Marc W. Holderied, Liam A. Thomas, and Carmi Korine
The acoustic arms race between insectivorous bats and their invertebrate prey has led to the convergent evolution of ultrasound hearing in seven orders of nocturnal insects. Upon hearing the echolocation calls of an approaching bat such insects take defensive action. Here we document an unknown sense of ultrasound hearing and phonotactic flight behaviour in the neuropteran family Myrmeleontidae (antlions). The antlion Myrmeleon hyalinus was presented with sound pulses at ultrasonic frequencies used by echolocating bats and its response thresholds in tethered flight determined. Behaviours included abdominal twitches, wing-flicks, brief pauses in flight and flight cessation. Such behaviours create erratic evasive flight manoeuvres in other eared insects, particularly mantids and lacewings. Antlions responded best to ultrasound between 60-80 kHz (75 dB peSPL at 80 kHz) showing response thresholds similar to the related lacewings (Neuroptera, Chrysopidae). Yet at lower ultrasonic frequencies (20-50 kHz) antlions were far less sensitive than lacewings. Based on calculated response distances we conclude that antlions respond only after having been detected by bats rather than using early evasive flights. We argue that the high response threshold for low frequency ultrasound is adaptive for an insect that is mainly active close to and within vegetation, because a behavioural response to the lower ultrasonic frequencies used by high-flying bats would result in evasive action in the absence of actual predation risk.