Marsat and Pollack Bursting and ultrasound avoidance often only 3–5 ms apart. The crickets must therefore be sensitive to ultrasound stimuli with a wide variety of temporal patterns, and experiments with tethered crickets show that this is indeed the case (Pollack and Elfeghaly, 1993). Bats and insects are engaged in an evolutionary “arms race” of auditory sensitivity, in which each combatant would beneﬁt from long-distance detection of the other. Bat echolocation is a relatively short-range modality; although echolocation calls can be very intense as they leave the bat (s120 dB SPL; Surlykke and Kalko, 2008), the amplitude of returning echoes is limited by the small reﬂective surface of an insect’s body, and by frictional loss to the air. As a result, many bats can detect their prey at an average distance of only s5–10 m depending on the species of bat and size of the prey (Kick, 1982; Holderied and Von Helversen, 2003; Surlykke and Kalko, 2008). By contrast, ultrasound sensitivity of crickets is such that they should be able to detect a bat at a dis- tance of 10–18 m (Nolen and Hoy, 1986). The best strategy for an insect that detects a distant bat, then, is to steer away from the sound source to escape detection, and indeed that is what both moths and crickets do in response to ultrasound stimuli of low-to- moderate intensity (Hoy, 1994). Once the bat detects its potential prey, however, a chase may ensue. Video recordings show that such “dog ﬁghts” between bats and beetles can last for many seconds, during which the beetle continues to exhibit evasive maneuvers (Simmons, 2005). The neural and behavioral responses during close range interactions are not well deﬁned. We know that, in some insects, very loud sound pulses (>95 dB SPL for crickets) will cause a different kind of evasive maneuver that is non-directional, a strategy that may serve to prevent the bat, which can out-ﬂy the insect, from predicting the trajectory of its prey (Roeder, 1967; Nolen and Hoy, 1986). However, considering that bats typically reduce the intensity of their vocalization as they approach their prey (Schnitzler and Kalko, 2001; Fullard et al., 2003; Schnitzler et al., 2003), it is unknown how commonly these evasive maneu- vers are triggered. Also, experiments with playbacks of recorded echolocation pulses to crickets and moths (Fullard et al., 2003, 2005) indicate that the temporal pattern and the frequency of the vocalizations of some bats during the ﬁnal attack phase might not stimulate the neurons very strongly. These experiments however were done in a ﬁxed spatial conﬁguration. The spatial dynamics of a bat-insect interaction can obviously cause large modulations in the intensity of ultrasound as it reaches each ear beyond those present in the pulse train that the bat produces. The consequence of this spatial dynamic on sensory processing remains unexplored and would be an interesting subject for future studies. ULTRASOUND CODING IN CRICKETS The cricket’s ears are located on the tibiae of the fore legs. Auditory information is carried from the ear to the prothoracic ganglion by auditory receptors, and is then relayed to the brain by ascending interneurons (Figure 2). The population of about 60–70 audi- tory receptors is composed of distinct types of cells based on their anatomy and tuning to sound frequency (Imaizumi and Pollack, 1999, 2001, 2005). About one-fourth of the receptors respond most strongly to high frequencies, including ultrasound. The remaining three-fourth are tuned to the low frequencies used during con- speciﬁc communication. The tuning of receptors to either high FIGURE 2 | Auditory neurons of the prothorax of crickets. (A) Auditory receptors project from the ears located on the tibiae of the forelegs to the prothoracic ganglion. A population of approximately 15 ultrasound receptors on each side provides inputs to the Ascending Neuron 2 (AN2) which carries the information to the brain, and to the Omega Neuron 1 (ON1), which inhibits the contralateral interneurons (both AN2 and ON1). (B) Morphologies of AN2 (red), ON1 (green), and an ultrasound receptor (blue) ﬁlled with ﬂuorescent dyes. Note that while the AN2 and ON1 neurons were imaged together, the receptor has been superimposed post hoc. or low frequencies forms the basis for categorical perception of sound frequency. This was demonstrated by using an habituation- dishabituation paradigm to show that stimuli with various carrier frequencies were categorized either as attractive or repulsive, with a sharp transition between the two at around 16 kHz (Wytten- bach et al., 1996). At this transition frequency, responses depended on the stimulus temporal pattern; non-cricket-like (2 pulses/s) stimuli consistently elicited avoidance steering, but a calling-song www.frontiersin.org July 2012 | Volume 6 | Article 95 | 3