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The Efficiency of Sound Production in Two Cricket Species, Gryllotalpa Australis and Teleogryllus Commodus (Orthoptera: Grylloidea)

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The Efficiency of Sound Production in Two Cricket Species, Gryllotalpa Australis and Teleogryllus Commodus (Orthoptera: Grylloidea) J. exp. Biol. 130, 107-119 (1987) 107 Printed in Great Britain © The Company of Biologists Limited 1987 THE EFFICIENCY OF SOUND PRODUCTION IN TWO CRICKET SPECIES, GRYLLOTALPA AUSTRALIS AND TELEOGRYLLUS COMMODUS (ORTHOPTERA: GRYLLOIDEA) BY MARK W. KAVANAGH Department of Zoology, University of Melbourne, Parkville, Victoria, 3052, Australia Accepted 27 February 1987 SUMMARY 1. Males of Gryllotalpa australis (Erichson) (Gryllotalpidae) and Teleogryllus commodus (Walter) (Gryllidae) produced their calling songs while confined in respirometers. 2. G. australis males used oxygen during calling at a mean rate of 4-637 ml O2h^', equivalent to 27-65mW of metabolic energy, which was 13 times higher than the resting metabolic rate. T. commodus males used oxygen during calling at a rate of 0-728 ml O2h~', equivalent to 4-34mW, which was four times the resting metabolic rate. 3. The sound field during calling by males represents a sound power output of 0-27 mW for G. australis and l-51XlO~3mW for T. commodus. 4. The efficiency of sound production was 1-05% for males of G. australis and 0-05 % for males of T. commodus. Comparison with other insect species suggests that none is more than a few percent efficient in sound production. INTRODUCTION Many insect species produce stereotyped acoustic signals that are important in intraspecific communication. In most species that communicate by sound, the male's calling song, which seems to attract conspecific females, is the most obvious and the most important component of the repertoire. Production of the calling song will involve a cost to the producer in the form of an increased use of metabolic energy. The energy required for sound production has been measured for a few insect species (Stevens & Josephson, 1977; Mac Nally & Young, 1981; Prestwich & Walker, 1981). Some part of the energy used during calling is converted into the acoustic energy contained in the call. The efficiency of sound production can be estimated by comparing the amount of energy used during calling with the amount of sound power produced. Few such estimates of the efficiency of sound production in insects have been performed (Counter, 1977; Mac Nally & Young, 1981; K. N. Prestwich, personal communication). It is the aim of Key words: cricket, mole cricket, efficiency, sound production. 108 M. W. KAVANAGH this study to measure this efficiency in two grylloid species, the mole cricket Gryllotalpa australis, and the gryllid Teleogryllus commodus. In grylloids, sound is produced by stridulation. The specialized forewings (tegmina) are opened and closed rapidly with sound being generated on the closing stroke (Michelsen & Nocke, 1974; Bennet-Clark, 1975). A hardened area (the scraper) on the leading edge of one wing contacts a row of sclerotized teeth (the file) on the underside of the opposite forewing. Contact of the scraper with the file teeth generates vibrations that set specialized regions of the forewing into resonant oscillation. Thus, each cycle of wing closing and opening (a wing stroke) generates one pulse of sound. In grylloid insects, the sound produced by a single wing stroke is usually termed a syllable, following Broughton (1963). The wing-stroke rate corresponds to the repetition frequency of syllables in the call. The two species to be used in this study were chosen because of the differences they exhibit in methods of sound production and in the songs they produce. G. australis, like other mole crickets, produces its call from a specialized burrow (Bennet-Clark, 1970; Ulagaraj, 1976) and the call consists of a continuous train of syllables, i.e. a trill. T. commodus produces its call without the use of a burrow and the call produced is a series of regularly repeated groups of syllables: a chirp is one group of syllables. MATERIALS AND METHODS All insects used in these experiments were captured as adults in the field. G. australis males were captured at the Royal Botanic Gardens, Melbourne, when they began to call at dusk. Males were located by homing in upon their calls and were quickly dug from their burrows. T. commodus males were captured by hand at Werribee, 30 km southwest of Melbourne. Respirometry measurements All respirometry trials were conducted in the laboratory. The oxygen consumption of calling males of both species was measured manometrically using a constant pressure compensating respirometer after the design of Mac Nally & Young (1981, their fig. 1). In this design two sealed chambers of the same volume are connected by a manometer bore. The experimental animal is placed in one of these chambers (the animal chamber), and as it respires it consumes oxygen and gives out CO2, which is absorbed by a small quantity of NaOH placed in each chamber. Initially the pressure in the two chambers is equal, but as the respired CO2 is absorbed, the pressure in the animal chamber decreases. This is indicated by the movement of a coloured fluid in the manometer bore. A micrometer with attached piston is then advanced to a level which compensates for this pressure difference. The volume of air displaced by the piston provides a measure of the volume of respired oxygen (Davies, 1966). Two sets of respirometry chambers were used. A larger set of chambers (24 X10-5X 11 cm) was used for G. australis. These were filled with 2-0kg of sterile soil rehydrated with 375 ml of distilled water. After addition of the soil the volume of Efficiency of cricket sound production 109 air in the chambers was 1-801. A smaller set of chambers was used for T. commodus. These measured 13x10-5x11 cm and had a volume of 1-251. A piece of cardboard (portion of egg carton) was added to the small chambers to provide cover for the experimental animal. Respirometry trials on both species followed the same protocol. The experimental animal was released into the animal chamber of the respirometer 24 h before the start of a trial. A Petri dish containing 45 ml of SmolP1 NaOH was placed in each chamber 6 h before the expected time of calling. The chambers were sealed and the complete assembly was transferred to a water bath at 23°C. Oxygen consumption in G. australis was measured from 5 min after calling began until cessation of calling, usually a period of 20—25 min. For T. commodus, the oxygen consumption of calling males was measured for 15-20 min when the males were calling consistently. Resting rates were measured in the same way during daylight hours, when males of both species were quiescent for long periods. The respirometers were tested regularly for leakages and faults by running trials without a cricket in the animal chamber. All measurements of oxygen consumption were converted to standard temperature and pressure (STP). Individuals involved in successful respirometry trials were marked and, at a later date, killed and fixed in alcoholic Bouin's solution. The mesothoracic musculature active during sound production (Bennet-Clark, 1970; Bentley & Kutsch, 1966) was dissected out of the fixed animals, placed in 70% alcohol, rehydrated in saline, blotted dry and weighed. Measurements of sound output The sound output of calling males of both species was measured using the following method. A microphone was moved around the calling male or, in the case of G. australis, the entrance to the male's burrow at a constant distance of 0-2 m. The apparatus was of similar design to that of Mac Nally & Young (1981, their fig. 2), and consisted of a semicircular rod mounted on steel spikes. The rod could be rotated through 180°. The microphone was mounted on this rod with a moveable clamp. By moving the microphone along the rod and by moving the rod itself, a full hemisphere of readings could be obtained. Readings of sound pressure level were taken at five different angles of elevation: anterior, 45° anterior, dorsal, 45° posterior and posterior. For each of these angles the sound pressure level was sampled at up to four azimuth positions, /r/2, 3JT/8, Jt/4 and Ji/8, and a lateral reading was taken. The sound field was sampled on only one side of each male (Mac Nally & Young, 1981, their fig. 3). Sound pressure levels around calling G. australis males were measured using a Bruel & Kjaer Type 4131 microphone connected to a Bruel & Kjaer Type 2203 sound pressure level meter via a 2-m extension lead. Slow root mean square (RMS) levels (time constant Is) were recorded in dB re. 2xlO~5Nm~2. All measurements on G. australis were made in the field. Soil temperatures were between 18 and 20°C. Background noise (all frequencies) was below 65 dB. To check for near-field 110 M. W. KAVANAGH effects the sound pressure levels of several G. australis males were measured at both 20 and 80cm directly above the burrow mouth. For T. commodus a Bruel & Kjaer Type 2230 sound pressure level meter was used with a Bruel & Kjaer Type 4155 microphone and a 3-m extension lead. This sound level meter was used in the Leq mode, which records the time-weighted average of a series of fast RMS recordings (time constant 125 ms). This gave a level in dB re. 2xlO5Nm~2 which was the equivalent continuous level with the same acoustic energy as the fluctuating (chirped) signal being recorded. A period of 20—30s was found to be sufficient to give a stable level for T. commodus. All the Bruel & Kjaer equipment was calibrated with a Bruel & Kjaer Type 4230 sound level calibrator. The sound output of T. commodus males was measured in the laboratory, because of the need for partial restraint. Males were placed in small cages of stainless steel mesh (5x5x5 cm; 2mm mesh).
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