By Ed Ricciuti
Scientists have clocked many hours studying how females of various parasitoid flies home in on the calls of cicadas, grasshoppers, and other insects that are their hosts. Research into the mechanics and workings of the ears that are used to do it, however, is far less glitzy and has received scant attention, but scientists in Germany has begun to fill that knowledge gap. Their research is described in a paper published in the Journal of Insect Science.
Mapping and diagramming the complex components of something as miniscule as a fly’s ear — not to mention figuring out how all that stuff works — required high-tech equipment and skills. In addition to dissecting the ear of a fly called Emblemasoma auditrix, the researchers stimulated and observed functions right down to the cellular level of neurons.
The investigation only scratches the surface of a very complex area of research, but hints at how finely the ear mechanism of the fly is attuned to the calls of its host. The findings also sketch out evidence of how fly ears may have evolved from an organ found in its relatives that is incapable of full-fledged hearing but does sense vibrations.
Located on the underside of the front-most portion of the thorax, the ear of E. auditrix is typical of insects that have the ability to hear. It contains a chitinous, tightly stretched membrane that functions much like the human eardrum, which vibrates when contacted by sound waves and transmits these to sensory cells. The “tympanal” insect ear has two other basic structures. Behind the eardrum is a tracheal air space that is part of the respiratory system, which is similar to the Eustachian tube behind the human eardrum. The other structure is the neural sensory apparatus, the chordotonal organ that processes and transmits sound in a form recognizable by the brain. Found only in insects and crustaceans, this organ contains units that sense mechanical vibrations, such as sound waves and movement, and converts them into nerve impulses.
E. auditrix belongs to the family Sarcophagidae, which are commonly called “flesh flies” because some species deposit larvae in carrion or wounds, while others, including E. auditrix, use insects as hosts. It targets a very specific host, the cicada called Okangana rimosa, which is found in the northeastern United States and southeastern Canada. Larvae that are deposited by the female feed inside the cicada, eventually killing it, before emerging to pupate in the soil. Curiously, the male E. auditrix has a fully developed ear but its function is unknown.
Techniques used to explore the labyrinth of the fly’s ear include micro-nano computer tomography, and X-ray imaging that scans and creates detailed cross-sections of internal organs, structures, and tissues. The dissection of the fly ear that occurred during the research required skills so deft and delicate that they would do credit to a brain surgeon. To expose the ear, flies were fixed with wax, dorsal side up, so that a portion of the cuticle — thin, flexible chitin of the exoskeleton over the thorax — could be removed. A hot needle was used to shrivel the flight muscles, which would have obstructed observation.
The researchers tested the response of individual neurons to various frequencies. They examined the impact of various frequencies on “auditory afferent neurons” and “auditory interneurons.” Afferent, or “sensory” neurons, sense stimuli and convert them to electronic nerve impulses that can be sent to the central nervous system. Interneurons bridge the communications gap, so to speak, between afferents and the neurons in the central nervous system that trigger a response to the stimuli.
Previous research has shown that insects with behavior triggered by sound stimuli respond according to the intensity, timing, and frequency of the sound. Although not much is known about how timing of the cicada’s call is sensed by the fly, it probably is involved in distinguishing between the host and other species.
The German researchers found that the responses of afferent neurons and interneurons in E. auditrix show that the ear is “broadly” tuned in respect to the cicada’s calling song. The song peaks at a frequency of 9 to 10 kHz, but the ear as a whole is most sensitive to a frequency of five kHz, about mid-range in human hearing. Some individual interneurons, however, seem to filter out the background noise and react most emphatically to the 9-to-10 kHz frequency peak of the cicada’s song.
The research also indicates that the tympanal ear of the group to which E. auditrix belongs evolved from a chordotonal organ in flies that are incapable of hearing. Several other findings about the ear of the fly open new areas of inquiry. Observations showed that the bigger the body of a fly, the bigger its ears, and that the right ear is about four percent larger than the left, on average. How these differences impact the fly’s ability to locate hosts is unknown.
“The research shows that even with a small ear, flies are able to locate precisely a sound source, the host,” said Dr. Lakes-Harlan, one of the co-authors. “The parasitoid shows a high degree of sensory, neural, and behavioral adaptation to one host species.”
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Ed Ricciuti is a journalist, author, and naturalist who has been writing for more than a half century. His latest book is called Bears in the Backyard: Big Animals, Sprawling Suburbs, and the New Urban Jungle (Countryman Press, June 2014). His assignments have taken him around the world. He specializes in nature, science, conservation issues, and law enforcement. A former curator at the New York Zoological Society, and now at the Wildlife Conservation Society, he may be the only man ever bitten by a coatimundi on Manhattan’s 57th Street.