The nightly chorus of chirping is a familiar sound that characterizes warm evenings. Far from being random noise, this consistent acoustic background is a highly organized form of communication. This symphony is a complex biological phenomenon, serving as an auditory window into the nocturnal lives of the creatures that produce it. Exploring the source and meaning of these sounds reveals a fascinating world of coded signals.
Identifying the Nocturnal Choristers
The majority of the chirping heard after dark originates from insects in the order Orthoptera, primarily crickets and katydids. Crickets typically produce a short, rhythmic, and steady sound, often heard from low-lying grass or the ground. They are responsible for the classic, pulsing soundtrack of summer evenings.
Katydids, which spend their time in trees and shrubs, contribute a different texture to the night chorus. Their call is often described as a longer, raspy buzz or a series of irregular clicks, sometimes said to sound like their name, “Katy-did.” While insects are the most common source, some amphibians, such as frogs and toads, also emit high-pitched, chirp-like mating calls near bodies of water. Distinguishing the various calls requires listening for the specific rhythm and pitch.
The Mechanism of Chirping
The production of sound in crickets and katydids is a mechanical process known as stridulation. This method involves the rubbing of specialized body parts against each other, similar to playing a violin. Only the male insects possess the necessary structures to create these acoustic signals.
To chirp, the male lifts its forewings and pulls a hardened, scraper-like edge on one wing across a file of tiny ridges on the underside of the other wing. This action causes the thin portions of the wings to vibrate, which amplifies the sound. The number of teeth struck per second determines the frequency of the chirp, which can range from 1,500 to nearly 10,000 cycles per second depending on the species. This mechanism contrasts with amphibians, which use air sacs and vocal cords to push air and create their calls.
The Purpose of Nighttime Songs
The primary function of the male’s nightly song is reproduction, specifically to attract a mate. Male crickets produce a distinct “calling song” designed to travel long distances and signal their location to receptive females. Females possess ear-like organs on their forelegs that are highly sensitive to these frequencies, allowing them to locate a partner.
Each species has a unique rhythmic pattern, which acts as species-specific identification to prevent cross-mating. Beyond attracting females, chirping serves as communication between males. A louder “rivalry song” may be used to warn off competing males who encroach on a territory.
A change in the acoustic signal to a “courtship song” occurs when a female is close, serving to induce copulation. Chirping at night offers a strategic advantage because the environment is quieter and safer from many daytime predators, allowing the acoustic signals to travel farther and more effectively.
Environmental Influences on the Symphony
The intensity and rate of chirping are controlled by external environmental factors, most notably temperature. Crickets and katydids are ectothermic, meaning their body temperature and activity level are directly influenced by the surrounding air temperature. As the temperature rises, the chemical reactions powering their muscle contractions speed up, enabling the insects to rub their wings together more rapidly.
Consequently, the rate of chirps per minute increases noticeably on warmer nights. This consistent relationship is so reliable that counting the number of cricket chirps within a specific timeframe can offer a close estimate of the ambient temperature. The chirping chorus becomes a near-constant presence during the peak warmth of late summer and early fall. Chirping will slow down significantly or stop completely when temperatures drop below a certain threshold, typically around 55 degrees Fahrenheit.