Nontympanal Hearing Organs

Until now we have focused on tympanal ears, which are sensitive to traveling waves of changing pressure in air and water, known as the acoustic far field. In the broadest sense of the word, however, hearing encompasses the detection of near-field sounds, as well as vibrations traveling through solid substrates. By and large, the near field can be thought of as a short distance, a few body lengths, from the sound source. Substrate vibrational signals have also been described as "seismic communication." In this larger sense, then, we could argue that most insects can hear. We highlight a few recent developments in the study of these alternative, but no doubt widespread, forms of hearing.

Detecting Near-Field Sounds

When a sound is produced, air particles are being pushed back and forth near the source of disturbance. These particle movements, or near-field sounds, are generally of low frequency (typically below 1 kHz) and, unlike pressure waves, do not travel far from the sound source, in many instances, just a few body lengths in distance. A small, light, and ponderable object occurring within the near-field sound will move in response to the vibrating air molecules. In insects, loosely attached setae or antennae are commonly used for detecting particle velocity. Depending on the structure of the receptor organ, and its position relative to the sound source, near-field receptors can offer information about the direction and the intensity of a sound source.

Some caterpillars can detect the near-field sounds produced by the beating wings of a flying wasp up to distances of 70 cm. Specialized hairs on the dorsal thorax of the caterpillar are displaced in the sound's near field, eliciting an evasive response, such as freezing or dropping from a leaf. Similar particle-displacement-sensitive setae on the cerci of crickets and cockroaches function in predator avoidance and possibly for close-range conspecific communication.

The antennae of many insects also function as near-field sound detectors. In many Diptera (mosquitos, chironomids, and fruit flies), for example, the males are attracted to the near-field "buzzing sounds" of females (Fig. 6). The long flagella of the antennae in male mosquitoes resonate to the tune of flying females, and these antennal movements in turn stimulate many thousands of scolopidia in the Johnston's organ, a chordotonal organ located at the base of the

FIGURE 6 The near-field sound hearing organs of a mosquito. (A) Micrograph of the head of a male Toxorhynchites brevipalpis, showing the plumous flagella of the antennae (blue arrows), which resonate in response to the near-field sounds of flying females. (B) A cross section through the antennal base, showing the location of the Johnston's organ (green arrows), where the auditory scolopidia are located. (A is courtesy of D. Huber. B was adapted, with permission, from M. C. Gopfert and D. Robert, 2001, Active auditory mechanics in mosquitoes, Proc. R. Soc. London B 268, 333—339.)

FIGURE 6 The near-field sound hearing organs of a mosquito. (A) Micrograph of the head of a male Toxorhynchites brevipalpis, showing the plumous flagella of the antennae (blue arrows), which resonate in response to the near-field sounds of flying females. (B) A cross section through the antennal base, showing the location of the Johnston's organ (green arrows), where the auditory scolopidia are located. (A is courtesy of D. Huber. B was adapted, with permission, from M. C. Gopfert and D. Robert, 2001, Active auditory mechanics in mosquitoes, Proc. R. Soc. London B 268, 333—339.)

antennae. The Johnston's organ may also be involved in the detection of near-field sounds produced by honey bee waggle dances. It is also the sensory organ by which the well-known Drosophila melanogaster detects the patterned wingbeats that make up the mating songs of these species.

Detecting Substrate-Borne Vibrations

The detection of vibrations traveling through solid substrates may be one of the most ubiquitous but least appreciated forms of acoustic communication in insects. With recent technological advances in the detection of substrate-borne signals (e.g., laser vibrometry and piezoelectric sensors), we are learning that a large number of insects can detect vibrations produced by both intentional and unintentional senders. In fact, most insect orders probably include species capable of detecting vibrations. At present, however, there are few complete studies on this subject. We provide two recent examples out of many possibilities.

Membracid treehoppers (Homoptera: Membracidae) transmit alarm calls through tree stems to communicate with con-specifics. Nymphs living in colonies of up to 100 individuals on the stem of a host plant produce coordinated waves of vibrations when threatened by a predator. The mother treehopper detects these alarm calls and rushes to defend her offspring, by kicking her hind legs and fanning her wings at the intruder (Fig. 7A). For many species of caterpillars that live in social or crowded conditions, vibrational signaling may be a principal means of communication. The common North American masked-birch caterpillar (Drepana arcuata) engages in acoustic "battles" with invading conspecifics. The nest owner drums and scrapes its mandibles and scrapes modified "oars" against the leaf in ritualized acoustic displays. Acoustic "duels" between residents and intruders can last

FIGURE 7 Acoustic communication through substrate-borne vibrations. (A) A group of treehopper nymphs (Umbonia crassicornis) use coordinated substrate-borne vibrations to warn their mother of an approaching predatory wasp. The mother detects the signals and rushes to the defense of her offspring. The waveform represents three group signals from an aggregation of nymphs. Scale bar, 560 ms. (Reproduced, with permission, from R. B. Cocroft, 2002, Antipredator defense as a limited resource: Unequal predation risk in broods of an insect with maternal care, Behav. Ecol. 13(1), 125—133. Waveform courtesy of R. B. Cocroft.) (B) Two masked birch caterpillars (Drepana arcuata) engaged in an acoustic "duel" over a silken nest on a birch leaf. The waveform depicts the three signal types (green, anal scrapes; blue, mandible drumming; orange, mandible scraping) used by the caterpillars. (Reproduced, with permission, from J. E. Yack et al, 2001, Caterpillar talk: Acoustically mediated territoriality in larval Lepidoptera, Proc. Natl. Acad. Sci. USA 98(20), 11371-11375.)

FIGURE 7 Acoustic communication through substrate-borne vibrations. (A) A group of treehopper nymphs (Umbonia crassicornis) use coordinated substrate-borne vibrations to warn their mother of an approaching predatory wasp. The mother detects the signals and rushes to the defense of her offspring. The waveform represents three group signals from an aggregation of nymphs. Scale bar, 560 ms. (Reproduced, with permission, from R. B. Cocroft, 2002, Antipredator defense as a limited resource: Unequal predation risk in broods of an insect with maternal care, Behav. Ecol. 13(1), 125—133. Waveform courtesy of R. B. Cocroft.) (B) Two masked birch caterpillars (Drepana arcuata) engaged in an acoustic "duel" over a silken nest on a birch leaf. The waveform depicts the three signal types (green, anal scrapes; blue, mandible drumming; orange, mandible scraping) used by the caterpillars. (Reproduced, with permission, from J. E. Yack et al, 2001, Caterpillar talk: Acoustically mediated territoriality in larval Lepidoptera, Proc. Natl. Acad. Sci. USA 98(20), 11371-11375.)

from a few minutes to a few hours (Fig. 7B). At present, we know little about how insects detect substrate vibrations. The subgenual organ (a chordotonal just "below the knee" in many insects) functions as a vibration receptor in some groups (like some crickets and termites), but for most insects, the receptor organs are yet to be identified. Clearly further research is required before we gain a full appreciation of this important form of communication in insects.

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