Becoming a parasite means that, one way or another, you’ve thrown in your lot with your host. Their food may become your food, their droppings (or flesh) the medium that moves your young to a new home, their habits a template for you to exploit. And an interesting paper published in the Journal of Experimental Biology last year shows that sometimes their dangers become yours, too.
Ormia ochracea is a small fly that’s a parasitoid of crickets – adults may fly around in the environment, but their larvae have to grow up inside a field cricket. This means that a female Ormia needs to find a living cricket to feed her young. Fortunately for her, crickets advertise.
Female Ormia exclusively target male crickets as host material. They find them by listening for their song: in fact, they’ve evolved specialized “ears” on their first pair of legs that are sensitive to the high-pitched cricket song most flies can’t hear. So on a summer night, as a male cricket sings to attract a mate, gravid female Ormia are also flying toward his song. When a fly gets close, she lands, walks until she finds the singing cricket, then deposits larvae on his back and on the ground around him. The maggots burrow into their new host and spend the next 10 days chowing down on the cricket’s interior. When they emerge to pupate, the cricket dies.
Obviously, male crickets are singing to attract female crickets, not female parasitoid flies. But both the female crickets and the female Ormia fly toward the male’s song, and they can both wind up as the victim of the same voracious nocturnal predator: bats. Some bats also use cricket song – in this case to locate their prey. Above one calling male cricket you might find a female cricket flying toward a mate, a female Ormia flying toward a new host, and a bat flying toward a meal. Bats aren’t picky – one plump insect is as good as another – so all the bugs in this scenario are at risk. Tying their life cycle to the crickets, the flies also became vulnerable to their predators.
Flying crickets are known to perform evasive maneuvers when they’re hit by a bat’s sonar pulse, and Merri Rosen and her colleagues at Cornell University wanted to see whether Ormia had evolved a similar behavior. Bear in mind that most flies targeted by a bat would be toast – their ears aren’t sensitive to anything as high-pitched as bat sonar, so they’d never hear the bat coming. But Ormia has those specialized ears for tuning in cricket song: can they also use them to detect bats? And if the flies hear a bat, what do they do about it?
To find out, the researchers tested how the flies behaved when they heard high-pitched sounds, both while they were walking and in flight. They found that when the flies were on the ground, they walked toward any high-pitched sound. The flies clearly could hear the ultra-high batlike pitches just as well as the cricket chirps; they just didn’t seem to distinguish between sounds that meant ‘food for babies’ versus sounds that meant “DANGER!” All that changed when the flies took wing. In the air, the flies were more discriminating -- steering toward the lower-pitched cricketlike sounds, and away from pitches more like bat sonar. It’s a pretty bit of convergent evolution in which fly and cricket, linked by parasitism, exposed to the same danger, evolve similar plans for making a getaway.Rosen, M., Levin, E., & Hoy, R. (2009). The cost of assuming the life history of a host: acoustic startle in the parasitoid fly Ormia ochracea Journal of Experimental Biology, 212 (24), 4056-4064 DOI: 10.1242/jeb.033183
PHOTO: Ron Hoy's lab: Cornell University