The chirping of crickets on a warm summer evening is one of the most evocative sounds of the natural world. But how do these small insects produce such a relatively loud and persistent call? The answer lies in a specialised mechanism called stridulation — a form of sound production that is far more sophisticated than it might first appear.
The Mechanism of Stridulation
Cricket sound is produced by rubbing specialised structures on the forewings (tegmina) together. Only male crickets sing; females are silent. The key structures involved are:
- The file (pars stridens): A row of tiny teeth on the underside of one forewing (typically the left in most species). This structure contains 50–300 evenly spaced ridges, similar to the teeth of a comb.
- The scraper (plectrum): A hardened ridge on the upper surface of the opposite forewing. As the cricket closes its wings, the scraper is drawn across the file, producing vibrations.
- The harp: A thin, membranous area of the forewing that acts as a resonator, amplifying the vibrations into audible sound. The harp vibrates at the same frequency as the tooth-strike rate, producing a clear, musical tone.
- The mirror: Another membrane near the harp that contributes to sound amplification.
How Loud Are Crickets?
- Field cricket: Up to 100 decibels at close range (comparable to a power tool)
- House cricket: Approximately 70–80 decibels
- Tree crickets: Softer, more musical; approximately 60–70 decibels
- For comparison: Normal conversation is approximately 60 decibels; a lawnmower is approximately 90 decibels
Types of Cricket Songs
Male crickets produce several distinct types of song, each serving a different purpose:
| Song Type | Purpose | Characteristics |
|---|---|---|
| Calling song | Attract distant females; establish territory | Loud, continuous, repetitive; produced at regular intervals |
| Courtship song | Stimulate a nearby female to mate | Quieter, more complex; produced when a female is close |
| Aggression song | Warn rival males; assert dominance | Short, sharp, aggressive bursts; produced during male encounters |
| Post-copulatory song | Encourage female to retain the sperm packet | Soft, brief; produced immediately after mating |
Cricket Songs and Temperature
One of the most fascinating aspects of cricket chirping is its relationship with temperature. Because crickets are ectothermic (cold-blooded), their muscle activity — and therefore their chirp rate — is directly influenced by ambient temperature. As temperature rises, crickets chirp faster; as it falls, they slow down.
This relationship is so predictable that it has been formalised as Dolbear's Law, first published by physicist Amos Dolbear in 1897:
Did you know? You can estimate the temperature in degrees Fahrenheit by counting the number of chirps a snowy tree cricket makes in 13 seconds and adding 40. This remarkably accurate relationship, known as Dolbear's Law, works because cricket muscle contractions are temperature-dependent, with each degree of warming producing a measurable increase in chirp rate.
How Do Crickets Hear?
Crickets detect sound using tympanal organs — thin membranes that function like eardrums. Remarkably, these ears are located not on the head but on the front legs, just below the knee joint (tibia). Each ear contains a tympanal membrane that vibrates in response to sound waves, transmitting signals to the nervous system.
Female crickets use these ears to locate calling males. They can discriminate between the songs of different species and assess male quality based on song characteristics such as loudness, frequency, and duration. Females preferentially approach males whose songs indicate larger body size and higher genetic quality.
UK Cricket Species
The UK has a limited but interesting cricket fauna:
- Dark bush-cricket (Pholidoptera griseoaptera): The most common; produces a brief, sharp chirp from hedgerows in late summer
- Roesel's bush-cricket (Roeseliana roeselii): Produces a continuous, high-pitched buzzing; expanding its range northward in the UK
- Great green bush-cricket (Tettigonia viridissima): One of the UK's largest insects; loud, sustained song audible over 50 metres
- Field cricket (Gryllus campestris): Extremely rare; confined to a few sites in southern England; subject of conservation programmes
- Speckled bush-cricket (Leptophyes punctatissima): Produces an ultrasonic call largely inaudible to humans
Stridulation vs. Other Insect Sounds
| Sound Method | Mechanism | Insects |
|---|---|---|
| Stridulation | Rubbing body parts together (file and scraper) | Crickets, grasshoppers, some beetles |
| Tymbals | Clicking of specialised abdominal membranes | Cicadas |
| Wing vibration | Rapid flight muscle contractions (buzz) | Bees, flies, mosquitoes |
| Substrate vibration | Drumming or vibrating plant surfaces | Planthoppers, some beetles |
Key Takeaway
Crickets produce sound through stridulation — rubbing a file on one forewing across a scraper on the other. Only males sing, producing distinct calling, courtship, and aggression songs. Chirp rate is temperature-dependent, described by Dolbear's Law. Females detect songs using tympanal ears located on their front legs and use song characteristics to select high-quality mates. The mechanism is a remarkable example of evolutionary engineering, producing clear, loud signals from tiny instruments.