There have been many changes to the way insects within the Orthoptera order hear and create sounds, but the lineage of their songs dates back around 350 million years, says Hojun Song, an associate professor in the entomology department at Texas A&M University.
“We may take it for granted when we hear crickets and katydids, but these insects are involved in very complex acts of communication,” he says. “Just like we use vocal cords and ears to relay messages in acoustic communication, these insects are engaged in a communication that is some of the oldest on Earth.”
The research in Nature Communications shows that the lineages of Orthoptera communicated to find mates, avoid predators, and navigate throughout 350 million years of diversification. Investigating these ancient communication methods can help us better understand how we ourselves communicate, Song says.
“We discovered a great deal, but there is a great deal that is yet to be discovered about acoustic communication in Orthoptera,” Song says. “But the practical applications of our study are also exciting because it is possible to develop novel tools, such as hearing aid devices, modeled after these ancient mechanisms used for acoustic communication.”
Communication with ‘wing teeth’
In terms of animal sounds, insects are the most diverse communicators on the planet, Song says. There are thousands of mammalian and avian species that communicate acoustically for mating, defense, and navigation, but there are tens of thousands of insect species that make and hear sounds.
Song and colleagues pored over extensive literature and used phylogeny, which refers to the evolutionary relationships among organisms. They also collected DNA- and RNA-quality samples of a wide range of Orthoptera species from around the globe and viewed curated species collections and fossil records.
Crickets and katydids have ears on their legs. Grasshoppers have ears on their abdomen.
The researchers deduced relationships and built a phylogeny based on DNA sampling and gene sequencing methods, including transcriptome analysis, which have improved exponentially over the past decade.
“A few data points can give a glimpse of the evolutionary relationships, but the amount of data we can generate now can give us a very clear view of the relationships among those species we sampled,” he says. “The transcriptome analysis helped us build a reliable family tree.”
The Orthoptera order encompasses about 28,000 known species, and around 16,000 of them communicate acoustically, Song says. These insects use various specialized mechanisms on their wings or legs and abdomens to create and hear sounds.
Some crickets’ wings have microscopic teeth on the underside that look like a file. When rubbed by a scraping mechanism on the other wing, the “teeth” emit the tell-tale chirp. Some grasshoppers scrape their legs with their wings. Some species of katydids communicate at frequencies at the ultrasound level, which is too high for the human ear to perceive.
Hearing mechanisms also vary from species to species, Song says. Crickets and katydids have ears on their legs. Grasshoppers have ears on their abdomen.
Orthoptera ‘time tree’
The authors sampled 239 species of Orthoptera for the analyses. The specimens exhibited a wide range of characteristics representing the diversity of acoustic communication within the Orthoptera order.
“There are 16,000 species that communicate acoustically, so we had to select species that covered a wide enough range of characteristics to build a robust evolutionary relationship phylogeny that investigated the use of hearing and sound producing mechanisms,” he says.
After scientists established relationships among the species, they used fossil data to calibrate the phylogeny and determine how old the lineages were, Song says. A paleontologist reviewed millions-year-old fossils of grasshoppers, katydids, and crickets to create a “time tree” representing how and when the different insect lineages diverged within the Orthoptera order.
Some of the fossils that the paleontologist reviewed included a cricket’s wing from the Triassic Period, 251 million–199 million years ago. The wing displays the same scraper and teeth that crickets currently use to create sound, Song says.
“What we found was that crickets were using the same mechanism to communicate as they do today,” he says. “So, if we had a time machine, we could go back 300 million years and hear a cricket song that is the same or very similar to what we hear today.”
Song and colleagues used that information as a calibration point to go back even further. They estimated, based on 272-million-year-old fossil evidence, that the crown-Orthoptera species appeared during the late Carboniferous period, around 350 million years ago.
Cricket evolution and communication
Song says some species within Orthoptera that appeared later likely evolved their communication to stand out in an increasingly noisy environment.
Communication became more complicated as animal life exploded during the Jurassic and Cretaceous periods. So, they developed advanced sound tempos, patterns, and frequencies to stand out among other species to attract mates, he says.
“There were more species entering the soundscape, and it was becoming saturated, so species probably had to find their narrow niche to communicate,” he says. “Their song had to stand out to mates, so they specialized, but they also had to avoid predators that were homing in via sound.”
The researchers determined very different divergences within two orthopteran suborders—Caelifera and Ensifera, Song says.
Within Ensifera, which includes crickets, katydids, mole crickets, and grigs, sound-producing mechanisms and ears likely co-evolved from about 300 million years ago, Song says.
Within Caelifera, which includes grasshoppers, the time tree shows most ancient grasshoppers evolved hearing mechanisms around 60 million years ago, prior to the evolution of sound production. Their hearing mechanism—the abdominal tympana—led to a great divergence in Orthoptera species.
“They may have developed hearing to avoid predators or to modulate their flight,” he says. “But the ability to hear opened up the possibility to communicate. Those evolutionary changes led to many divergences over millions of years.”
Song has recently received another round of funding from National Science Foundation to use more species and more sophisticated technology. His team will next increase the number of species sequenced to 1,600 and apply biomechanics and biophysics to investigate how the functions of stridulatory wings and tympanal ears have evolved.
Despite many studies, the evolution of acoustic communication is still an open field to scientists, Song says. The research has major implications for insect species’ sustainability in the changing environment and acoustic communication throughout the animal kingdom, including humans.
“Insects, like many species, are adapted to their environment,” he says. “Urbanization is changing the soundscape these insects communicate in. If we can understand these mechanisms and how Orthoptera make sound and hear, we will know more about how we hear and maybe develop novel technology that improves the acoustic environment and acoustic communication for all communicating life, from Orthoptera species to humans.”