Page 30 - Summer2017

Insect Bioacoustics
case for many insects that thus experience very small IIDs. Despite these challenges, parasitoid flies (Mason et al., 2001; Figure 1C) and crickets (Schönich and Hedwig, 2010) can discriminate between sound sources positioned as little as 1° to the left or right of the midline, an acuity similar to that of humans.
In this article, I present a few examples in which clever acoustical engineering allows insects to overcome some of the constraints imposed by their small size. Note that by “en- gineering,” I mean adaptation effected through natural selec- tion, not willful design. First, though, I briefly describe the roles that sound plays in insect lives. Readers should consult Gerhardt and Huber (2002), Balakrishnan (2016), and Pol- lack (2016) for more thorough reviews of how insects use sound signals.
Why Insects Listen
Predator Detection and Avoidance
loud acoustic signals, known as calling songs, that adver- tise their species identity, their location, and, in some cases, their “quality” as prospective mates. In many cases, females respond by walking or flying toward the sound source (see video at http://acousticstoday.org/ptaxis) of a Texas field cricket [Gryllus texensis] walking on a spherical treadmill toward a loudspeaker situated approximately 45° to her left). In other cases, females respond with their own songs that males then use as an acoustic guide to approach the female.
Host Localization
A third function of hearing in insects, host detection and localization, has so far been described only for parasitoid flies (Figure 1C), where it has evolved independently at least twice (Lakes-Harlan et al., 1999). The flies deposit larvae on or near their singing host (crickets, katydids, or cicadas, de- pending on the species of fly) that, like the singer’s intended audience, they locate by homing in on its song. The larvae then burrow into the host and consume it from the inside.
Being Small Yet Loud
As mentioned above, it is often advantageous for an adver- tising male’s signal to reach the largest possible number of receivers. Insects have evolved a number of mechanisms that boost their acoustic output.
Resonance
The loudest insect sounds are produced in one of two ways: stridulation, which involves rubbing of one body part against another, or tymbalation, which is a snapping or buckling of specialized regions of the exoskeleton known as tym- bals. Crickets, katydids, and grasshoppers are stridulators, whereas cicadas are tymbalators. The radiation efficiency of sounds produced by both mechanisms is often enhanced through resonance (Bennet-Clark, 1999).
Stridulating crickets rub a plectrum, a hardened region on the edge of one front wing, against a row of “teeth,” hardened ridges on the underside of the opposite wing, in a manner similar to stroking the teeth of a comb with one’s thumbnail (Figure 2).
Each tooth strike produces a brief click that excites a reso- nance determined by the size, shape, and material properties of the wings. Sound is radiated mainly from a region of the wing called the harp, which resonates at a frequency close to that of the cricket’s song, which is typically 3-5 kHz depend- ing on species (Montealegre-Z et al., 2011). The input of en- ergy to the system through successive tooth strikes is coor-
Hearing has evolved independently in insects at least 24 times (Greenfield, 2016). Phylogenetic analysis shows that most insect ears evolved at around the time that echolo- cating bats appeared in the fossil record, about 65 million years ago, suggesting that hearing in these cases evolved in response to the selection pressure exerted by these preda- tors. Hunting bats emit ultrasonic calls and detect their prey from the echoes that are returned from their bodies (Fenton et al., 2016). Moths, crickets, katydids, locusts, beetles, man- tises, lacewings, flies, and perhaps others respond to bat-like ultrasound with behaviors that reduce the probability of capture, such as flying away from the sound source, diving into vegetation or, in some cases, jamming the bat’s echolo- cation system with their own ultrasound emissions. In many cases, antibat defense was the primitive (and still dominant) function of hearing, whereas in others, such as crickets and katydids, intraspecific communication, often using low-fre- quency sounds (<10 kHz), predated the evolution of echolo- cating bats by more than 100 million years. Extant insects in these groups hear bat-like frequencies in addition to those used for communication and respond to them defensively, suggesting that bat detection and avoidance evolved as “add- ons” to an already functioning auditory system.
Reproduction
The insect sounds most familiar to humans are produced by males to attract sexually receptive females. Crickets, ci- cadas, katydids (Figure 1D), and grasshoppers produce
28 | Acoustics Today | Summer 2017