, it's a hijacking,” says Kathirithamby. And it's a simple ruse that can work in many hosts.
Biologists who study insect parasites say Kathirithamby has identified a novel strategy for insect parasites. It's quite a contrast from hardline parasites, for example, which bombard the host's immune system, says John Edwards, who studies parasitic wasps at the Central Science Laboratory in York, U.K. "This is sneakier,” he says, “more like a camouflage effect."