Ultrasound avoidance is an escape or avoidance reflex displayed by certain animal species that are preyed upon by echolocating predators. Ultrasound avoidance is known for several groups of insects that have independently evolved mechanisms for ultrasonic hearing. Insects have evolved a variety of ultrasound-sensitive ears based upon a vibrating tympanic membrane tuned to sense the bat's echolocating calls. The ultrasonic hearing is coupled to a motor response that causes evasion of the bat during flight. Although ultrasonic signals are used for echolocation by toothed whales, no known examples of ultrasonic avoidance in their prey have been found to date. Ultrasonic hearing has evolved multiple times in insects: a total of 19 times. Bats appeared in the Eocene era, (about 50 million years ago); anti-bat tactics should have evolved then. Antibat tactics are known in four orders of Insecta: moths (Lepidoptera), crickets (Orthoptera), mantises (Dictyoptera), and green lacewings (Neuroptera). There are hypotheses of ultrasound avoidance being present in Diptera (flies) and Coleoptera (beetles).
Ultrasound avoidance in moths The idea that moths were able to hear the cries of echolocating bats dates back to the late 19th century. F. Buchanan White, in an 1877 letter to Nature made the association between the moth's high-pitched sounds and the high-pitched bat calls and wondered whether the moths would be able to hear it. However, it was not until the early 1960s that Kenneth Roeder et al. made the first electrophysiological recordings of a noctuid moth's auditory nerve and were able to confirm this suspicion. Later research showed that moths responded to ultrasound with evasive movements. Moths, as do crickets and most insects that display bat avoidance behaviors, have tympanic organs that display phonotactic and directional hearing; they fly away from the source of the sound and will only have the diving behavior considered above when the sound is too loud—or when, in a natural setting, the bat would be presumably too close to simply fly away. It was found that the moths' responses vary according to ultrasound intensity, diving towards the ground if the pulse was of a high amplitude or flying directly away from the sound source if the sound amplitude was low (if the sound was softer). Acoustic sensory receptors in noctuid moths are mechanoreceptors located in a chamber formed by the wall of the abdomen and the tympanic membrane, and they are most sensitive to lower frequencies of ultrasound (between 20 and 30 kHz.). The moth's body axis allows it to be more sensitive to sounds coming from particular directions. Their ears, on either side of the metathorax, have two sensory cells within the membranes. Though the tuning curves of these cells are identical, the sensitivity thresholds differ, allowing for sound localization and a wider range of sensitivity to sound. The movement of the wings during flight also plays a role, since sound thresholds change with wing position. The neural mechanisms for triggering the acoustic startle response are partially understood. However, there is little known about the motor control of flight that ultrasound initiates. Further research has shown that many species of moths are sensitive to ultrasound. Sensitivities for ultrasound change according to the environment the moth thrives in, and the moth can even change its own sensitivity if it is preyed upon by bats with different echolocating calls. Such is the case of the Australian noctuid moth, Speiredonia spectans, which adapts its acoustic sensitivity according to the characteristics of the call of the bat inside the cave with them.
Ultrasound avoidance in crickets
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