The warty birch caterpillar Drepana bilineata produces two distinct types of vibrational signals (mandible drumming and anal scraping) during interactions with conspecifics. Vibrational signalling is characterized using standard and high‐speed videography synchronized with laser‐doppler vibrometry, and behavioural experiments test the hypothesis that signalling functions to advertise occupancy of birch (Betula) leaves. Drumming involves raising the head and striking the leaf with the sharp edges of the open mandibles. Anal scraping involves dragging a pair of specialized oar‐shaped setae against the leaf surface. Staged encounters between leaf residents and conspecific intruders result in the resident signalling, with rates increasing as the intruder moves closer. Intruders signal significantly less often than residents. Conflicts are typically resolved within a few minutes, with the resident winning in 61% of the trials, and the intruder winning in 6%. Contests that last more than 30 min are deemed ‘ties’ and comprise the remaining 33% of trials. The results support the hypothesis that vibrational signals function to advertise leaf occupancy. Vibrational communication is believed to be widespread in Drepanoidea caterpillars, but has only been described in two species to date: D. bilineata (present study) and Drepana arcuata. It is proposed that differences in territorial behaviour and signalling between these species are related to their relative investments in silk leaf mats and shelters. The proximate and ultimate bases for the evolution of vibrational communication in caterpillars are discussed.
Drepana bilineata moths were collected from the wild at 15‐W ultraviolet collecting lights (Bioquip, Rancho Dominguez, California) between May and August 2003 and 2004 throughout the National Capital Region of Eastern Ontario, Canada, and at the Queen’s Biological Station, near Kingston, Ontario, Canada. Females oviposited on cuttings of paper birch (Betula papyrifera ) and the larvae were reared indoors under an LD 16 : 8 h photoperiod at 21–26 °C. Early‐ (first and second) and late‐ (third to fifth) instar larvae were used for life‐history and behavioural observations, morphological studies of sound producing mechanisms, and laser vibrometry recordings.
Signalling was monitored and characterized using two recording methods. The first involved recording the signals of late‐instar resident caterpillars using a videocamera and microphone during staged encounters with intruders (see below). These recordings were used to initially describe structures and movements associated with signalling, and to measure temporal characteristics of signalling. Only late‐instars were recorded using this method because early‐instar signals are not detectable with a microphone. Videos were imported to an Apple eMac (G4; Apple Corp., Cupertino, California) as Apple iMovie 3.03 files, and videoclips were saved as Apple Quicktime Pro 7.2.0 files, where sounds were subsequently extracted as ‘aiff’ files. Temporal characteristics, including signal durations, complex and bout durations, and number of signals per complex/bout, were measured using Canary or Raven Bioacoustics Research Programs (Cornell Laboratory of Ornithology, Ithaca, New York). Signal durations were obtained from 16 fourth‐ and fifth‐instar larvae that were selected at random throughout the summer from the broods of five wild‐caught females. Durations of 70 mandible drums (five drums per individual) were measured from 14 individuals, and 65 anal scrapes (five scrapes per individual) were measured from 13 individuals. Means of signal durations were calculated per individual and then used to calculate a grand mean. Using high‐speed video analysis (see below), it was found that, although the two signals sometimes occurred separately, they are most often partially overlapping. Because it is often difficult to distinguish where an anal scraping signal ends, anal scrapes were measured from the beginning of the signal to the end of the mandible drum. Temporal analysis of signal bouts and complexes were analyzed from ten residents signalling in the presence of a conspecific intruder. Bouts were defined as any combination of signals that are flanked by feeding, walking, or at least 1 s of inactivity. Complexes were defined as a combination of an anal scrape immediately followed by one or more mandible drums, or a single signal. Mean complex and bout durations, mean number of complexes per bout, and mean number of signals per complex were calculated from 18 bouts (one to three bouts per individual), comprising 104 complexes (number of complexes per bout varied).
The second method for characterizing signals employed laser‐doppler vibrometry (LDV) in conjunction with high‐speed video, and was used for describing the mechanisms and spectral components of signalling. All five instars were recorded using this method. Vibrations were recorded using a LDV (Polytec OFV 511 sensor head and OFV 3001 controller; Polytec GmbH, Germany) at the University of Toronto at Scarborough. The laser was reflected by a piece of reflective tape (approximately 1 mm2) positioned 1.5–2.5 cm from the resident caterpillar. Vibrations perpendicular to the leaf surface were measured at the location of the reflective tape. Signals were digitized (PCI‐6023E; National Instruments, Austin, Texas; 20 or 50 kHz sampling rate) simultaneously with the capture of digital high‐speed video (500 frames s−1; PCI 1000; RedLake Motionscope, San Diego, California), using Midas software (Xcitex, Cambridge, Massachusetts). All recordings were made on a vibration‐isolated table. Spectral characteristics of 17 mandible drums and 17 anal scrapes were measured from six individuals (two to three signals per individual) in Raven Bioacoustics Research Program using a 1024‐point Fourier transform (DFT, Rectangular window).