J. exp. Biol. 130, 107-119 (1987) 107Printed in Great Britain © The Company of Biologists Limited 1987

THE EFFICIENCY OF SOUND PRODUCTION IN TWOCRICKET 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 Teleogrylluscommodus (Walter) (Gryllidae) produced their calling songs while confined inrespirometers.

2. G. australis males used oxygen during calling at a mean rate of4-637 ml O2h^', equivalent to 27-65mW of metabolic energy, which was 13 timeshigher than the resting metabolic rate. T. commodus males used oxygen duringcalling at a rate of 0-728 ml O2h~', equivalent to 4-34mW, which was four times theresting metabolic rate.

3. The sound field during calling by males represents a sound power output of0-27 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 and0-05 % for males of T. commodus. Comparison with other insect species suggests thatnone is more than a few percent efficient in sound production.

INTRODUCTION

Many insect species produce stereotyped acoustic signals that are important inintraspecific communication. In most species that communicate by sound, the male'scalling song, which seems to attract conspecific females, is the most obvious and themost important component of the repertoire.

Production of the calling song will involve a cost to the producer in the form of anincreased use of metabolic energy. The energy required for sound production hasbeen measured for a few insect species (Stevens & Josephson, 1977; Mac Nally &Young, 1981; Prestwich & Walker, 1981). Some part of the energy used duringcalling is converted into the acoustic energy contained in the call. The efficiency ofsound production can be estimated by comparing the amount of energy used duringcalling with the amount of sound power produced. Few such estimates of theefficiency of sound production in insects have been performed (Counter, 1977; MacNally & 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 cricketGryllotalpa 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 closingstroke (Michelsen & Nocke, 1974; Bennet-Clark, 1975). A hardened area (thescraper) 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 teethgenerates vibrations that set specialized regions of the forewing into resonantoscillation. Thus, each cycle of wing closing and opening (a wing stroke) generatesone pulse of sound. In grylloid insects, the sound produced by a single wing stroke isusually termed a syllable, following Broughton (1963). The wing-stroke ratecorresponds to the repetition frequency of syllables in the call.

The two species to be used in this study were chosen because of the differencesthey 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 ofsyllables, i.e. a trill. T. commodus produces its call without the use of a burrow andthe call produced is a series of regularly repeated groups of syllables: a chirp is onegroup 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, whenthey began to call at dusk. Males were located by homing in upon their calls and werequickly dug from their burrows. T. commodus males were captured by hand atWerribee, 30 km southwest of Melbourne.

Respirometry measurements

All respirometry trials were conducted in the laboratory. The oxygen consumptionof calling males of both species was measured manometrically using a constantpressure 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 bya manometer bore. The experimental animal is placed in one of these chambers (theanimal chamber), and as it respires it consumes oxygen and gives out CO2, which isabsorbed by a small quantity of NaOH placed in each chamber. Initially the pressurein the two chambers is equal, but as the respired CO2 is absorbed, the pressure in theanimal chamber decreases. This is indicated by the movement of a coloured fluid inthe manometer bore. A micrometer with attached piston is then advanced to a levelwhich compensates for this pressure difference. The volume of air displaced by thepiston 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 sterilesoil 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 theexperimental animal.

Respirometry trials on both species followed the same protocol. The experimentalanimal was released into the animal chamber of the respirometer 24 h before the startof a trial. A Petri dish containing 45 ml of SmolP1 NaOH was placed in eachchamber 6 h before the expected time of calling. The chambers were sealed and thecomplete assembly was transferred to a water bath at 23°C.

Oxygen consumption in G. australis was measured from 5 min after calling beganuntil cessation of calling, usually a period of 20—25 min. For T. commodus, theoxygen consumption of calling males was measured for 15-20 min when the maleswere calling consistently. Resting rates were measured in the same way duringdaylight hours, when males of both species were quiescent for long periods. Therespirometers were tested regularly for leakages and faults by running trials without acricket in the animal chamber. All measurements of oxygen consumption wereconverted to standard temperature and pressure (STP).

Individuals involved in successful respirometry trials were marked and, at a laterdate, killed and fixed in alcoholic Bouin's solution. The mesothoracic musculatureactive during sound production (Bennet-Clark, 1970; Bentley & Kutsch, 1966) wasdissected 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 thefollowing method. A microphone was moved around the calling male or, in the caseof G. australis, the entrance to the male's burrow at a constant distance of 0-2 m. Theapparatus was of similar design to that of Mac Nally & Young (1981, their fig. 2), andconsisted of a semicircular rod mounted on steel spikes. The rod could be rotatedthrough 180°. The microphone was mounted on this rod with a moveable clamp. Bymoving the microphone along the rod and by moving the rod itself, a full hemisphereof 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 anglesthe sound pressure level was sampled at up to four azimuth positions, /r/2, 3JT/8, Jt/4and Ji/8, and a lateral reading was taken. The sound field was sampled on only oneside of each male (Mac Nally & Young, 1981, their fig. 3).

Sound pressure levels around calling G. australis males were measured using aBruel & Kjaer Type 4131 microphone connected to a Bruel & Kjaer Type 2203 soundpressure 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 onG. australis were made in the field. Soil temperatures were between 18 and20°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 both20 and 80cm directly above the burrow mouth.

For T. commodus a Bruel & Kjaer Type 2230 sound pressure level meter was usedwith a Bruel & Kjaer Type 4155 microphone and a 3-m extension lead. This soundlevel meter was used in the Leq mode, which records the time-weighted average of aseries 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 acousticenergy as the fluctuating (chirped) signal being recorded. A period of 20—30s wasfound to be sufficient to give a stable level for T. commodus. All the Bruel & Kjaerequipment was calibrated with a Bruel & Kjaer Type 4230 sound level calibrator.

The sound output of T. commodus males was measured in the laboratory, becauseof the need for partial restraint. Males were placed in small cages of stainless steelmesh (5x5x5 cm; 2mm mesh). Measurements were made when the male calledconsistently while standing on the floor of the cage. The microphone was moved tothe exact position required. The cages were placed on a tray of damp soil 3 cm deep.The temperature was 21-5 ± 1-5°C and background noise in the room was below60dB.

Additional males of both species were used to measure peak and RMS soundpressure levels 20 cm dorsal to the calling male. The measurements were made withthe Bruel & Kjaer Type 2230 sound level meter. Similar measurements were alsomade on synthesized G. australis calls. Signals were synthesized with the TektronixFG501 function generator and a homemade synthesizer. They were broadcastthrough a Phillips Dome Tweeter (AD01610T8) using a Toshiba SB-M30 amplifier.Signals were produced with different duty cycles and measurements were made ofthe difference between peak and RMS values for the same signal for a range of dutycycles.

RESULTS

Sound production in Gryllotalpa australis

Males of G. australis produce a loud calling song from specialized burrows similarto those constructed by G. vineae (Bennet-Clark, 1970). The calling song ofG. australis is a loud trill produced for 20—30min at dusk on summer nights, whenthe soil temperature is above 15°C. At 23°C the call has a carrier frequency of2-54 kHz and a pulse repetition frequency of about 70 Hz.

Respirometry

G. australis males constructed their specialized burrows in the respirometers.Most burrow construction took place in the 2 h immediately before the malecommenced calling. From 45 respirometry trials, eight successful measurements ofoxygen consumption during calling and 11 measurements of oxygen consumption atrest were made. The mean rate of oxygen consumption for calling males was4-637 ml O2h~' (s.E. = 0-258). This was an increase of about 13 times the mean

Efficiency of cricket sound production 111

resting rate of 0-345 ml 0 2 h ' (s.E. = 0-017). Mass-specific rates of oxygen con-sumption were calculated from both total body mass and the mass of muscle involvedin sound production. The mean rates for calling males were 5-303 ml C^h"1 gbodymass ' (S.E. = 0-307) and 117-66mlO2h~'gmuscle"

1 (S.E. = 6-266).The oxygen consumption rates of resting and calling males were converted to

energy equivalents using the oxycalorific conversion factor 19-796 J ml C^"1 at sir(Elliot & Davison, 1975). The energy required for production of the calling song wasthe total energy used during calling minus the metabolic (resting) rate. ForG. australis this was 27-65-2-06 = 25-59mW.

The sound field and power output

Four successful measurements of the sound field were made to the right side of themale's burrow and four to the left. The sound field was found to be symmetricalaround the saggital plane, and so no distinction was made between measurementsfrom the right and left sides. Measurements on six males at 80 cm confirmed thatthere were no near-field effects associated with measuring at 20 cm from the mouth ofthe burrow. Differences between the sound pressure levels at the two positions were11-9 ± 0-3 dB, which conforms to the inverse square law.

Means and standard errors of sound pressure levels for each sampling positionaround calling males are presented in Table 1. These values were used to reconstructthe shape of the sound field around the burrow of a calling G. australis male (Fig. 1).The shape of the sound field in this species is similar to that described for G. vineae(Bennet-Clark, 1970) except that proportionately more of the sound produced isprojected posteriorly in G. australis. The differences in the shape of the sound fieldbetween these two species may arise from differences in the design of the burrow.

90 dB

Reconstruction of the sound field around the burrow of a calling Gryllotalpaauslralis male. (A) The dorsal aspect of the horizontal plane; (B) the lateral aspect of thesaggital plane. The solid circles and bars are the means and standard errors of soundpressure levels from Table 1. The concentric rings represent sound pressure level on alinear scale. A, anterior; L, lateral; D, dorsal; P, posterior.

Tab

le 1

. So

und

leve

ls a

roun

d ca

lling

Gry

llota

lpa

aust

rali

s m

ales

42

3n

/8

S.E

. S

.E.

4 4 S

.E.

48

S.E

. X

-

+ X

+ X

+ Y

-

-

-

+ A

nter

ior

85.1

1.

1 0.

9 84

.8

1-2

1.0

84.3

1.

1 1.

0 83

.4

1.0

0.9

45'

ante

rior

87

.5

0.9

0.5

86.7

0.

8 0.

8 86

.0

0.8

0.7

85.6

0.

9 0.

7 D

orsa

l 89

.8

0.8

0.8

89.4

0.

9 0.

8 88

.7

0.9

0.9

86.8

1.

1 1.

0 45

" po

ster

ior

89.4

0.

8 0.

7 88

.6

0.8

0.8

87.8

0.

8 0.

7 87

.0

0.9

0.8

Post

erio

r 87

-6

0.8

0.7

87.1

0.

9 0.

8 86

.6

0.9

0.7

85.7

0.

8 0.

7 L

ater

al

85.9

2.

5 1.

9

Sou

nd p

ress

ure

leve

ls a

re g

iven

in

dB.

As

the

dB s

cale

for

sou

nd p

ress

ure

leve

ls is

rel

ativ

e, m

eans

and

sta

ndar

d er

rors

wer

e ca

lcul

ated

aft

er c

on

ver

tin

g S

PL

mea

sure

men

ts to

abs

olut

e un

its

of s

ound

pre

ssur

e. T

he

mea

ns a

nd s

tand

ard

erro

rs o

f so

und

pres

sure

wer

e th

en c

onve

rted

bac

k to

the

soun

d pr

essu

re l

evel

s sh

own.

Bec

ause

all

conv

ersi

ons

are

done

in

rela

tion

to

a se

t re

fere

nce

valu

e, t

he s

tand

ard

erro

rs a

re a

sym

met

rica

l ab

out

the

mea

n.

Efficiency of cricket sound production 113

G. australis males construct a burrow which has four openings to the surface,whereas the G. vineae burrow has only two openings.

The sound power output of calling males was calculated from the formula:

P = 2jrr2[antilog(SPL- 120)/10]

(Olson, 1957), where P is the amount of acoustic power passing through ahemisphere of radius r. In this case, r was 0-2 m. The power was calculated for eachsampling point of each male (note that the 3JI/8, JT/4, JI/8 and lateral positions arecounted twice), and the power output was the mean of all these values. ForG. australis the mean sound power output was 0-27 mW (S.E. = 0-001).

In G. australis, calling involved the use of 25-59mW of metabolic energy toproduce a sound power output of 0-27 mW. Thus, the efficiency of males of thisspecies at converting metabolic to acoustic energy was 0-27/25-59X 100 = 1-05 %.

That the measures of sound output used in determining this overall efficiencyrating, i.e. slow RMS levels, were true integrations over time of the sound producedwas confirmed by measuring both peak and RMS levels on a number of G. australismales and on synthesized G. australis calls. The RMS level of an unmodulated orpure tone is, by definition, half of the peak level, i.e. 3 dB less than the peak level. Asthe call of G. australis is a pure tone divided into pulses, a true RMS value should beproportional to the 'on time' of the pulsed signal, i.e. to the duty cycle. For a dutycycle of 50 %, the RMS level would be half the RMS level for a pure tone of the sameamplitude, i.e. 6dB less. This would give an RMS level 9dB below the correspond-ing peak level. The duty cycle of G. australis males for temperatures between 18 and25°C varies between 48 and 70% (M. W. Kavanagh & S.-A. Tagney, in prep-aration). This should give differences between peak and RMS levels of between 9-4and 6-1 dB. Fig. 2 shows a plot of synthesized G. australis calls with different dutycycles and the difference between peak and RMS levels recorded for these signals.From Fig. 2 duty cycles of 48—70% gave differences between peak and RMS levelsof between 9-3 and 6-7 dB, which fits the predictions. Measurements on the calls ofG. australis males revealed differences between peak and RMS levels of between 8-0and 111 dB (peak levels; 98-7±l-0dB, N = 15). These differences are slightlyhigher than predicted, but this is explained by the shape of the pulses in the call ofsome G. australis males. Some males produce pulses with long attacks and decays,which reduces the effective duty cycle by up to 10-15 %.

Sound production in Teleogryllus commodus

Males of T. commodus produce a complex calling song that contains two types ofchirps, as described previously (Leroy, 1966; Hill, Loftus-Hills & Gartside, 1972).For the purposes of the present study, the call of T. commodus at 23 °C is assumed toconsist of one complex chirp of five high-amplitude pulses with a repetitionfrequency of 15 Hz, and 12 smaller pulses at 25 Hz followed by a simple chirp of 12small pulses, also at 25 Hz. This sequence of two chirps is repeated for the durationof the call at a rate of 30min-1. The carrier frequency of the call is 3-8 kHz at 23 °C(Hill, 1974).

114 M. W. KAVANAGH

100

80

60

20

8 10Peak-RMS (dB, SPL)

12 14

Fig. 2. The difference between peak and root mean square (RMS) sound pressure levelsfor synthesized Gryllotalpa austrails calls plotted against the duty cycle of the call. Linearregression analysis of the data gave the line of best fit y = — 8-4x +1266 and a correlationcoefficient of r = —0-96.

Respirometry

From 35 respirometry trials, eight successful measurements of oxygen consump-tion during calling were obtained. Eleven measurements of oxygen consumption forresting males were also made. The mean rate of oxygen consumption in calling maleswas 0-728 ml C^h"1 (S.E. = 0-048). This was nearly four times the resting rate of0-187 ml Ozh"1 (s.E. = 0-012). Mass-specific rates were calculated as for G. austra-lis. For calling males the mass-specific rates were 1-209 ml O2 h~' g body mass"

1

(S.E. =0-048) and 76-652mlO2h~1gmuscle~1 (s.E. = 3-949). The energy used

during sound production by T. commodus was calculated by using the same methodas for G. australis and yielded a value of 3-22mW.

Power output

Three successful measurements of the sound field were made to the right of a maleand two to the left. The sound field was found to be symmetrical. Table 2 shows themeans and ranges of sound pressure levels around calling T. commodus males. Areconstruction of the sound field was not attempted due to the low number of SPLmeasurements obtained for this more mobile species. The sound power output ofT. commodus was calculated as for G. australis. The mean sound power output wasl-51XlO~3mW (S.E. = 2-5x10-4). Therefore, T. commodus males are 1-51X10"3/3-22X 100 = 0-05 % efficient at converting metabolic to acoustic energy.

Measurements of peak and RMS levels on T. commodus males revealed that, aswith G. australis the RMS levels used were good measures of the average sound

Efficiency of cricket sound production 115

Table 2. Sound levels around calling Teleogryllus commodus males

Anterior45° anteriorDorsal45° posteriorPosteriorLateral

Sound pressure levels

X

70-468-969-667-566-964-8

are given in

S.E.-

2-01-72-01-81-52-2

dB.

+1-7

• 4

•5

1-51-3•8

X

68-565-967065-966-2

S.E.-

2-21-72-21-41-3

+1-71-51-81-211

output. Peak levels for T. commodus males were 8-8-11-5 dB above the fast RMSlevels (peak levels, x = 83-3 ± 1-1 dB, A*= 14). However, in the case of T. com-modus, which produces a chirped call, a series of these fast RMS levels was averagedover a longer period to give a level for the call as a whole. These Leq values were usedin all calculations.

DISCUSSION

The cost of sound productionSound production in males of G. australis and T. commodus incurs a considerable

cost in the form of increased metabolic expenditure. Table 3 compares the increasesin oxygen consumption rate observed in these two species with those found in otherspecies of sound-producing insect. All the other species in Table 3 produce trilledcalls. As noted earlier, G. australis produces a trilled call, and its mass-specificoxygen consumption rate is consistent with the comparatively high rates recordedfrom the other trilling species. T. commodus produces a chirped call, and shows acomparatively low rate of oxygen consumption during calling.

The cost of sound production depends on many inter-related factors, the most

accessible of which is the wing-stroke rate (Prestwich & Walker, 1981). A high wing-

Table 3. Mass-specific rates of oxygen consumption for several insect species

Species

Oxygen consumption rates(mlO2h"'g (mlO2h~'g

body mass ) muscle"')Calling Resting Calling Resting Source

Gryllotalpa australisTeleogryllus commodusAnurogryllus arboreusOecanthus celerinictusOecanthus quadripunctatusCystosoma saundersii

Xeoconocephalus robustusEuconocephalus nasulus

5-3031-2093-8913-3683-8186-281

15-8018-40

0-4200-3090-3150-4370-4760-301

1-922-61

117-6676-65———

97-80

90-099-0

8-451918

———4-20

——

This paperThis paperPrestwich & Walker (1981)Prestwich & Walker (1981)Prestwich & Walker (1981)Mac Nally & Young (1981);

Mac Nally&Doolan (1982)Stevens & Josephson (1977)Stevens & Josephson (1977)

116 M. W. KAVANAGH

Table 4. Mass-specific oxygen consumption per wing stroke for several insect species

Species

Wing-strokerate

(min"1) gbodymass"') gmuscle

Oxygen consumption rates1 1

Source

Gryllotalpa auslralisTeleogryllus commodusAnurogryllus arboreusOecanthus celerinictusOecanthus quadripunctatusCystosoma saundersii

Neoconocephalus robustusEuconocephalus nasutus

•See text.fNumber of tymbal muscle contractions per minute.

4200Chirp*4440342022802520t

114009600

2-lOx2-32x1-46X1-64X2-79x4-15X

2-69X2-97X

10"5

lO-5

10-5lO-5

lO-5

lO-5

ID"5

ID"5

4-1-

6-

1-1-

67X10"4

47X10"3

—

47X10"1

49X10"4

72X10-4

This paperThis paperPrestwich & Walker (1981)Prestwich & Walker (1981)Prestwich & Walker (1981)MacNally & Young (1981);

Mac Nally&Doolan (1982)Stevens & Josephson (1977)Stevens & Josephson (1977)

stroke rate requires a greater amount of metabolic energy, and so variation in wing-stroke rate may explain some of the variation in oxygen consumption rates for thespecies in Table 3. Oxygen consumption rates per wing stroke for the same group ofsound-producing insects are displayed in Table 4. For the trilling species, the rate ofoxygen consumption per wing stroke is calculated using the formula from Prestwich& Walker (1981):

VO2WS-' = Vo^WStime"1,

where WS is wing-stroke rate. For T. commodus, the chirping species, the followingformula, also from Prestwich & Walker (1981), is used:

Vo2WS"' = VO2/(WS chirp"1) X (chirp time-').

Both formulae assume that the cost per wing stroke is constant and that the cost ofmaintaining the forewings raised is negligible (Prestwich & Walker, 1981).

The cost of calling expressed as oxygen consumption per wing stroke is similar forseveral species when the calculations are based on rates of oxygen consumption pergram body mass (Table 4). However, these similarities disappear when the mass ofsound-producing muscle is used. As the total body mass includes many structuresnot directly responsible for sound production (e.g. reproductive structures), thismay not be as reliable a measure as the mass of sound-producing muscle. Therefore,there is still a degree of variation in the cost of sound production that is not directlydependent on the wing-stroke rate. This variation may arise from several sources,such as differences in the structure of the file teeth (density, depth, rigidity andpitch), which will affect the amount of force that must be produced to overcome theresistance of these teeth (Prestwich & Walker, 1981).

While there is variation in the cost of sound production between insect species, allthe recorded values fall within a fairly narrow range. The range of oxygenconsumpion rates per wing stroke for sound-producing insect species overlaps with

Efficiency of cricket sound production 117

the range of oxygen consumption rates per wing beat for flying insects (Stevens &Josephson, 1977).

The efficiency of sound production

Neither G. australis nor T. commodus males are efficient at converting metabolicenergy used during calling into sound output (1-05% and 0-05% efficient,respectively). There are only two other direct measures of the efficiency of soundproduction in insects available to compare with the results obtained here. Mac Nally& Young (1981) calculated an efficiency of 0-82 % for the bladder cicada, Cvstosomasaundersii while K. N. Prestwich (personal communication) has calculated a value of0-23 % for the gryttid Anumgryllus arboreus. The similarity of these values indicatesthat sound production is an inefficient process in the species of insects studied so far.

While there are no further direct measures of efficiency in insects, Bennet-Clark(1970) has estimated the efficiency of the conversion of mechanical energy generatedby sound-producing muscle to sound power output for two species of mole crickets.Since Bennet-Clark (1970) gives the wing-stroke rates (i.e. pulse repetition fre-quencies) and the masses of the sound-producing muscles (i.e. wing closers andopeners) for G. vineae and G. gryllotalpa, we can estimate the probable energeticcost of calling in these two species if we assume that they use oxygen during calling atthe same rate per wing stroke per gram muscle as does G. australis. Using thesefigures it is estimated that the amount of energy used by G. vineae during calling is37-94 mW. A resting rate can be derived from the resting rate per gram muscle foundfor G. australis and correcting for the muscle mass of G. vineae. This gives a restingrate for G. vineae equivalent to 2-72 mW. Thus, the increase in metabolic energyused during calling by G. vineae is estimated to be about 35-22 mW. As Bennet-Clark(1970) has measured the sound output in these two species, we can also estimate theoverall efficiency of calling in G. vineae and G. gryllotalpa. The sound power outputof G. vineae is l-2mW (Bennet-Clark, 1970) and thus the efficiency of this species isprobably about 1-2/35-22X 100 = 3-41 %. Similar calculations for G. gryllotalpa givean efficiency of 0-5 %. If these calculations are based on oxygen consumption rates ofG. australis per gram body mass, rather than per gram muscle mass, the valuesobtained are 1-36% efficiency for G. vineae and 0-06% for G. gryllotalpa. Mac Nally& Young (1981) have also estimated an overall efficiency for G. vineae by a differentmethod, arriving at a value of 5 %.

These estimates for G. vineae and G. gryllotalpa illustrate two points whencompared with the calculated efficiency of G. australis. First, even in the mostefficient of these species, G. vineae, the efficiency of converting metabolic to acousticenergy is still low. Second, there are quite large differences in the efficiencies ofclosely related species.

The efficiency of sound production calculated for G. australis and those estimatedfor G. vineae and G. gryllotalpa are higher than those calculated for the gryllidsT. commodus and A. arboreus. The gryllid values are also lower than the 0-82%calculated for the cicada C. saundersii. As already mentioned, the efficiency of soundproduction in mole crickets is improved by the use of specialized burrows.

118 M. W. KAVANAGH

C. saundersii employs a similar strategy. A large, air-filled cavity in the abdomen actsas a large resonant sound radiator which improves sound radiation and, hence,overall efficiency (Mac Nally & Young, 1981; Alexander, 1983). T. commodus andA. arboreus use no such devices to improve the effectiveness of sound radiation andthus, in this respect, would be expected to be less efficient producers of sound.

The measurements of efficiency calculated for G. australis and T. commodusmales may be slight underestimates of the true value due to some absorption of thesound output by the substrate. However, in the case of the mole cricket, the shapeand function of the burrow makes it unlikely that much sound is lost to the soil ordown the burrow (Bennet-Clark, 1970). T. commodus males do not have the benefitof a burrow to direct their sound output, so it is possible that some of the soundoutput was absorbed by the soil. However, even if only half the sound output wasmeasured, which seems very unlikely, the overall efficiency of converting metabolicto acoustic energy would still be only 0-18%. This inflated value remains much lessthan those for G. australis (this paper) or C. saundersii (Mac Nally & Young, 1981).

It should be noted that these estimates of the efficiency of sound productionobtained for G. australis and T. commodus are confined to energy transfer during thebrief periods when the animals are actually calling. Obviously additional factorswould need to be taken into account to estimate the cost of calling as a fraction of theanimals' total energy budget. In particular, the construction of a burrow by males ofG. australis is likely to be an energetically expensive activity, which would add to thecost of sound production. Although T. commodus does not expend energy onconstructing a burrow, it does call for much longer periods than G. australis and thiswould add to the cost of sound production. Hence it might turn out that soundproduction involves a similar proportion of the total energy budget in these twospecies in spite of their different calling strategies.

I wish to thank Drs David Young, Ralph Mac Nally and Jane Doolan for their helpand for critically reading this manuscript, also Dr Murray Littlejohn and Bill Hopperfor helpful discussions on sound measurement. My thanks go to the management andstaff of the Royal Botanic Gardens, Melbourne. Financial support for this work wasprovided by a grant to Dr David Young from the Faculty of Science, University ofMelbourne. The author was in receipt of a Commonwealth Postgraduate ResearchAward.

REFERENCES

ALEXANDER, R. M C N . (1983). Animal Mechanics. Oxford: Blackwell.BENNET-CLARK, H. C. (1970). The mechanism and efficiency of sound production in mole

crickets. J. exp. Biol. 52, 619-652.BENNET-CLARK, H. C. (1975). Sound production in insects. Scienl. Prog., Oxford 62, 263-283.BENTLEY, D. R. & KUTSCH, W. (1966). The neuromuscular mechanism of stridulation in crickets

(Orthoptera: Gryllidae). J. exp. Biol. 45, 151-164.BROUGHTON, W. B. (1963). Methods in bioacoustic terminology. In Acoustic Behaviour ofAnimals

(ed. R. G. Busnel), pp. 3-24. Amsterdam: Elsevier.COUNTER, S. A., JR (1977). Bioacoustics and neurobiology of communication in the tettigoniid

Xeoconocephalus robustus.J. Insect Physiol. 23, 993-1008.

Efficiency of cricket sound production 119

DAVIES, P. S. (1966). A constant pressure respirometer for medium sized animals. Oikos 17,108-112.

ELLIOT, J. M. & DAVISON, W. (1975). Energy equivalents of oxygen consumption in animalenergetics. Oecologia 19, 195-201.

HILL, K. G. (1974). Acoustic communication in the Australian field crickets Teleogrylluscommodus and T. oceanicus (Orthoptera: Gryllidae). Ph.D. thesis, University of Melbourne.

HILL, K. G., LOFTUS-HILLS, J. J. & GARTSIDE, D. F. (1972). Pre-mating isolation between theAustralian field crickets Teleogryllus commodus and T. oceanicus (Orthoptera: Gryllidae). Aust.J . Zoo/. 20, 153-163.

LEROY, Y. (1966). Signaux acoustique, comportement et syst£matique de quelques especes degryllides (Orthopteres, Ensiferes). Bull. Biol. Fr. Belg. 100, 1-134.

MAC NALLY, R. C. & DOOLAN, J. M. (1982). Comparative reproductive energetics of the sexes inthe cicada Cystosoma saundersii. Oikos 39, 179-186.

MAC NALLY, R. & YOUNG, D. (1981). Song energetics of the bladder cicada Cvstosoma saundersii.J. exp. Biol. 90, 185-196.

MICHELSEN, A. & NoCKE, H. (1974). Biophysical aspects of sound communication in insects. Adv.Insect Physiol. 10, 247-296.

OLSON, H. F. (1957). Elements in Acoustic Engineering. New York: van Nostrand.PRESTWICH, K. N. & WALKER, T. J. (1981). Energetics of singing in crickets: effects of

temperature in three trilling species (Orthoptera: Gryllidae). J . comp. Physiol. 143, 199-212.STEVENS, E. D. & JOSEPHSON, R. K. (1977). Metabolic rate and body temperature in singing

katydids. Physiol. Zool. 50, 31-42.ULAGARAJ, S. M. (1976). Sound production in mole crickets (Orthoptera: Gryllotalpidae:

Scaptenscus). Ann. ent. Soc. Am. 69, 299-306.