Information in sonar echoes of fluttering insects available for echolocating bats

Abstract

Insects were mounted in the acoustical beam of an ultrasonic loudspeaker transmitting either an 80-kHz continuous constant frequency (CF) tone or a short frequency-modulated (FM) signal (82-18 kHz). The returning CF or FM echoes were recorded and analyzed. Short amplitude peaks or amplitude glints, which occur in the echo in the rhythm of the wing beat, are produced each time some part of the wing surface moves into a plane perpendicular to the impinging sound waves. The amplitude glints are represented in spectrograms of echoes as transitory spectral broadenings or frequency glints. In CF echoes, the composition of positive and negative frequency shifts associated with glints encode the attitude of the flying insect relative to the sound source independent of the insect species. Further information in CF echoes allows a classification of insects as to wing beat frequency, wing structure, wing beat type, wing length, and insect size. Only the wing beat rate of a fluttering insect could be decoded from FM echoes of sounds delivered at fixed duration and repetition rate toward a beetle. The information in the insect echoes described here explains much of the performance of bats concerning their ability to identify insects in behavioral experiments. Whether bats are able to classify insects in the field is discussed. The data from this study allow the computation of the maximum echo sound-pressure levels for insects.