Earlier studies of phonotaxis by female crickets describe this selective behavioural response as being important in the females' choices of conspecific males, leading to reproduction. In the present study, moderate (30+) to very large data sets of phonotactic behaviour by female Acheta domesticus L., Gryllus bimaculatus DeGeer, Gryllus pennsylvanicus Burmeister and Gryllus veletis Alexander demonstrate substantially greater plasticity in the behavioural choices, as made by females of each species, for the syllable periods (SP) of model calling songs (CS) than has been previously described. Phonotactic choices by each species range from the very selective (i.e. responding to only one or two SPs) to very unselective (i.e. responding to all SPs presented). Some females that do not respond to all SPs prefer a range that includes either the longest or shortest SP tested, which fall outside the range of SPs produced by conspecific males. Old female A. domesticus and G. pennsylvanicus are more likely to be unselective for SPs than are young females. Each species includes females that do not respond to a particular SP when responding to CSs with longer and shorter SPs. The results suggest that the plasticity of phonotactic behaviour collectively exhibited by the females of each species does not ensure that choices of a male's CS effectively focus the female's phonotactic responses on CSs that represent the conspecific male. The phonotactic behaviour collectively exhibited by females of each species does not readily fit any of the models for selective processing by central auditory neurones that have been proposed to underlie phonotactic choice.

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1. Inactivating one L 1 results in angular errors and circling during orientation toward the side having the intact L1 in response to calling songs (CSs) whose intensities are below the threshold for L3 (Figs. 2, 3A). When song intensities are increased above the threshold of L3, circling decreases (Fig. 3B).2. Following inactivation of one L1 and occlusion of the ear providing input to the intact L1, no phonotaxis occurs in response to CSs at 60 dB (below the threshold of L3; Fig. 4A) demonstrating the necessity of L1 for phonotaxis. Orientation, at large and consistent angular errors (Fig. 5A) and circling (Fig. 5B) to the unoccluded side resumes when song intensities are increased above the threshold of L3 (Fig. 4B) suggesting that a single L3 can induce phonotactic responses.3. Inactivation of one L3 causes angular errors in orientation when CSs are above its threshold (Figs. 6A, 7), which are not apparent when CSs are below L3's threshold (Figs. 6B, 7).4. Inactivation of one L3 and occlusion of the ear providing input to the contralateral L1 and L3, leave only one L1 functioning. This results in turning and circling toward the unoccluded side containing the one functioning L1 (Figs. 6C, 8), thus confirming the sufficiency of L 1 for phonotaxis.

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In response to model calling songs (CSs), the phonotaxis of female Acheta domesticus ranges from being very selective to unselective. Within 15 min of nanoinjecting juvenile hormone III (JHIII) or picrotoxin (PTX) into the prothoracic ganglion, females become more selective for syllable period (SP) than in pre-tests. Controls for JHIII experiments, including nanoinjection of acetone into the prothoracic ganglion or nanoinjection of JHIII into the metathoracic ganglion, do not influence selectivity. Similarly, nanoinjection of saline into the prothoracic ganglion and nanoinjection of PTX outside of the prothoracic ganglion does not change the overall selectivity of the female's phonotaxis. These results indicate that circuits in the prothoracic ganglion modulate the SP-selectivity of phonotaxis.Photoinactivating both of the ON1 prothoracic auditory interneurones in old females that were previously unselective for SP also results in greater SP-selectivity during phonotaxis. Evidence suggesting that ON1 has this effect via its inhibitory input to L3 (another prothoracic auditory neurone) includes: photoinactivation of one ON1 neurone causes angular errors in the female's orientation to CSs at 85 dB (above the threshold of the L3), stimulation with 60 dB CSs (above the threshold of ON1 but below the threshold of L3) does not induce errors in angular orientation, inactivation of ON1 in old crickets results in greater angular errors (85 dB stimulus) than it does when ON1 is inactivated in young females, and photoinactivation of ON1 increases the firing rate of the L3 neurone. Fig. 4. Controls for picrotoxin (PTX) injection. (a) Tables showing positive phonotaxis (shaded boxes) in response to syllable periods (SPs)ranging from 30 to 90 ms (columns) for 26 females (each row is one female) before and after nanoinjection of 9.2 nL of saline into the ventral portion of the prothoracic ganglion. (b) Graphs showing the females in (a) grouped by the number of SPs they responded to in the pre-test: those that responded to only one to three of the SPs presented in the pre-test (left graph, n ϭ 5), those that responded to four or five SPs during the pre-tests (middle graph, n ϭ 18), and those that responded to six or seven of the SPs presented during the pre-test (right graph, n ϭ 6). (c) More controls for PTX injection. Tables showing positive phonotaxis (shaded boxes) in response to SPs ranging from 30 -90 ms (columns) for 12 females (each row is one female) before and after nanoinjection of 50 pg of PTX in 9.2 nL of saline into the haemolymph outside of the prothoracic ganglion. (d) Mean results from the females in (c). 330 G. Atkins et al.

1. When tested with legphone stimulation at 5 and 16 kHz, two prothoracic 'low-frequency neurons', ON1 and L1 of Acheta domesticus females, receive mainly excitation from one side (soma-ipsilateral in ON1, soma-contralateral in L1) and inhibition from the opposite side as is described for other cricket species (Figs. 2,3). While thresholds at 5 kHz are similar in L1 and ON1, L1 receives 16 kHz excitation with a 15 -20 dB higher threshold (lower than in other cricket species) than ON1. Stimulation of L1 with lower intensity 16 kHz sound on the side of its major input results in a clear IPSP visible in dendritic recordings (Figs. 3,4). In L1 and ON1 the intensity response at 16 kHz rises steeper than that at 5 kHz. 2. The most sensitive auditory low-frequency receptors recorded have similar thresholds as ON1 and L1 at 5 kHz. Responses of the most sensitive auditory highfrequency receptors recorded show an intensity dependence which is similar to that of ON1 at 16 k Hz (Fig. 1C). 3. Results of two-tone experiments show a tuning of inhibition in ON1 and L1 which is similar to excitatory tuning of ON1 (Fig. 4), however with about 10 to 15 dB higher thresholds. In contrast, in Gryllus bimaculatus an exact match between ONl-excitation and ON1/AN1 inhibition has been described.

L3, an auditory interneuron in the prothoracic ganglion of female crickets (Acheta domesticus) exhibited two kinds of responses to models of the male's calling song (CS): a previously described, phasically encoded immediate response; a more tonically encoded prolonged response. The onset of the prolonged response required 3-8 sec of stimulation to reach its maximum spiking rate and 6-20 sec to decay once the calling song ceased. It did not encode the syllables of the chirp. The prolonged response was sharply selective for the 4-5 kHz carrier frequency of the male's calling songs and its threshold tuning matched the threshold tuning of phonotaxis, while the immediate response of the same neuron was broadly tuned to a wide range of carrier frequencies. The thresholds for the prolonged response covaried with the changing phonotactic thresholds of 2- and 5-day-old females. Treatment of females with juvenile hormone reduced the thresholds for both phonotaxis and the prolonged response by equivalent amounts. Of the 3 types of responses to CSs provided by the ascending L1 and L3 auditory interneurons, the threshold for L3's prolonged response, on average, best matched the same females phonotactic threshold. The prolonged response was stimulated by inputs from both ears while L3's immediate response was driven only from its axon-ipsilateral ear. The prolonged response was not selective for either the CS's syllable period or chirp rate.

The omega neurons (ON1s) are a mirror-symmetrical pair of identified prothoracic auditory interneurons of crickets which have been previously described as intraganglionic. Using intracellular techniques we stained ON1s of female Teleogryllus oceanicus and found that many ON1s have axons which project anteriorly out of the prothoracic ganglion. The ascending axon arises contralateral to the soma at the most anteriolateral bend of the bow-shaped process of an otherwise "archetypical" ON1 and travels up the neck connective in a ventral position just inside the connective tissue sheath. The occurrence of the ascending axon is age-dependent. Seventy-five percent of ON1s stained in late nymphal stages and in young adults had an ascending axon while only 30% of ON1s in older adults had an ascending axon. Evidence is presented to show that ON1s having ascending axons are developmental variants of the "archetypical" ON1 and do not represent a separate neuron type. The two morphological types of ON1s are not distinguishable on the basis of their responses to sound stimuli having carrier frequencies of 3.5-60 kHz. Although we know that the ascending axon conducts action potentials, its target and terminal morphology are not yet known.