A Fly with Ears? German Researchers Study Local Parasite
For over a decade, German researchers have been coming to the Biological Station to study a parasitic fly, Emblemasoma auditrix. Nanina Tron, a Ph.D. student from the University of Giessen explains why. "It can hear. It uses sound to find its host. For flies, this is very unusual." Only a handful of fly species are known to have functional ears.
A cicada, Okanagana rimosa, is the fly's host. Males of the species use a harsh, metallic buzz to attract mates. Emblemasoma follows this same call to locate its host. It then places a larva in the cicada's tymbal, the sound-producing organ. The larva eats the cicada's soft tissues, emerges from the body, then falls to the ground where it pupates.
"We want to know how it can locate the cicada," says Liesa Beuter, a master's student who is part of the research team led by Dr. Reinhard Lakes-Harlan. It was Lakes-Harlan, a sensory neurobiologist, who first described Emblemasoma's ear in 1999, following fieldwork at UMBS (Convergent evolution of insect hearing organs from a preadaptive structure, Proceedings of the Royal Society London B. 1999; 266:1161-67). He or members of his lab have returned to the station eleven of the past thirteen summers.
Understanding how Emblemasoma locates its host involves studying both the fly's behavior in the field and its physiology in the lab. At UMBS field sites, Tron and Beuter play recordings of Okanagana's call and film the flies' approach to the speakers. They also note how accurately the flies land at the source of sound. Based on field work done at UMBS in 2012, Tron says, "We could see they have a bouquet of different approaching modes. They can orient while sitting or flying." The flies' preference is to hopscotch in, using plants as resting spots. But even without vegetation, they could fly directly to the cicadas.
To learn more about the mechanics of the fly's ear, the team takes live species back to Germany. Most animals hear by triangulating the sound delays between ears spaced some distance apart. But with Emblemasoma, the ears are too close together to permit a difference to register. Lakes-Harlan says, "The flies 'know' the elevation of the sound source. We ask what mechanism might be responsible for this ability. The biophysics are still not fully understood: whether the ear is a pressure receiver or a pressure difference receiver." In a one experiment, they destroy one of the flies' two ears and see how this affects their orientation. This research is still in progress and might lead to new designs for miniature microphones.
After nearly a dozen field seasons in northern Michigan, Lakes-Harlan's work still focuses primarily on mechanisms of Emblemasoma's hearing in three-dimensional space. Yet he admits that what began as a neurobiologist's inquiry into anatomy and physiology has, "expanded from sensory physiology to behavior, parasitology, ecology and other fields." So when asked about how long he plans to keep coming to Biological Station, he responds, "Questions of localization of sound sources in space might be answered in one or two years. But then other questions, like population dynamics or signal evolution are still open."
It would appear that Lakes-Harlan and his students will be returning to the Station for many summers to come.
NOTE: Questions about Dr. Lakes-Harlan's research should be directed to his e-mail address.