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Review articles Neural analysis of sound frequency in insects Gerald S. Pollack* and Kazuo Imaizumi Summary Insects, like other hearing animals, must extract information from the sounds they hear so that they may respond appropriately. One parameter of sound that carries information is its frequency content. Insects analyze sound frequency to identify mates, to judge the distance to potential competitors, and to detect predators and prey. We review how frequency is analyzed in the insect nervous system, focusing on two taxa in which this problem has been studied most intensively, katydids and crickets. BioEssays 21:295–303, 1999. r 1999 John Wiley & Sons, Inc. Introduction An important function of nervous systems is sensory processing, that is, the extraction of behaviorally relevant information from the environment. Insects provide favorable model systems for studying sensory processing, for a number of reasons. First, they offer the usual advantage of invertebrate model systems: relatively few neurons, many of which are ‘‘identifiable.’’ An insect nervous system may contain a few hundred thousand neurons (and a single segmental ganglion, which is often the appropriate level of analysis, may have only a few thousand), whereas a mammalian brain contains billions of neurons. Moreover, many of the neurons of insects can be identified by their physiology and anatomy as unique cells that can be recognized and studied in specimen after specimen. Second, many behaviors of insects are tightly related to specific sensory inputs in a robust manner, allowing one to formulate clear hypotheses about the sorts of information processing that might be taking place. Third, insects are a highly diverse group in which sensory capabilities have evolved repeatedly; hearing, for example, has arisen independently at least 14 times.(1) This diversity provides a rich source of material for a comparative approach that can help demonstrate how neural circuits are shaped by natural selection. In this review we examine the neural processing of one important feature of auditory stimuli, sound frequency. Al- Department of Biology, McGill University, Montreal, Canada. Funding agency: NSERC; Funding agency: Whitehall Foundation. *Correspondence to: Gerald S. Pollack, Department of Biology, McGill University, 1205 Doctor Penfield Avenue, Montreal QC H3A B1, Canada. E-mail: [email protected] BioEssays 21:295–303, r 1999 John Wiley & Sons, Inc. though analysis of sound frequency is important in many insect groups, this problem, and in particular its relationship to behavior, has been studied most thoroughly in Ensifera, a suborder of Orthoptera that includes katydids (Tettigoniidae) and crickets (Gryllidae). These insects will form the focus of this review. Ensiferan auditory systems The ears of ensiferans are situated on the front pair of legs. (Insect ears occur in a variety of locations that seem strange from a vertebrate perspective, e.g., wings, mouths, abdomens(2).) Externally, the ear consists of a pair of tympanal membranes, which serve as the animal’s physical interface with its auditory world. Sound causes the thin membranes to vibrate, and these vibrations are passed on to a population of sense organs, called scolopoid sensilla, which are the basic units of hearing. Each scolopoid sensillum consists of a receptor neuron and several accessory cells, and the ear includes a few dozen sensilla arrayed along the length of the leg (Fig. 1A). A single receptor is most sensitive, or ‘‘tuned,’’ to a particular sound frequency (see Fig. 2B, for examples). Different receptors may be tuned to different sound frequencies, and there is a systematic relationship between the position of the receptor within the ear and the frequency to which it is most sensitive,(3–6) similar to the tonotopic organization of frequency sensitivity along the basilar membrane of the vertebrate cochlea.(7) What determines the frequency sensitivities of different receptors? In some insects, notably mole crickets (which are also ensiferans) and locusts, the underlying mechanism resides in the tympanal membrane itself.(8–10) The mechanical BioEssays 21.4 295 Review articles Figure 1. Ensiferan auditory systems. A: Auditory receptors are arrayed along the surface of a tracheal branch in the prothoracic tibia.(16) The ear of a katydid is illustrated, but cricket ears are similar in overall structure. The receptor neurons project to the prothoracic ganglion, where they interact with several interneurons, some of which are shown in B–E. B,C: Ascending neurons from a cricket(65) (B) and a katydid(31) (C). Both neurons are named ascending neuron 1 (AN1). D,E: Local interneurons of a cricket(65) (D) and a katydid(30) (E). Both neurons are designated omega neuron 1 (ON1). properties of the tympanal membrane and associated structures are such that different sound frequencies cause different regions of the membrane to vibrate most strongly. Auditory sensilla attach at different sites on the membrane, and thus receptor neurons acquire different frequency sensitivity. In crickets and katydids, however, the tympanum responds in a spatially uniform way with respect to sound frequency.(11–14) Moreover, the scolopoid sensilla do not attach directly to the tympanal membrane. Instead, sound energy is transmitted to them through internal membranes and air cavities,(15,16) the mechanical properties of which are unknown. There are two main views about how individual receptors become tuned, each with some supporting evidence. One is that the mechanics of the inner ear as a whole (i.e., the membranes, air cavities, etc.) respond in a spatially nonuniform way to sounds of different frequencies, so that, for example, one end of the structure would be most efficiently stimulated by high sound frequencies, and the other by lower frequencies.(17) This sort of mechanism is similar to what occurs in the inner ears of higher vertebrates, where the mechanics of the basilar membrane results in a spatial separation of sound frequencies along its length.(7) The alternative view is that the tuning of each receptor neuron is 296 BioEssays 21.4 determined by factors intrinsic to the scolopoid sensillum in which it resides, rather than by the structures that deliver sound energy to the sensillum.(18) Resolution of this issue will require detailed measurements of the movements of structures within the inner ear as they respond to sound; such measurements are not yet available. An extensive treatment of the biomechanics of insect hearing is beyond the scope of this brief review. However, several more thorough discussions of this topic are available.(8,19,20) Although many of the acoustical details of katydid and cricket ears are still somewhat murky, the structural similarities suggest that the ears are similar in overall operation. The similarities extend into the central nervous system, where the ‘‘same’’ identified auditory interneurons can be recognized in both taxa (Fig. 1B–E). It is unclear whether these similarities of cricket and katydid auditory systems represent an instance of homology(21) or of duplicate, but independent, evolution of similar auditory systems from the same exaptations.(22) Auditory behaviors of ensifera Foremost among the sounds that ensiferans listen to are the songs of their own species. Their songs are of course conspicuous to humans as well, the words ‘‘cricket’’ and ‘‘katydid’’ being onomatopoeic descriptions of the songs of particular species. Songs are used to attract mates, to induce copulation, and to declare and defend territories.(20,23) A second type of behaviorally relevant sound is produced by echolocating bats, which hunt insects by emitting ultrasonic cries and detecting the echoes that are reflected from the bodies of their prey. Many insects, including ensiferans, exhibit escape responses when stimulated with bat-like sounds, which they recognize in part by their ultrasonic (1) frequencies. The remainder of this review presents a few examples that illustrate how sound frequency affects behavior, and how it is analyzed in the nervous system. Frequency analysis in katydids One probable role of frequency analysis is estimation of distance to the sound source. As sound propagates through the environment, its spectrum changes. The attenuation of sound with distance increases with sound frequency, both because high frequencies are better blocked by small objects between the source and target, such as twigs and leaves, and because frictional loss of energy is greater for higher frequencies.(24,25) In the katydid species Mygalopsis marki, males situate their territories according to their acoustical judgments of the distances to other singing males,(26,27) and it is likely that they make this assessment by analyzing song spectrum. The song has a broad spectrum (ca. 8–30 kHz) close to its Review articles source, but the higher-frequency components become increasingly faint with distance. At a distance of 20 m, frequencies above 12 kHz are barely distinguishable from ambient noise, even though the lower frequency components of the song are still prominent.(28) Neural analysis of song spectrum depends both on the anatomical organization of the receptor neuron terminals in the central nervous system and on circuitry among interneurons. As in most sensory systems, the central target regions of receptor neurons are related to their positions at the periphery. Auditory receptors form a tonotopic ‘‘map’’ in the central nervous system, mirroring the systematic relationship in the ear between a receptor neuron’s location and its frequency sensitivity. The receptors terminate in a region of the prothoracic ganglion called the auditory neuropil, where their branches form a curved band that starts at the anterior edge of the neuropil and proceeds posteriorly along its ventral border, eventually wrapping dorsally around the posterior edge and continuing anteriorly. Receptors that respond best to low sound frequencies terminate at the anterior end of the band and receptors tuned to progressively higher frequencies terminate at progressively posterior (and eventually anterodorsal) positions(5,6,28–30) (Fig. 2A). This anatomical organization provides the framework within which receptors make synaptic connections with interneurons. Different interneurons sample different regions of the array of receptor terminals with their dendrites, and thus receive input from receptors tuned to different sound frequencies.(30) Auditory receptors differ not only in frequency tuning but also in absolute sensitivity. Minimum thresholds of receptors tuned to different frequencies can vary by as much as 40 dB (Fig. 2B). When these sensitivity differences are considered together with the distance-dependent spectrum of the song, it becomes apparent that the tonotopic organization of receptor terminals can also be interpreted as an odotopic map: there is a systematic relationship between the distance from which a song arises and the region of auditory neuropil where receptor activity is most intense (Fig. 2C). Similarly, the frequency selectivity of interneurons can also be seen as selectivity for songs arising from different distances. For example, a particular interneuron’s dendrites are restricted to the low-frequency portion of the tonotopic map, and the neuron is accordingly tuned to low frequencies. The intensity of low frequency components in the song is low, and receptors tuned to low frequencies are rather insensitive. Consequently, this interneuron only responds to songs arising within 10–15 m. By contrast, another interneuron with dendrites throughout most of the tonotopic array responds well to songs 30 m away.(31) Perhaps the most interesting interneurons, from the point of view of distance selectivity, are a group (of undetermined Figure 2. Tonotopic and odotopic organization of katydid receptor neurons. A: Drawings of parasagittal sections of the auditory neuropil, showing the terminal branches of receptors tuned to different frequencies, shown at right.(30) B: Threshold tuning curves of different receptors in M. markii. Each curve plots, for a single receptor neuron, the minumum sound intensity (threshold) needed to excite the receptor as a function of sound frequency. Each receptor is ‘‘tuned’’ to that frequency at which threshold is lowest.(31) C: The effect of distance to the source on the activation of receptors. Each curve in the graph corresponds to a different receptor. The sketches below illustrate the extent to which different regions of the auditory neuropil (shown in sagittal view) are activated by songs arising from different distances; darker areas are more heavily activated.(31) D: Distance selectivity of three different interneurons.(31) size) of morphologically indistinguishable neurons similar to that shown in Figure 1C. Even though their dendrites invade most of the auditory neuropil, different neurons within the group are maximally sensitive to sounds from different distances (Fig. 2D). This selectivity thus comes about not through selective sampling of the receptor array, but instead is ‘‘sculpted’’ by inhibitory inputs from other interneurons (as yet unidentified) that are ‘‘tuned’’ to distances above and below a particular cell’s optimum distance.(31) Another role of frequency analysis is to sharpen selectivity for conspecific signals. The male’s calling song in another species of katydid, Ancistrura nigrovittata, has a fairly narrow spectrum, with energy concentrated between 10 and 17 kHz. Behavioral measurements show that females are most sensitive to frequencies in this range.(32) Thus, although a female can hear sounds over a wider range (perhaps allowing her to hear bats or other predators), her nervous system is nevertheless specialized to detect and analyze frequencies in the narrower band used for intraspecific communication. A particu- BioEssays 21.4 297 Review articles lar interneuron, designated AN1 (Fig. 1C), is sharply tuned to the conspecific frequency range and may play a role in tuning the behavioral responses of females. AN1 is morphologically similar to the distance-selective interneurons of M. marki, and one is tempted to speculate that it may be a homologue of these. In both species, AN1’s dendrites overlap with the terminals of most receptor neurons and frequency selectivity results mainly from inhibition by neurons tuned to higher and lower frequencies.(31,33,34) It thus appears likely that similar (or identical?) neural circuitry is used for different tasks in the two species; distance estimation in M. markii, and selectivity for conspecific song spectrum in A. nigrovittata. Frequency analysis in crickets Cricket songs are dominated by a single narrow frequency band. This spectral purity (which is responsible for the songs’ musical quality) may be an adaptation that facilitates localization of the singer by conspecific listeners. Crickets determine sound direction by comparing sound intensity at the two ears.(35,36) The insects are small with respect to the wavelength of the dominant frequency in their songs (5–10 cm) and because of this a song originating on one side can readily diffract around the body and reach the offside ear with little or no attenuation. Interaural intensity differences are thus too small for accurate sound localization. Crickets solve this problem with an acoustical trick (the operation of which is beyond the scope of this article) that works only over a very narrow frequency range,(37) and the energy of the song is concentrated within this range. (The localization problem is presumably less severe for katydids, the songs of which generally include short-wavelength components that generate larger interaural intensity differences.) The dominant frequency in the songs of the cricket species Teleogryllus oceanicus is about 4.5 kHz(38) and, not surprisingly, the insects are most sensitive to this frequency.(35) They are also highly sensitive to frequencies within the range 25–50 kHz, however, even though these are present only at very low intensity in intraspecific signals. These ultrasonic frequencies are used by echolocating bats, which hunt insect prey on the wing by emitting ultrasonic cries and analyzing the echoes reflected by the insects’ bodies. When stimulated with ultrasound, T. oceanicus, like a number of other insects, exhibit escape responses.(1,35) Behavioral experiments indicate that T. oceanicus divide the entire audible range of frequencies, which extends at least from 3 to 100 kHz, into only two frequency classes, namely low (cricket-like) and high (bat-like) frequencies, with the border between the two classes at around 15 kHz.(39) The dual specialization of the auditory system to analyze both cricket-like and bat-like sound frequencies is apparent at the level of interneurons, most of which show enhanced 298 BioEssays 21.4 sensitivity to one or both of these frequency ranges.(40–43) At the level of receptor neurons, representation of the two ranges is heavily skewed in favor of cricket-like stimuli. Recordings from the whole auditory nerve consist of compound action potentials (CAPs), which represent the summed extracellular potentials produced by synchronously firing receptor neurons. CAPs evoked by cricket-like frequencies are 4–5 times larger than those evoked by ultrasound, suggesting that cricket-like frequencies excite more receptors(44)(Fig. 3A). A difference in CAP amplitude could also arise, however, if extracellular potentials were larger for low-frequency receptors, as would be the case if they had larger-diameter axons.(45) Larger axons would also be expected to conduct action potentials more rapidly.(46) However, conduction velocity does not differ between low- and highfrequency CAPs (Fig. 3B), ruling out this explanation for the difference in their amplitude. Another possible explanation is that receptors that respond to low frequencies might be more tightly synchronized than those responding to ultrasound, resulting in more efficient summation of their individual potentials. If this were the case, low-frequency CAPs would be expected to be shorter in duration, but they are not (Fig. 3C). The difference in CAP amplitude thus arises simply because more receptors respond to low frequencies than to ultrasound. This conclusion is supported by single-unit recordings. In a sample of 86 individual receptor neurons (recorded in as many crickets), 55 were tuned within the range 4–5.5 kHz, whereas only 14 were tuned to ⱖ20 kHz.(47) Thus, although more than 80% of the neurons were tuned to one or the other of the two behaviorally relevant ranges, four times as many were tuned to the lower range. The numerical imbalance in receptors tuned to cricket-like and bat-like sounds is at odds with the equally robust behavioral and interneuronal responses to these stimuli. Crickets perform positive phonotaxis (locomotion toward the sound source) when stimulated with conspecific songs, and negative phonotaxis (locomotion away from the sound source) when stimulated with ultrasound.(35) Both responses are strong and reliable (although the threshold for negative phonotaxis is somewhat higher), and there is little hint of a four- to fivefold difference in the numbers of sensory neurons involved. Two main questions arise from this situation: (1) why are these two ranges represented unequally by receptors, and (2) how can only a few ultrasound-tuned receptors drive behavior as strongly as a much larger number of neurons tuned to low-frequencies? Why are so few receptors tuned to ultrasound? There are no obvious functional advantages to detecting bat sounds with only a few receptors while devoting many more Review articles Figure 3. Unequal representation of cricket-like and bat-like sounds in cricket ears. A: Compound action potentials (CAPs) recorded from the auditory nerve are larger for stimuli with a cricket-like frequency (4.5 kHz) than for those with a bat-like frequency (30 kHz). Numbers to the left indicate stimulus intensity relative to threshold.(44) B: Conduction velocities of CAPs are similar for the two frequencies, indicating that the difference in their amplitude is not due to differences in axon diameter.(44) C: CAPs evoked by 4.5 kHz (solid line) and 30 kHz (dashed line) are similar in duration, indicating that the difference in amplitude (note scaling factors) is not due to a difference in the degree of synchronization among the contributing receptor neurons.(44) to encoding intraspecific signals. Both tasks present similar problems, namely detecting, recognizing, and localizing the stimulus, and one might expect that these could be better accomplished with more rather than fewer receptors, if only because a larger number of receptor neurons would allow greater dynamic range and increased signal-to-noise ratio. Analysis of intraspecific signals does appear to be more demanding than bat detection, however, perhaps accounting in part for the differing numbers of receptors involved. For example, behavioral responses to cricket song, in addition to showing frequency selectivity, are tuned to the speciesspecific temporal structures of the signals, whereas the escape response elicited by ultrasound is less selective for temporal pattern.(48) Responses to intraspecific signals may also be oriented more precisely than escape responses. Although no direct comparison of the directional accuracy of these two responses is yet available, experiments on other model systems suggest that escape responses may be less precise than target-oriented behaviors.(49) Direct evidence that the sensory requirements to detect and evade bats are minimal is provided by some moths, which accomplish this with only a single receptor neuron in each ear.(50) So, one reason for the small number of ultrasound receptor neurons in crickets may simply be that only a few receptors are needed to get the job done. Another reason may be the different evolutionary histories of listening to conspecifics and listening to bats. Ears and sound-producing structures were present in the very earliest crickets, which appeared some 250–300 million years ago.(21,51,52) Hearing in crickets has thus been associated with intraspecific communication throughout the entire course of this long evolutionary history. Bats, however, appeared only about 50 million years ago.(53) Selective pressure to detect and respond to bat cries thus arose relatively recently and must have been incorporated into a system that was already highly specialized for the analysis of intraspecific signals. One can imagine that this was accomplished by coopting some of the preexisting circuitry for this new task, and perhaps only a few receptors could be ‘‘spared’’ for this. How do only a few ultrasound receptors drive behavior as strongly as many low-frequency receptors? That a small number of ultrasound-tuned receptors can influence behavior as strongly as a much larger number that are tuned to low frequencies implies that each ultrasound neuron must ‘‘speak more loudly’’ to central neural circuits. Recent studies on an identified interneuron, ON1, have confirmed that this indeed takes place. The ON1 neurons (Fig. 1D) exist as a bilateral pair in the prothoracic ganglion. Each neuron receives auditory input on one side (the side on which its cell body is located) and provides inhibitory input to a number of contralateral targets, including its mirror-image partner. ON1 enhances bilateral auditory contrast, thereby facilitating the cricket’s determination of the direction of the sound source.(54–56) ON1’s frequency sensitivity mirrors that of behavioral responses,(41) suggesting that it receives input from the entire array of receptors. Although ON1 responds strongly to both cricket-like and bat-like sounds, recordings of its membrane potential show that the underlying physiological events differ for the two frequency ranges.(57) Cricket-like frequencies evoke excitatory postsynaptic potentials (EPSPs) with smooth waveforms that increase in amplitude in a graded manner with increasing stimulus intensity (Fig. 4A). Ultrasound, by contrast, evokes trains of large, discrete EPSPs that increase in frequency with increasing intensity (Fig. 4B). A simple hypothesis to account for this difference in waveform is that the EPSPs evoked by low frequencies are compound responses, formed by the BioEssays 21.4 299 Review articles Figure 4. Difference in excitatory postsynaptic potentials (EPSPs) evoked by cricket-like and bat-like sounds. A: EPSPs evoked in omega neuron 1 (ON1) by cricket-like stimuli are relatively smooth and are graded in intensity.(57) B: Bat-like sounds elicit summating trains of large, discrete EPSPs.(57) C: Large, ultrasound-elicited EPSPs (top trace) are preceded, with constant latency, by action potentials in the auditory nerve (bottom trace). This figure was prepared by aligning successive segments of the recording according to the timing of the EPSPs, and then averaging across these segments. A total of 770 segments of recording were averaged, derived from responses to a series of 50-dB, 30-kHz stimuli (G. Pollack, unpublished observations). D: Temporal coupling of spikes in two ultrasound-sensitive interneurons. Extracellular recordings were made both from omega neuron 1 (ON1) (top two traces) and from another identified interneuron, ascending neuron 2 (AN2) (bottom trace), in response to both 5-kHz and 30-kHz stimuli.(57) The traces are aligned according to the timing of the spikes in AN2. The top trace shows 23 superimposed sweeps in a response to a 5-kHz, 100-dB stimulus. The spikes of ON1 occur nearly randomly with respect to those of AN2. The middle trace shows 25 sweeps in a response to 30 kHz, 90 dB. Here, ON1 spikes are clustered around those of AN2, revealing temporal coupling of spikes in the two neurons. summation of a number of smaller EPSPs that cannot be resolved individually, but that the discrete EPSPs observed in response to ultrasound are unitary responses, each resulting from a single action potential in a single receptor neuron. The unitary nature of ultrasound-elicited EPSPs is confirmed in by simultaneous recordings from the auditory nerve and ON1. In Figure 4C, the large EPSPs were used as time markers to divide a long recording into segments, each corresponding to a single EPSP. These segments were then aligned according to the timing of the EPSP and averaged. The clear action potential in the nerve recording seen in the lower trace of Figure 4C ‘‘survives’’ the averaging because it is time-locked to the EPSP; other events in the nerve that occur randomly with respect to the EPSP average to zero. The constant latency between action potentials in the nerve and EPSPs in ON1 indicates that receptors tuned to ultrasound synapse 300 BioEssays 21.4 directly onto ON1, and the large size of the EPSPs indicates that these connections are powerful. Another indication of their potency comes from comparing the timing of action potentials in ON1 and another ultrasound-sensitive interneuron, AN2. If single EPSPs are sufficiently large to bring both interneurons to firing threshold, the timing of action potentials in the interneurons would be determined in part by the timing of the EPSPs. If the same small population of receptors is responsible for the responses of both interneurons to ultrasound, it follows that action potentials in the two interneurons would be temporally correlated. This is in fact observed, but only when the neurons are driven by ultrasound (Fig. 4D). These experiments show that the paucity of ultrasound receptor neurons is compensated by their increased synaptic potency. The cellular mechanisms for increased synaptic efficacy are unknown, but several possibilities present themselves. Perhaps the most obvious of these is differential passive filtering of inputs from low- and high-frequency receptors. The transmission of synaptic potentials through a neuron’s dendritic arbor is affected by factors such as the diameters and lengths of neuronal branches. If ultrasound-tuned receptors terminated closer to the site at which action potentials are initiated in ON1, or if they terminated on larger-diameter processes, the EPSPs they produce would suffer less attenuation by the passive electrical properties of the postsynaptic cell and would thus have a greater influence on ON1’s behavior. Recent anatomical studies have begun to address this possibility. Unlike katydids, in which receptor neurons form a continuous map in the central nervous system, cricket auditory receptor neurons fall into two distinct anatomical classes. One class terminates in a cluster of branches near the midline, in the same region where the best known auditory interneurons arborize most extensively (Fig. 5A). These receptor neurons are thus well positioned to make monosynaptic connections with the interneurons.(58,59)(Fig. 4C). Preliminary data indicate that all the ultrasound-tuned receptors, and about one-third of those tuned to low frequencies, fall into this class.(60) Both low- and high-frequency neurons of this anatomical type terminate in essentially the same region, making differential passive filtering of their inputs unlikely (Fig. 5A). The second anatomical type, comprising the remaining lowfrequency receptor neurons, has a characteristic bifurcating projection that has little or no overlap with the dendrites of the best known auditory interneurons (Fig. 5B,C). Interestingly, this is the most frequently encountered type of receptor, suggesting that a substantial amount of sensory input to auditory interneurons may come not directly from sensory neurons but rather from interposed interneurons. So, part of the explanation for the differing potency of ultrasound and Review articles varicosities than those tuned to low-frequencies,(60) suggesting that each action potential may release a greater amount of neurotransmitter. In addition, firing rates (i.e., number of action potentials per unit time) are about 20–30% higher for ultrasound receptors.(47) This too might enhance their influence on interneurons. Concluding remarks The examples summarized illustrate a few ways in which insect auditory systems implement the specializations that are required to analyze behaviorally important signals. Although this review has focused on only one suborder of insects, similar work is being pursued in other groups as well, including grasshoppers,(30,62) mantises(63) and flies.(64) And whereas we have focused here only on frequency analysis, these same insect models are being exploited to study other problems of hearing, such as the analysis of sound direction and of the temporal structure of stimuli.(23) What ties these various insects and investigations together is the explicit focus on the role of the auditory system in behavior. Natural selection operates at the level of phenotype which, in the case of the nervous system, is behavior. Insects, with their wide array of auditory systems, each with their own special features that can be assigned clear behavioral functions, provide ideal material for studying the cellular mechanisms by which nervous systems respond to selection pressure. Figure 5. Central projections of cricket auditory receptors. A: All ultrasound receptors (right) and some low-frequency receptors (left) terminate in a dense cluster of branches close to the midline, in the same region of the auditory neuropil in which well-described auditory interneurons arborize. B: The majority of low-frequency receptors terminate more laterally and posteriorly, and have few, if any, branches that overlap with the dendrites of the well-described auditory interneurons. C: Confocal reconstruction of a lowfrequency receptor of the type shown in B (green) and of processes of omega neuron 1 (ON1) (red), shown in horizontal view. The receptor neuron was stained with Lucifer Yellow, and ON1 was stained with neurobiotin/Texas-Red avidin. Most of the receptor terminals are lateral and posterior of ON1 processes. The few receptor branches that appear to overlap with ON1 are, in fact, situated dorsal of ON1 dendrites (G. Pollack, unpublished observations). a, anterior; p, posterior; m, medial; l, lateral. low-frequency receptors may lie in differences in the design of the afferent circuitry.(61) Another part of the explanation may reside in the cellular properties of the receptors. Sensory terminals are characterized by numerous swellings, or varicosities, which are believed to be sites of transmitter release. Preliminary data indicate that receptors that are tuned to ultrasound have more Acknowledgments We thank Dr. Zen Faulkes, Dr. Adam Wilkins and two anonymous referees for their helpful comments. Research not previously published was supported by grants (to G.S.P.) from NSERC and from the Whitehall Foundation. References 1. Hoy RR, Robert D. Tympanal hearing in insects. Annu Rev Entomol 1996;41:433–450. 2. Fullard JE, Yack JE. The evolutionary biology of insect hearing. Trends Ecol Evol 1993;8:248–252. 3. Oldfield BP. Tonotopic organization of auditory receptors in Tettigoniidae (Orthoptera: Ensifera). J Comp Physiol A 1982;147:461–469. 4. Oldfield BP, Kleindienst HU, Huber F. Physiology and tonotopic organization of auditory receptors in the cricket Gryllus bimaculatus DeGeer. J Comp Physiol A 1986;159:457–464. 5. Stumpner A. Tonotopic organization of the hearing organ in a bushcricket. Physiological characterization and complete staining of auditory receptor cells. Naturwissenschaften 1996;83:81–84. 6. Stölting H, Stumpner A. Tonotopic organization of auditory receptor cells in the bushcricket Pholidoptera griseoaptera (De Geer 1771) (Tettigoniidae, Decticini). Cell Tissue Res 1998;294:377–386. 7. Bekesy G von. Experiments in hearing. New York: McGraw-Hill; 1960. p 745. 8. Michelsen A. The physiology of the locust ear. I. Frequency sensitivity of single cells in the isolated ear. Z Vergl Physiol 1971;71:49–62. 9. Michelsen A. The physiology of the locust ear. II. Frequency discrimination based upon resonances in the tympanum. Z Vergl Physiol 1971;71:63– 101. BioEssays 21.4 301 Review articles 10. Michelsen A. Insect ears as mechanical systems. Am Sci 1979;67:696– 706. 11. Paton JA, Capranica RR, Dragsten PR, Webb WW. Physical basis for auditory frequency analysis in field crickets (Gryllidae). J Comp Physiol A 1977;119:221–240. 12. Michelsen A, Larsen ON. Biophysics of the ensiferan ear. I. Tympanal vibrations in bushcrickets (Tettigoniidae) studied with laser vibrometry. J Comp Physiol A 1978;123:193–203. 13. Larsen ON, Michelsen A. Biophysics of the ensiferan ear. III. The cricket ear as a four-input system. J Comp Physiol A 1978;123:217–227. 14. Bangert M, Kalmring K, Sickmann T, Stephen R, Jatho M, Lakes-Harlan R. Stimulus transmission in the auditory receptor organs of the forleg of bushcrickets (Tettigoniidae). I. The role of the tympana. Hearing Res 1998;115:27–38. 15. Ball EE, Oldfield BP, Michel-Rudolph K. Auditory organ structure, development, and function. In: Huber F, Moore TE, Loher W, editors. Cricket behavior and neurobiology. Ithaca: Comstock; 1989. p 391–422. 16. Lakes R, Schikorski T. Neuroanatomy of tettigoniids. In: Bailey WJ, Rentz DCF, editors. The Tettigoniidae: biology, systematics and evolution. Berlin: Springer-Verlag; 1990. p 166–190. 17. Kalmring K, Jatho M. The effects of blocking inputs of the acoustic trachea on the frequency tuning of primary auditory receptors in two species of tettigoniids. J Exp Zool 1994;270:360–371. 18. Oldfield BP. The tuning of auditory receptors in bushcrickets. Hearing Res 1985;17:27–35. 19. Michelsen A, Larsen ON. Hearing and sound. In: Kerkut GA, Gilbert LJ, editors. Comprehensive insect physiology, biochemistry and pharmacology, Vol 6. New York: Pergamon Press; 1985. p 495–556. 20. Ewing AR. Arthropod bioacoustics. Neurobiology and behaviour. Ithaca, NY: Cornell University Press; 1989. 260 pp. 21. Sharov AG. Phylogeny of the Orthopteroidea. (Transl 1971, Israel Program for Scientific Translation, Jerusalem.) Trans Paleontol Inst Acad Sci USSR 1968;118:1–216. 22. Gwynne DR. Phylogeny of the Ensifera (Orthoptera): a hypothesis supporting multiple origins of acoustical signalling, complex spermatophores and maternal care in crickets, katydids, and weta. J Orthop Res 1995;4:203– 218. 23. Pollack GS. Neural processing of acoustic signals. In: Hoy RR, Popper AN, Fay RR, editors. Comparative hearing: insects. Springer handbook of auditory research. Berlin: Springer-Verlag; 1998. p 139–196. 24. Römer H. 1993;Environmental and biological constraints for the evolution of long-range signalling and hearing in acoustic insects. Philos Trans R Soc Lond B 340:179–185. 25. Bennet-Clark HC. Size and scale effects as constraints in insect sound communication. Philos Trans R Soc Lond B 1998;353:407–419. 26. Thiele D, Bailey WJ. The function of sound in male spacing behaviour in bush-crickets (Tettigoniidae, Orthoptera). Aust J Ecol 1980;5:275–286. 27. Römer H, Bailey WJ. Insect hearing in the field. II. Male spacing behaviour and correlated acoustic cues in the bushcricket Mygalopsis marki. J Comp Physiol A 1986;159:627–638. 28. Oldfield BP. Central projections of primary auditory fibres in Tettigoniidae (Orthoptera; Ensifera). J Comp Physiol A 1983;151:389–395. 29. Römer H. Tonotopic organization of the auditory neuropile in the bushcricket Tettigonia viridissima. Nature 1983;306:60–62. 30. Römer H, Marquart V, Hardt M. Organization of a sensory neuropile in the auditory pathway of two groups of orthoptera. J Comp Neurol 1988;275: 201–215. 31. Römer H. Representation of auditory distance within a central neuropil of the bushcricket Mygalopsis marki. J Comp Physiol A 1987;161:33–42. 32. Dobler S, Stumpner A, Heller K-G. Acoustic communication in the duetting bushcricket Ancistrura nigrovittata (Orthoptera, Phaneropteridae). II. Sex specific behavioural tuning for the partner’s song. J Comp Physiol A 1994;175:303–310. 33. Stumpner A. An auditory interneurone tuned to the male song frequency in the duetting bushcricket Ancistrura nigrovittata (Orthopera, Phaneropteridae). J Exp Biol 1997;200:1089–1011. 302 BioEssays 21.4 34. Stumpner A. Picrotoxin eliminates frequency selectivity of an auditory interneuron in a bushcricket. J Neurophysiol 1998;79:2408–2415. 35. Moiseff A, Pollack GS, Hoy RR. Steering responses of flying crickets to sound and ultrasound: mate attraction and predator avoidance. Proc Natl Acad Sci USA 1978;75:4052–4056. 36. Schmitz B, Scharstein H, Wendler G. Phonotaxis in Gryllus campestris L. (Orthoptera, Gryllidae). II. Acoustic orientation of female crickets after occlusion of single sound entrances. J Comp Physiol A 1982;152:257– 264. 37. Michelsen A, Popov AV, Lewis B. Physics of directional hearing in the cricket Gryllus bimaculatus. J Comp Physiol A 1994;175:153–164. 38. Balakrishnan R, Pollack GS. Recognition of courtship song in the field cricket, Teleogryllus oceanicus. Anim Behav 1996;51:353–366. 39. Wyttenbach RA, May ML, Hoy RR. Categorical perception of sound frequency by crickets. Science 1996;273:1542–1544. 40. Moiseff A, Hoy RR. Sensitivity to ultrasound in an identified auditory interneuron in the cricket: a possible neural link to phonotactic behavior. J Comp Physiol A 1983;131:113–120. 41. Atkins G, Pollack GS. Age dependent occurrence of an ascending axon on the omega neuron of the cricket, Teleogryllus oceanicus. J Comp Neurol 1986;243:527–534. 42. Atkins G, Pollack GS. Response properties of prothoracic, interganglionic, sound-activated interneurons in the cricket Teleogryllus oceanicus. J Comp Physiol A 1987;161:681–693. 43. Brodfuehrer PD, Hoy RR. Ultrasound sensitive neurons in the cricket brain. J Comp Physiol A 1990;166:651–662. 44. Pollack GS, Faulkes Z. Representation of behaviorally relevant sound frequencies by auditory receptors in the cricket Teleogryllus oceanicus. J Exp Biol 1998;201:155–163. 45. Rushton WAH. A theory of the effects of fibre size in medullated nerve. J Physiol 1951;115:101–122. 46. Hodgkin AL. A note on conduction velocity. J Physiol 1954;125:221–224. 47. Imaizumi K, Pollack GS. Physiological properties of auditory receptors in the Australian field cricket Teleogryllus oceanicus. Soc Neurosci Abs 1997;23:1571. 48. Pollack GS, El-Feghaly E. Calling song recognition in the cricket Teleogryllus oceanicus: comparison of the effects of stimulus intensity and sound spectrum on selectivity for temporal pattern. J Comp Physiol A 1993;171: 759–765. 49. Roche King J, Comer CM. Visually elicited turning behavior in Rana pipien: comparative organization and neural control of escape and prey capture. J Comp Physiol A 1996;178:293–305. 50. Surlykke A. Hearing in notodontid moths: a tympanic organ with a single auditory neurone. J Exp Biol 1984;113:11–28. 51. Hoy RR. The evolution of hearing in insects as an adaptation to predation from bats. In: Webster DB, Fay RR, Popper AN, editors. The evolutionary biology of hearing. Berlin: Springer-Verlag; 1992. p 115–129. 52. Otte D. Evolution of cricket songs. J Orthop Res 1992;1:24–46. 53. Novacek MJ. Mammalian phylogeny: shaking the tree. Nature 1992;356: 121–125. 54. Selverston AI, Kleindienst HU, Huber F. Synaptic connectivity between cricket auditory interneurons as studied by selective photoinactivation. J Neurosci 1985; 5:1283–1292. 55. Horseman G, Huber F. Sound localisation in crickets. I. Contralateral inhibition of an ascending auditory interneuron (AN1) in the cricket Gryllus bimaculatus. J Comp Physiol A 1994a; 174:389–398. 56. Horseman G, Huber F. Sound localisation in crickets. II. Modeling the role of a simple neural network in the prothoracic ganglion. J Comp Physiol A 1994b;175:399–413. 57. Pollack GS. Synaptic inputs to the omega neuron of the cricket Teleogryllus oceanicus: differences in EPSP waveforms evoked by low and high sound frequencies. J Comp Physiol A 1994;174:83–89. 58. Hennig RM. Ascending auditory interneurons in the cricket Teleogryllus commodus (Walker): comparative physiology and direct connections with afferents. J Comp Physiol A 1988;163:135–143. Review articles 59. Hirtz R, Wiese K. Ultrastructure of synaptic contacts between identified neurons of the auditory pathway in Gryllus bimaculatus De Geer. J Comp Neurol 1997;386:347–357. 60. Imaizumi K, Pollack GS. Anatomy and physiology of auditory receptors in the Australian field cricket Teleogryllus oceanicus. Soc Neurosci Abs 1996;22:1082. 61. Faulkes Z, Pollack GS. Sound frequency specific responses of cricket omega neuron. Soc Neurosci Abs 1997;23:1570. 62. Halex H, Kaiser W, Kalmring K. Projection areas and branching patterns of the tympanal receptor cells in migratory locusts, Locusta migratoria and Schistocerca gregaria. Cell Tissue Res 1988;253:517–528. 63. Yager DD. Serially homologous ears perform frequency range fractionation in the paying mantis, Creobroter (Mantodea, Hymenopodidae). J Comp Physiol A 1996;178:463–475. 64. Oshinksy ML, Hoy RR. Response properties of auditory afferents in the fly Ormia ochracea. In: Burrows M, Matheson T, Newland PL, Schuppe J, editors. Nervous systems and behaviour. Proceedings of the fourth international congress of neuroethology. Stuttgart: Georg Thieme Verlag; 1995. p 359. 65. Wohlers DW, Huber F. Intracellular recording and staining of cricket auditory interneurons (Gryllus campestris L., Gryllus bimaculatus DeGeer). J Comp Physiol 1978;127:11–28. BioEssays 21.4 303