How Does an Insect Make That Noise With No Voice? Rub Two Rough Parts Together

The arrangement has two parts. One is a file, a row of closely spaced ridges on a hard surface, and the other is a scraper, a stiff edge drawn across it. Each ridge the scraper passes produces a click, and because the ridges are spaced regularly and the movement is fast, those clicks merge into a continuous tone whose pitch is set by how many ridges pass per second. On its own that sound would be very quiet, so a resonating structure is essential, and in crickets the wing surrounding the file acts as it, vibrating at a particular frequency and radiating the sound efficiently.

The same principle is implemented with whatever anatomy is available:

• •Crickets and bush crickets draw one wing across the other
• •Grasshoppers draw a ridged hind leg across a wing vein
• •Many beetles rub the head or thorax against the abdomen
• •Some ants rub abdominal segments together
• •Spiders, millipedes and scorpions all have their own versions
• •Some fish rub bones or teeth together inside the body

The uses divide clearly and the songs are usually distinguishable. Attracting a mate is the commonest, and those songs are long, loud and species-specific, which prevents wasted effort between species sharing a field and lets researchers identify species by sound alone. Courtship songs are quieter and produced only when a female is close. Rivalry songs are produced between competing males. Alarm sounds are produced when an animal is seized, and in several beetles and ants have been shown to startle a predator or to summon nest mates. Some larvae use the sound to communicate with attending ants that protect them.

Producing a tone is only half the problem, since a small animal is a poor radiator of sound and most of the energy fails to leave it. Several species solve that with structures that would look excessive if the purpose were not understood. Mole crickets dig a burrow shaped as a double horn, positioning themselves at the throat so the burrow acts as a megaphone matched to their song frequency, and the resulting sound carries for hundreds of metres. Tree crickets cut a hole in a leaf and sing through it, using the leaf as a baffle that stops the sound cancelling itself. Some bush crickets have inflated wing cavities acting as resonators.

The rate at which the scraper is drawn across the file depends on muscle activity, and insect muscle works faster when warm, so the pitch and the repetition rate of the song both rise with temperature. That relationship is consistent enough to be used in reverse. Counting the chirps of a snowy tree cricket over a period and applying a simple formula gives the air temperature to within a degree or so, a relationship published by Amos Dolbear in 1897 and still quoted. It also means a female must account for temperature when judging a male's song, since a warm male sounds more vigorous than a cool one of the same quality.

Drawing a stiff scraper across a row of regular ridges produces a click per ridge, and at speed those merge into a tone whose pitch depends on how fast the file is crossed, with a resonating wing or body panel making it audible. Wings, legs, abdominal segments and even bones are used across different groups. The rate rises with temperature reliably enough that cricket chirps give the air temperature.