A group of parasitic insects tough enough to kill fire ants has shown researchers a nasty new trick. The creatures hide inside their victim by making the host form a protective bag of its own skin.
The order of tiny insects called Strepsiptera can live only as parasitoids inside the bodies of other insects, says Jeyaraney Kathirithamby of the University of Oxford in England. When she began studying the life cycle of a Pacific species, she noticed that the insects become enclosed in a sheath of thin tissue as they jab their way into the victim.
DNA tests showed that this bag of skin comes from the host, probably disguising the invader from the host’s immune defenses, Kathirithamby and her colleagues report in an upcoming Proceedings of the National Academy of Sciences. Investigations of other Strepsiptera species are turning up the same strategy, says Kathirithamby.
Understanding how Strepsiptera parasitoids outwit host defenses is particularly exciting, she says, because the order’s 500-plus species manage to prey on insects in 34 families. Some of these parasite-host relationships are quite unusual. For example, in some of the parasitic species, the males infiltrate ants while the females take up residence in grasshoppers. Also, Kathirithamby says, uncovering the biology of these superb insect killers may lead to biological controls for pests.
The particular species (Stichotrema dallatorreanum) that Kathirithamby discovered in skin bags targets the katydids of Papua New Guinea, which damage oil palms. The parasitoid’s youngest larval stage attacks the host. The larva burrows through the katydid’s outer cuticle into the first layer of living tissue, the epidermis. The larva wriggles around for a day or two at this level until a loose sheath of epidermal tissue encloses it.
In effect, Kathirithamby says, “the parasitoid hijacks the [host’s] wound-repair system.”
Some insects kill invading parasitoids by encapsulating them. The host’s skin bag works differently, says Kathirithamby, noting that larvae thrive within these thin and apparently permeable sheaths.
When S. dallatorreanum finally matures, the top of its body grows to the surface of the victim and pokes out. The species has no males, so some 800,000 larvae develop without fertilization inside each adult. They emerge from her body, which remains embedded in the host. This new generation moves on to new victims, and the old host dies.
Strepsiptera don’t attack people. Even so, notes Michael Strand of the University of Georgia in Athens, “there’s a real recognition now that understanding more about parasite-host interactions really does matter.” The strategies in their battles, as well as the underlying genetic and molecular mechanisms, are pervasive in nature, he says. Besides, he adds, some of these interactions “are really elegant.”
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