As an echolocating bat closes in on a flying insect, it increases call emission to rates beyond 160 calls per second. This high call rate phase, dubbed the terminal buzz, has proven enigmatic because it is unknown how bats are able to produce calls so quickly. We found that previously unknown and highly specialized superfast muscles power rapid call rates in the terminal buzz. Additionally, we show that laryngeal motor performance, not overlap between call production and the arrival of echoes at the bat's ears, limits maximum call rate. Superfast muscles are rare in vertebrates and always associated with extraordinary motor demands on acoustic communication. We propose that the advantages of rapid auditory updates on prey movement selected for superfast laryngeal muscle in echolocating bats.

Most echolocating bats exhibit a strong correlation between body size and the frequency of maximum energy in their echolocation calls (peak frequency), with smaller species using signals of higher frequency than larger ones. Size-signal allometry or acoustic detection constraints imposed on wavelength by preferred prey size have been used to explain this relationship. Here we propose the hypothesis that smaller bats emit higher frequencies to achieve directional sonar beams, and that variable beam width is critical for bats. Shorter wavelengths relative to the size of the emitter translate into more directional sound beams. Therefore, bats that emit their calls through their mouths should show a relationship between mouth size and wavelength, driving smaller bats to signals of higher frequency. We found that in a flight room mimicking a closed habitat, six aerial hawking vespertilionid species (ranging in size from 4 to 21 g, ref. 5) produced sonar beams of extraordinarily similar shape and volume. Each species had a directivity index of 11 ± 1 dB (a half-amplitude angle of approximately 37°) and an on-axis sound level of 108 ± 4 dB sound pressure level referenced to 20 μPa root mean square at 10 cm. Thus all bats adapted their calls to achieve similar acoustic fields of view. We propose that the necessity for high directionality has been a key constraint on the evolution of echolocation, which explains the relationship between bat size and echolocation call frequency. Our results suggest that echolocation is a dynamic system that allows different species, regardless of their body size, to converge on optimal fields of view in response to habitat and task.

SummaryDuty cycle describes the relative ʻon timeʼ of a periodic signal. In bats, we argue that high duty cycle (HDC) echolocation was selected for and evolved from low duty cycle (LDC) echolocation because increasing call duty cycle enhanced the ability of echolocating bats to detect, lock onto and track fluttering insects. Most echolocators (most bats and all birds and odontocete cetaceans) use LDC echolocation, separating pulse and echo in time to avoid forward masking. They emit short duration, broadband, downward frequency modulated (FM) signals separated by relatively long periods of silence. In contrast, bats using HDC echolocation emit long duration, narrowband calls dominated by a single constant frequency (CF) separated by relatively short periods of silence. HDC bats separate pulse and echo in frequency by exploiting information contained in Doppler-shifted echoes arising from their movements relative to background objects and their prey. HDC echolocators are particularly sensitive to amplitude and frequency glints generated by the wings of fluttering insects. We hypothesize that narrowband/CF calls produced at high duty cycle, and combined with neurobiological specializations for processing Doppler-shifted echoes, were essential to the evolution of HDC echolocation because they allowed bats to detect, lock onto and track fluttering targets. This advantage was especially important in habitats with dense vegetation that produce overlapping, time-smeared echoes (i.e. background acoustic clutter). We make four specific, testable predictions arising from this hypothesis.Key words: Chiroptera, calling behaviour, signal design, Yinpterochiroptera, Yangochiroptera, flutter detection, Doppler shift compensation. Fig.1A-C). For the purpose of this paper, we define LDC bats as those producing signals with a duration <25% of their signal period during the search phase of echolocation. Most LDC bats produce echolocation calls with their larynx, although a handful of species in the family Pteropodidae use tongue clicks (Griffin et al., 1958;Yovel et al., 2010).HDC bats avoid auditory masking by separating pulse and echo in frequency, allowing them to broadcast calls and receive echoes at the same time (Schuller, 1974;Schuller, 1977). HDC bats take advantage of information contained in Doppler-shifted echoes generated by the relative movements of bat and target, including acoustic glints generated by the wingbeats of fluttering insects. Echolocation calls of HDC bats consist of a long CF component followed by a brief, downward FM sweep. In some species, the initial portion of the call also contains a short, upward FM sweep (Henson et al., 1987; Jones and Rayner, 1989). Narrowband calls of HDC bats are typically multi-harmonic with the highest signal energy in the second acoustic element (Pye and Roberts, 1970;Schnitzler and Denzinger, 2011). HDC bats emit long duration calls (e.g. 10 to >50ms) relative to their call period ( Fig.1D-F). We operationally define HDC bats as those whose signal durations are ≥25% of...

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The external morphology of the terminal region of the fifth tarsal segment of the blowfly, Calliphora vomitoria (L.) has been studied using light and scanning electron microscopy (S.E.M.). The pulvilli, with their numerous tenent hairs of spatulate form projecting from the ventral surface, are responsible for adhesion to smooth surfaces. The two large claws are believed to be important in clinging to irregularities in surfaces. Two footplates, possibly sensory organs, lie in close association with the base of the large ventral seta, the empodium. Blowflies release a non‐volatile lipid secretion on to the spatulate ends of the tenent hairs and this secretion is essential to the adhesion process on smooth surfaces. The force of adhesion has been measured for tethered blowflies on glass using both vertical and lateral pulls; lateral pulls gave much greater forces. It is concluded that surface tension of the lipid secretion under tenent hairs is sufficient to enable successful adhesion to smooth surfaces by blowflies.