Why Do Crickets Chirp? And Why Kauai's Fell Silent

In 2001, the evolutionary biologist Marlene Zuk and her colleagues worked their Kauai field sites listening for Pacific field crickets, a species they had been recording on the island since 1991. Across intensive searches they heard a single calling male. Two years later the crickets were abundant again, and none of them were singing.

Working out why that is strange starts with what a chirp actually is, and most people have that part wrong.

Why do crickets chirp?

Adult male crickets chirp to broadcast a calling song that draws females in from a distance. The sound is stridulation: a male rubs his forewings together, raking a hardened scraper on one wing across a toothed file on the other. Female crickets do not chirp, because their wings carry no file and no scraper.

The leg-rubbing version belongs to grasshoppers

The picture most people carry is a cricket sawing its back legs together. Crickets do not rub their back legs together. They rake one wing against the other, and the leg version describes a different insect.

The Amateur Entomologists’ Society puts the two mechanisms side by side: a grasshopper has “a series of small pegs on the inside of their hind legs” that are “rubbed against the fore wing to produce the call”, while a cricket keeps both parts on its forewings. Rich Zack, an entomologist at Washington State University, describes the cricket version as a file and scraper - bumps like comb teeth on one wing, the hardened edge of the other wing dragged across them. Zack also confirms the sex rule: “Only adult male crickets make sound.”

Tidy explanations that nobody checked are the normal state of general knowledge, which is worth remembering when you ask why Paris ended up with catacombs as well.

She is listening through her knees

The audience is equipped to match. Cricket ears sit on the tibia of the front legs, so a cricket hears the world through its knees. Schneider and colleagues, writing in Scientific Reports in 2017, describe each ear as two oval translucent membranes, one at the front of the leg and one at the back, fed by a spiracle on each side of the thorax and an internal sound-guiding tracheal system that makes “a system of internally-coupled ears that are important for sound localization”. In one of the species they measured, the front membrane ran as thin as 0.35 micrometres, well below the 2 to 5 micrometres known from other crickets.

That arrangement is what lets a female locate a singing male across a dark field. It comes with a matched transmitter: the same paper puts most cricket calling songs between 2 and 8 kHz, and the Singing Insects of North America catalogue, founded by Thomas J. Walker, puts a number on each species.

Species	Dominant frequency of the calling song
Fall field cricket (Gryllus pennsylvanicus)	4.7 kHz
House cricket (Acheta domesticus)	4.8 kHz
Snowy tree cricket (Oecanthus fultoni)	2.9 kHz

The same male keeps a second song in reserve. SINA’s description of a fall field cricket courting a female has soft shuffling sounds, two-pulse chirps, and brief periodic ticks strongest around 12 kHz, well clear of the 4.7 kHz of his calling song. One song is the advertisement, the other is what he plays once she is within close range.

The chirp thermometer is real, for one cricket

In 1897 Amos Dolbear, a professor of physics and astronomy at Tufts College, published a two-page note in The American Naturalist titled “The Cricket as a Thermometer” (volume 31, pages 970 to 971), setting out a formula for reading air temperature off the chirp rate. Nowhere in those two pages does he name the cricket he had been listening to.

The version that holds up belongs to the snowy tree cricket, nicknamed the thermometer cricket. SINA’s species page gives the working rule: in the eastern United States, count chirps for 13 seconds and add 40 to get degrees Fahrenheit; west of the Great Plains, count for 12.5 seconds and add 38. The same page adds the caveat that gets stripped off everywhere else: “if you are really interested in using the snowy tree cricket as a thermometer, you should calibrate your local crickets against a thermometer placed near where they sing.”

So the arithmetic is species-specific. The rasping field cricket under a porch step is not the insect the formula was written for. Rules of thumb drift a long way from the thing they originally described, which is roughly what happened to the story that the clocks get shifted for farmers. A mood ring needs the same calibration: its green can mean about 23 C on one liquid-crystal mixture and between 35 C and 36 C on another.

Everything that can hear the song is listening

A calling song has to carry. That makes it useful to more than the intended recipient, and on Kauai the extra listener was Ormia ochracea, a small parasitoid fly. Zuk, Rotenberry and Tinghitella, reporting in Biology Letters in 2006, note that the fly is North American in origin, uses song to find its hosts, and deposits larvae that “burrow into the cricket and develop inside, killing the host upon emergence”. Close to 30% of calling males on Kauai were carrying it.

Kauai switched the song off in 12 to 20 generations

Somewhere between the late 1990s and 2003, a mutation swept the island’s population. In these “flatwing” males the file “is reduced in size and relocated at an angle precluding sound production”, and the paper’s verdict is blunt: “Flatwings are thus unable to call.” Zuk and her co-authors put the sweep at 12 to 20 generations, assuming four generations a year.

Of 133 male crickets collected on Kauai in 2004, twelve still had wings that could make a sound. An island population had stopped transmitting the signal its entire mating system was built on.

Silent males still reproduce. They home in on other crickets’ songs, stopping at a mean distance of 75.5 cm from a speaker playing a calling song, then wait beside the few remaining singers and intercept the females who come. Sonia Pascoal and colleagues, reporting in Current Biology in 2014, found that the Kauai and Oahu flatwings show “different genomic architectures consistent with the timing of two mutational events on the X chromosome”. The silent morph turned up on Oahu two years after Kauai, the same silence invented twice.

Predators and parasites shape an animal’s behaviour more forcefully than its own preferences do, which is the uncomfortable arithmetic behind asking whether any insect is expendable.

Step outside tonight and the noise in the grass is males dragging a wing across a comb, a fall field cricket near 4.7 kHz, aimed at females listening through their front knees. On one Hawaiian island the identical broadcast worked as a delivery address for a fly, and within twenty generations the population had gone off the air. A calling song is a location as much as an invitation. Strictly FYI.