Research from St. Olaf College and the University of Denver, published in Current Biology , found that a parasitic fly in Hawaii has evolved to eavesdrop on the mating calls of Pacific field crickets. The flies were likely introduced to the Islands by Polynesian settlers and European cargo ships.
The research found that in Hawaii, the acoustic parasitoid fly has quickly evolved more sensitive hearing across a range of sound frequencies that are prominent in recently evolved and rapidly changing cricket calling songs. This adaptation improves the flies' ability to locate hosts for their larvae, increasing their chances of survival.
"It's important to understand how the sensory systems of eavesdroppers evolve because it reveals the fascinating ways animals adapt to survive and thrive," said co-corresponding and co-senior author Norman Lee , an associate professor of biology at St. Olaf and the director of the Neuroscience Program. "Eavesdroppers, like flies that listen in on cricket songs, show how some species develop incredible abilities to detect sounds or signals that aren't meant for them."
In previous studies, co-corresponding and co-senior author Robin Tinghitella, an associate professor of biology at the University of Denver, found that some male Pacific field crickets were evolving new songs through wing mutations, so as not to become prey to carrying the larvae of female Ormia ochracea, the parasitic flies. In turn, the parasitic flies have improved in their ability to find the location of male crickets for the larvae to incubate and develop in –– leading to the decline of crickets that do not have novel songs.
"This now appears to be a classic example of adaptation and counteradaptation, back and forth, between the crickets and flies," Tinghitella said. "Will the crickets evolve new songs, yet again, to evade the parasitic flies? Will the flies develop new ways of finding hosts? We can't wait to see what will happen next."
Building off of previous academic work, the team collected Ormia ochracea flies from Hawaii and Florida as a comparison population. The research team ran two experiment types, behavioral and neural, that focused on measuring the female parasitic flies' response to cricket song types (such as purring, rattling, and typical). They found that Hawaiian flies had developed a more sensitive auditory system, allowing them to better locate these novel songs. In the field, they found that while the parasitic flies in Hawaii preferred the louder typical cricket songs, they were still able to detect the less intense purring and rattling songs.
"This research also helps us understand broader questions, like how animals navigate their environments, find food, or avoid predators," Lee said. "Plus, the unique strategies these eavesdroppers use can inspire innovations in technology. In essence, it's about uncovering the hidden ways nature works and using that knowledge to benefit science and society."
The research team recommends future studies to compare the neural threshold of the Hawaiian fly and cricket populations. Further research will help to better understand if female crickets' auditory systems have evolved to better detect new songs by the males.
This research was funded by the National Science Foundation, from their Division of Integrative Organismal Systems and Division of Environmental Biology; an NSF Graduate Research Fellowship; and the Collaborative Undergraduate Research and Inquiry (CURI) program at St. Olaf. The research team was composed of faculty and graduate students from St. Olaf and the University of Denver.
Through CURI, this work received undergraduate research support from Mikayla Carlson ' 23 and Mackenzie Farrell ' 23 in the summer of 2022 and Quang Vu ' 25 in the summer of 2023 . "Engaging in cutting-edge, hands-on research allows St. Olaf undergraduates to apply their coursework, develop transferable technical skills, and experience how science is truly done while contributing to meaningful discoveries," Lee said.
