Summary
Caterpillar defenses have been researched extensively, and, although most studies focus on visually communicated signals, little is known about the role that sounds play in defense. We report on whistling, a novel form of sound production for caterpillars and rare for insects in general. The North American walnut sphinx (Amorpha juglandis) produces whistle ‘trains’ ranging from 44 to 2060 ms in duration and comprising one to eight whistles. Sounds were categorized into three types: broadband, pure whistles and multi-harmonic plus broadband, with mean dominant frequencies at 15 kHz, 9 kHz and 22 kHz, respectively. The mechanism of sound production was determined by selectively obstructing abdominal spiracles, monitoring air flow at different spiracles using a laser vibrometer and recording body movements associated with sound production using high-speed video. Contractions of the anterior body segments always accompanied sound production, forcing air through a pair of enlarged spiracles on the eighth abdominal segment. We tested the hypothesis that sounds function in defense using simulated attacks with blunt forceps and natural attacks with an avian predator – the yellow warbler (Dendroica petechia). In simulated attacks, 94% of caterpillars responded with whistle trains that were frequently accompanied by directed thrashing but no obvious chemical defense. In predator trials, all birds readily attacked the caterpillar, eliciting whistle trains each time. Birds responded to whistling by hesitating, jumping back or diving away from the sound source. We conclude that caterpillar whistles are defensive and propose that they function specifically as acoustic ‘eye spots’ to startle predators.
Methodology
Larvae of the walnut sphinx, Amorpha juglandis (J. E. Smith 1797), were reared from eggs obtained from moths captured at ultraviolet lights at the Queen's University Biological Station (QUBS; Lake Opinicon, Ontario, Canada; 44°33′ 55.34″ N, 76°19′ 26.59″ W) in July 2008 and August 2009. Larvae were reared on cuttings of hop hornbeam (Ostyra sp.), alder (Alnus sp.), walnut (Juglans sp.) or beech (Fagus sp.). All experiments were performed on late (fourth or fifth) instar larvae. Caterpillars of Paonias myops (J. E. Smith), used for comparative purposes, were reared from eggs of moths captured at QUBS on cuttings of cherry (Prunus sp.).
Sounds analyzed for spectral and temporal characteristics were recorded from 10 caterpillars using a Brüel & Kjær (B&K; Naerum, Denmark) 1/4 in microphone type 4939 (grid on), amplified with a Brüel & Kjær Nexxus conditioning amplifier type 2690 and recorded to a Fostex FR-2 Field Memory Recorder (Gardena, CA, USA) at a sampling rate of 192 kHz. Recordings were analyzed using RavenPro Bioacoustics Research Program 1.4 (Cornell Laboratory of Ornithology, Ithaca, NY, USA). Sound production was induced by placing an individual on a cutting of a host plant and delivering an attack to the abdomen with blunt forceps (see ‘Attack trials’ for details). All recordings were performed in an acoustic chamber (Eckel Industries, Cambridge, MA, USA).
Temporal characteristics, including train duration, number of pulses in a train, inter-pulse interval, pulse duration and the element (waveform) repetition rate, were measured from the first three trains of ten individuals. A train was defined as a series of sound pulses following an attack, until sound production ceased. A pulse is a group of uninterrupted waves, or elements. Mann–Whitney U tests were performed to compare the durations of the first and third trains of 10 individuals to determine whether sensitization occurred and to compare the repetition rate of waveforms in different sound types. Measured spectral characteristics included the fundamental and dominant frequencies and number of harmonics at –40 dB. Power spectra and spectrograms were produced using a 1024-point Fast Fourier Transform (Hann window, 50% overlap). As pulses differed in their spectral qualities, they were categorized as types 1–3 (see Results for details). In order to determine where different pulse types occurred within a train, pulse trains of 11 animals were analyzed.