Page 28 - Summer2017

Gerald S. Pollack
Postal:
400 Walmer Road, #805 Toronto, Ontario M5P 2X7 Canada
Email:
gerald.pollack@mcgill.ca
Insect Bioacoustics
Despite being small, acoustical specializations allow insects to produce, detect, and localize sound for communication, predator detection, and host localization.
Introduction
Hearing and acoustic communication are widespread among vertebrate animals, but insects are the only invertebrate group in which sound production and hear- ing are widespread (sound is taken here to mean airborne sound; the sensing of substrate vibration is essentially ubiquitous among terrestrial invertebrates, and although some aquatic invertebrates produce or detect sound, they will not be considered here). Insects listen to, detect, and locate sound-producing predators, hosts, mates, and rivals, and they emit sound to attract, repel, or threaten members of their own species and to startle and evade predators.
Insects are small (Figure 1), and this constrains their use of acoustics (Michelsen, 1992; Bennet-Clark, 1998). For example, when an insect sings to attract a mate, it is advantageous for his song (and in most cases, it is the males that sing) to be heard over long distances, thereby maximizing the chance of its reaching a recep- tive partner. Sound production requires the transfer of energy from moving body parts to the air and is, fundamentally, powered by muscle contractions. Even large insects weigh only a few grams, limiting the available muscle power and resulting sound amplitude. Moreover, because of impedance-mismatch penalties, the effi- cient transfer of energy from insect to air is possible only if the wavelength of the sound is not too large relative to the size of the sound-radiating structure. Accord- ingly, most insect sounds have wavelengths of a few centimeters or less. But short wavelength sounds are readily reflected and/or absorbed by objects such as leaves and twigs, further limiting their effective range.
Hearing requires the reverse transfer of energy, from the environment to the re- ceiver. The acoustical force exerted on the receiver, which is then available for transmission to and excitation of auditory nerve cells, is proportional to its surface area. The ability of small insect ears to detect very faint sounds is thus limited. In- deed, minimum auditory thresholds of insects are typically in the range of 30- to 50-dB sound pressure level (SPL), which is well above the minimum threshold of humans.
One way in which some insects rival other animals is in their ability to localize sound. Close to a sound source, air particles oscillate in the direction of wave prop- agation; thus particle velocity carries information about sound-source direction. Some insect ears, such as the antennae of mosquitoes or fruit flies, are sensitive to particle velocity and can extract information about the direction of the sound source directly. Other insects use eardrums to detect the sound pressure compo- nent, which is dominant at distances greater than a wavelength from the source. Sound pressure reflects local variation in the density of air and in itself carries no directional information; rather, that must be derived from the direction of sound propagation.
26 | Acoustics Today | Summer 2017 | volume 13, issue 2 ©2017 Acoustical Society of America. All rights reserved.