Beetles, a group containing nearly 400,000 described species, are often perceived as silent, yet many species engage in acoustic communication. This communication, which often occurs through vibrations traveling through the ground or wood, serves as a fundamental method for coordinating social interactions. Sound allows these insects to convey information over short distances when visual or chemical signals are ineffective, especially in dark or dense habitats. This sonic biology reveals a complex language governing daily life, from finding mates to warning off predators.
The Mechanics of Beetle Sound Production
The majority of beetle species that produce sound rely on a mechanism known as stridulation, which involves rubbing two specialized body parts together to create friction. This process is analogous to a musician drawing a bow across a violin string, utilizing a “file” structure containing ridges and a “scraper” structure that moves across it. The location of these stridulatory organs varies significantly across different families, leading to a diverse range of sounds. For instance, many longhorn beetles produce a sound by moving their head up and down, causing a scraper on the back of the head to rub against a file on the pronotum, the plate covering the thorax.
Other species utilize different body junctions, such as the elytro-tergal mechanism, where the underside of the hardened forewings (elytra) rubs against the surface of the abdomen. Sound characteristics, including pulse duration and frequency, are determined by the spacing and size of the ridges on the file. These sounds are often broadband and pulsatile, meaning they contain a wide range of frequencies and are emitted in short bursts.
A completely different method of sound production is percussion, or drumming, where the beetle deliberately strikes a hard substrate with a part of its body. The deathwatch beetle, for example, makes a ticking noise by repeatedly tapping its head against the walls of its wooden tunnel. This transmits vibrations through the wood, traveling further than airborne sound and signaling a nearby recipient. Some beetles also produce a distinct hissing or squeaking sound by forcing air or fluid from their bodies. While less common than stridulation, this method is primarily used as a sudden, loud defensive display to startle potential threats.
Decoding the Messages
Beetle sounds serve distinct, context-specific functions, translating mechanical noise into meaningful behavioral communication. A common purpose is courtship and mate attraction, where species-specific acoustic signals ensure individuals find and recognize appropriate partners. Male deathwatch beetles initiate a tapping pattern of four to eleven strikes per bout, and a receptive female responds with her own tapping sequence, allowing the male to locate her within the timber. Similarly, bark beetles use specific “love songs” or chirps that convey mating intentions and help in species recognition, particularly over the short distances within a tree tunnel.
Sound also functions as a signal of alarm or defense, often employed when a beetle is disturbed or attacked. The sudden squeaking noise produced by certain beetles when handled acts as a startle display to momentarily deter a predator. This acoustic defense may also serve as an aposematic warning, indicating that the insect is unpalatable or chemically protected, reinforcing the message of danger.
Acoustic signals are also employed for territorial defense and group coordination, especially in species that live in close proximity. In bark beetles, vibratory signals traveling through the wood substrate help to space out individuals, preventing overcrowding within the limited resources of a host tree. Parental burying beetles use stridulation during brood care, and research suggests this acoustic communication is necessary for the successful coordination of feeding and tending to the larvae.
Notable Sound-Producing Beetles
The Deathwatch Beetle (Xestobium rufovillosum) provides a classic example of percussion used for reproductive success. This wood-boring beetle is famous for the ticking or tapping sound it creates, which was historically associated with omens of impending death in old houses. Adult beetles emerge in the spring and use this head-tapping behavior against wood surfaces as a crucial component of their mating ritual. The rhythmic signal is a vibrational courtship call that allows males and females to pinpoint each other’s location inside wooden structures.
The Bess Beetle (Odontotaenius disjunctus), also known as the patent leather beetle, exhibits an elaborate acoustic repertoire. These social insects live in decaying logs and have been documented to produce up to fourteen different acoustic signals through stridulation. Adults create their squeaking sounds by rubbing the upper surface of the abdomen against their folded hind wings, while the larvae produce sound by rubbing their modified legs together. This complex communication system is fundamental to their subsocial life, allowing adults to coordinate parental care and enabling the larvae to signal their needs, such as hunger.
The Hercules Beetle (Dynastes hercules), one of the world’s longest beetle species, uses sound primarily for defense. When threatened, the adult male will produce a loud “huffing” sound through stridulation. This noise is generated by rubbing the abdomen against the underside of the elytra, serving as an abrupt auditory warning to potential predators. The sheer size of the insect, combined with the sudden, startling sound, makes this acoustic display an effective defense mechanism in its tropical rainforest habitat.