Journal J. Comp. Physiol. 135, 259-268 (1980) of Comparative Physiology, A by Springer-Verlag 1980 Insect Disturbance Stridulation: Characterization of Airborne and Vibrational Components of the Sound W. Mitchell Masters* Section of Neurobiology and Behavior, Cornell University, Ithaca, New York 14853, USA Accepted October 18, 1979 Summary. Some insects stridulate when attacked by 1 kHz. The average decrease in power above this fre- a predator. This behavior has been interpreted as quency is about 12 dB/octave. The maximum peak-to- a defensive response, the sound being a warning to peak amplitude of cuticular motion is about 1 to predators of the insect's noxiousness. Since to humans 10 gm. many such disturbance sounds are audibly similar, These common characteristics may lead predators it is possible that they may in fact be mutually mi- to treat insects producing disturbance sounds simi- metic. This idea was investigated through analysis larly, although this possibility should be tested empiri- of the temporal and spectral characteristics of the cally. If acoustic mimicry exists, the communicatory disturbance sounds of a variety of insects that stridu- interchange between predator and prey may be subtler late by a file-and-scraper device. Properties of both than is commonly appreciated. the airborne sound and the underlying cuticular vibra- tion (detected by a special vibration measuring instru- ment) were examined, and four characteristic features found: Introduction 1. The temporal pattern is simple. Bursts of tooth- strike impulses are about 80 ms long, and are sepa- When insects are handled or restrained, some respond rated by pauses about 90 ms long. Bursts occur at by emitting sounds, variously known as disturbance, a rate of about 5 to 10/s. distress, alarm or warning cries. Alexander (1967), 2. The temporal pattern is irregular. For tooth- in his classification of insect acoustic signals, reported strike interval, burst duration, pause duration and that more examples fell into the category of distur- interburst interval, the standard deviation is usually bance sounds than into any other. Yet, despite the > 30% of the mean. Much of the irregularity is pre- great number of disturbance sounds known, their sumably caused by the insect struggling at the same function has not been well established. They have time it stridulates. Some insects show less variability, been thought to aid insects when they are attacked and these appear to lack tight coupling between stri- by predators (see Haskell, 1961), either because the dulatory movements and struggling movements, so sound startles the attacker and causes it to hesitate struggling does not interfere with stridulation. momentarily, thus increasing the chance of the in- 3. The airborne sound pressure waveform is im- sect's escape, or because the sound is a warning, in pulsive. The frequency coverage of the sounds is quite the manner of an aposematic visual signal, that the broad with an average 10-dB bandwidth of about insect has an effective and perhaps punishing defense. 40 kHz centered at 25 kHz. The sounds are not in- Only recently have experimental studies shown that tense, ranging from about 10 to 60 dB (re disturbance sounds apparently do deter some preda- 20x 10 .6 Pa) at 10 cm. tors (Bauer, 1976; Sandow and Bailey, 1978; Smith 4. The cuticular vibration waveform is sharply and Langley, 1978; Masters, 1979a). peaked and contains maximum energy at a frequency To the human ear, many insect disturbance sounds determined by the tooth-strike rate, usually about are remarkably similar. This raises the interesting pos- sibility that predators also perceive these sounds as * Present address: Fakult/it ffir Biologie, Universit/it Konstanz, being alike, and that disturbance sounds might be Postfach 5560, D-7750 Konstanz, Federal Republic of Germany the functional mode linking a large mimetic assem- 0340-7594/80/0135/0259/$02.00 260 W.M. Masters: Insect Disturbance Stridulation blage. The advantage to members of such an assem- bined frequency response of the microphone-amplifier-tape record- blage, supposing that disturbance sounds are sematic, er combination was approximately +2.5 dB over the range 0.5 would be much the same as for visual mimicry (see to 100 kHz. Sound recordings were made inside a sound attenuating room Edmunds, 1974): members could share the burden at a temperature of 22,+ 2 ~ In order to reduce sound reflections of educating predators, and at the same time simplify inside the room, urethane foam pads (about 2 x 1 x 0.1 m) were the learning task required of predators by standardiz- hung approximately 10 cm from the walls and ceiling. To reduce ing the warning stimulus. However, the idea of acous- reflections at high frequencies still further, wedgeshaped fingers of foam (base 4x2.5 cm, height 10 cm) were glued to the inside tic mimicry requires more rigorous substantiation of a corner cube formed by three sides of a corrugated cardboard than subjective appraisal by the unaided ear. In this box 80 cm on a side. This corner was placed in front of the ring- paper I analyse the physical characteristics, both air- stand mounted microphone at a distance of about 1 m. For sound borne and vibrational, of the disturbance sounds of recording, insects were held in front of the microphone by their a number of insects. The study is restricted to those legs, either by hand or with forceps, at measured distances marked on a 1 mm diameter wire extending out from the microphone. insects that use a file-and-scraper mechanism, proba- Insect surface vibration was measured using a non-contact, bly the most common method of sound production optical technique (Masters, 1979b). The vibration response of the among insects (Dumortier, 1963 a). The results of the instrument was _+ 1.5 dB between 10 Hz and 120 kHz. The signal analysis suggest that, generally, disturbance sounds and calibration outputs of the vibration detector were recorded on the two channels of a stereo tape recorder (Uher 4400 Report possess several characteristics in common, which to- Stereo) having a frequency response of _+ 1.5 dB from 20 Hz to gether might identify the sounds. This might lead 18 kHz, down 5.5 dB at 20 kHz. Usually vibration at only one predators to treat the sounds, and the insects that point on the cuticle was measured for each insect. On beetles produce them, similarly. the point was on one wing cover, whereas on mutillids it was in the middle of the second abdominal tergite, to which the scraper is attached. Setae around the point of vibration measurement were scraped off so that they would not interfere with light reflection from the cuticle. In order to record vibrations from those insects Materials and Methods with non-shiny cuticle, it was usually necessary to stick a small (~ 1 mm 2) piece of aluminized mylar (video splicing tape) to the The insects used in this study were collected at the Archbold Biolog- point on the insect which was to be examined. This increased ical Station, 12 km south of Lake Placid, Florida, during the sum- the sensitivity of the technique (Masters, 1979b), and for some mers of 1976 and 1977. One or more species from the families insects it was the only way to obtain an adequate signal-to-noise Reduviidae, Cydnidae, Carabidae, Hydrophylidae, Passalidae, Sca- ratio. Addition of the tape should have had negligible influence rabidae, Cerambycidae and Mutillidae were used, with the majority on the vibration of the cuticle, since the mass of the tape was of the specimens being cerambycids or mutillids. Most insects were small (24 l.tg/mm2) and its stiffness much less than that of the taken at light traps at night, but velvet ants (Mutillidae) were cuticle. collected during the day as they wandered over sandy, cleared areas. Temporal Analysis. Disturbance sounds produced by insects having The files and scrapers of representative insects were investi- file-and-scraper devices are generally structured as a series of im- gated using a scanning electron microscope (AMR-1000A, Ad- pulses, called tooth strikes, each produced by impact of the scraper vanced Metals Research, Bedford, MA). The specimens were dried against a ridge, or tooth, of the file. A single pass of the scraper by the critical point method (Samdri PVT-3, Tousims Research along the file (or some portion of it) gives rise to a series of Corp., Rockville, MD) and sputter coated with palladium-gold tooth strikes which I call a burst. Between bursts are pauses caused (Samsputter 2A, Tousims Research). In mutillids the file is located when the file stops or reverses its motion, or when the file and ventromedially on the third abdominal tergite. The scraper is on scraper are disengaged. (In some cases the file is pulled across the caudal edge of the second tergite and lies over the central the scraper rather than vice versa, but throughout this paper I portion of the file. The third abdominal segment fits partially will assume that the file is stationary and the scraper is pulled inside the second and during stridulation it is moved in and out across it.) The interval from the beginning of one burst to the of the second segment like a piston in a cylinder. The file of beginning of the next I call the interburst interval and its reciprocal cerambycids is located ventromedially on the anterior portion of the burst rate. the mesothorax just forward of the points of attachment of the For temporal analysis of disturbance sounds, the tooth-strike elytra. The scraper is found on the caudal edge of the prothorax rate, burst duration, pause duration and interburst interval were and lies over the file.