Stepping outside on a late summer or early autumn evening is often accompanied by a loud chorus of insect noises. These sounds, which range from a steady rhythm to a high-pitched rasp, are carefully orchestrated biological communication. The insects producing this nocturnal symphony are primarily males, and their loud calls serve a singular purpose in the life cycle of their species.
Identifying the Nocturnal Noisemakers
The majority of rhythmic, persistent sounds heard close to the ground or in low shrubs come from crickets. These insects produce a musical, low-frequency chirp with a steady, metronome-like pattern. While crickets may chirp throughout the day, their activity and volume become most noticeable after sunset.
A distinctly different, often louder noise originates from katydids, which are typically found higher up in trees and bushes. Their sound is characterized by a high-pitched, buzzy, or raspy series of clicks that can sound like a sharp “katy-did” or a rapid “tic-tic-tic” phrase. Katydids are among the loudest purely nocturnal insects.
Though often mistaken for nocturnal singers, cicadas are generally not the source of the late-night chorus. These insects produce a continuous, intense drone or whine that can reach over 100 decibels. They call almost exclusively during the heat of the day and into the early evening dusk. By true nightfall, the continuous buzzing of the cicada typically quiets down, leaving the crickets and katydids to dominate the darkness.
The Science Behind Insect Sound Production
Crickets and katydids create their sounds through a process known as stridulation. This mechanism involves rubbing two specialized body parts together, much like drawing a bow across a violin string. The sound is produced when a structure called the “scraper,” a hardened edge on one forewing, is rapidly swept across a “file,” a vein on the opposite forewing containing a row of tiny teeth.
This friction causes the forewings to vibrate, and the entire wing structure acts as an acoustic amplifier to broadcast the sound. The speed of the movement and the number of teeth struck per second determine the frequency and pulse rate of the chirp, which varies dramatically between species.
Cicadas, by contrast, generate their loud calls using a different structure entirely. They employ a pair of ribbed membranes called tymbals located on the sides of their abdomen. Muscles buckle these membranes inward and outward, creating a rapid series of clicks. These clicks resonate through hollow air sacs in the abdomen to produce their characteristic, continuous buzz.
Why Insects Sing at Night
The intense sound production is almost entirely a reproductive strategy. The calls function as species-specific mating signals, performed by male insects as a “calling song” to attract a receptive female of the same species. The female uses specialized hearing organs, located on her forelegs in crickets and katydids, to locate the source of the song.
The choice of night for this activity is an adaptation that balances the need for communication with the risk of predation. Calling in the dark helps these insects avoid many visual predators, such as insectivorous birds that hunt during the day.
The cooler nighttime temperatures allow the calls to carry farther, and the absence of daytime environmental noise reduces acoustic competition. This nocturnal timing maximizes the male’s chance of attracting a mate while minimizing his exposure to detection.