Crickets and flies face off in a quiet evolutionary battle

Hawaii’s male crickets can’t hide from their buzzing boogeymen for long.

In just a handful of years, cricket-killing parasitic flies have evolved hearing that’s more sensitive to their prey’s new, covert love songs, researchers report February 20 in Current Biology.

The nocturnal fly Ormia ochracea lays its eggs on crickets, which hatch into larvae that eventually make a meal of their host. Native to continental North America, the flies were introduced to Hawaii around 1989 and began targeting the islands’ Pacific field crickets (Teleogryllus oceanicus), eavesdropping on their chirps to find them.

Soon after the flies were introduced, the wing shape of some male Pacific field crickets rapidly evolved, giving the insects an unusual purring or rattling call. That may have allowed males to “sing” to females without alerting the flies. But researchers wondered if the flies would push back.

“Is there going to be a response from the fly or is this really going to turn out to be a system where they evolve sort of a private mode of communication that the eavesdropping [fly] isn’t ever going to be able to find?” says Robin Tinghitella, an evolutionary and behavioral ecologist at the University of Denver.

In the lab, Tinghitella and colleagues compared O. ochracea flies from Hawaii and Florida. The team measured how flies’ hearing neurons responded to specific sound frequencies, and how the flies reacted to the different cricket songs.

Hawaiian flies were more sensitive to frequencies at around 5 and 10 kilohertz, two frequencies that dominate normal and purring cricket songs, than their Floridian counterparts. Hawaiian flies were also more likely to move in response to cricket purrs.

In Hawaii, the team lured flies into traps with recordings of cricket songs. Nearly 20 percent of the flies were caught using purring or rattling songs, suggesting the crickets’ freshly released tracks — not even a decade old — are already detectable by many of the flies. 

The findings point to adaptation and coadaptation, Tinghitella says, which may eventually develop into a game of evolutionary cat and mouse between cricket and fly. 

More data on the flies’ hearing could provide a starting point for understanding how the crickets might respond, says study coauthor Norman Lee, a neuroethologist at St. Olaf College in Northfield, Minn. From there, researchers may be able to make some predictions about how an innovation race between crickets and flies is likely to play out.