Caterpillars are often perceived as quiet creatures, known primarily for diligently munching on leaves. This common perception suggests a silent existence. However, a closer look reveals a surprising aspect of their biology: certain species produce sounds. This uncovers a fascinating realm of acoustic communication and defense mechanisms.
The Surprising Sounds of Caterpillars
Contrary to popular belief, certain caterpillar species are not silent and produce a variety of sounds. These sounds are often subtle and not easily detectable by the human ear without focused attention. They range from distinct clicks and chirps to more complex hisses, whistles, and scraping noises. While not all species exhibit this behavior, it is documented in specific groups.
For example, some silkmoth and hawkmoth caterpillars produce clicks and whistles. The Nessus sphinx hawkmoth caterpillar, when disturbed, emits a sound described as a “scream.” Swallowtail caterpillars also produce clicking noises.
How Caterpillars Produce Sound
Caterpillars employ diverse biological mechanisms to generate sounds, often utilizing structures associated with feeding or respiration. One method involves forced airflow, where air is pushed through specialized internal passages. The Nessus sphinx hawkmoth caterpillar, for instance, “vocalizes” by forcing air into and out of its gut, creating sound from the oral cavity. This process is analogous to a whistling kettle, where air flowing between two foregut chambers, the crop and esophagus, amplifies certain frequencies through a Helmholtz resonator effect.
Another mechanism is stridulation, which involves rubbing body parts together to create friction. Some caterpillars use their mandibles, or jaw-like mouthparts, to produce clicks or scraping sounds against surfaces like leaves. The common hook-tip moth caterpillar also uses its mandibles for drumming and scraping, and can even use specialized anal “oars” to generate sound. Additionally, species like the Walnut sphinx caterpillar and the squeaking silkmoth (Rhodinia fugax) produce sounds by expelling air through their spiracles, small openings on their bodies used for breathing.
Why Caterpillars Make Noise
The sounds produced by caterpillars serve various adaptive functions, primarily centered on survival and interaction with their environment. A prominent reason for sound production is predator deterrence. Many caterpillar sounds act as a warning or startling mechanism against potential threats, especially birds. For example, the Nessus sphinx hawkmoth caterpillar’s “scream” is a defensive response when attacked.
These sounds can also function as aposematic signals, indicating to predators that the caterpillar may be unpalatable or toxic. Some caterpillar whistles might even mimic alarm calls of other animals, startling predators. Beyond defense, some caterpillars use acoustic signals for communication within their own species. The common hook-tip moth caterpillar, for instance, uses vibrational signals to defend its silk nest sites from other caterpillars, engaging in acoustic “duels” over territory. Western tent caterpillars respond to specific sound frequencies similar to those made by parasitic flies, indicating a defense mechanism to avoid parasitism.
Hearing the Unheard: Observing Caterpillar Sounds
Observing caterpillar sounds can be challenging due to their subtle nature and small size. The sounds are frequently low in amplitude and may fall outside the typical range of human hearing. Consequently, specialized equipment is often necessary for clear detection and study. Scientists commonly use sensitive microphones, sometimes paired with bat detectors that convert high-frequency sounds into an audible range, to record and analyze these acoustic signals.
While direct observation of faint sounds is possible in extremely quiet environments and with close proximity, sophisticated tools provide the detailed data required for research. Ongoing studies in acoustic entomology continue to uncover more about the diverse soundscapes of these creatures.