, 20130528, published 27 March 2013280 2013 Proc. R. Soc. B

Martin C. Gpfert and Randolf MenzelUwe Greggers, Gesche Koch, Viola Schmidt, Aron Drr, Amalia Floriou-Servou, David Piepenbrock,

Reception and learning of electric fields in bees

Supplementary datatml http://rspb.royalsocietypublishing.org/content/suppl/2013/03/26/rspb.2013.0528.DC1.h

"Data Supplement"

References http://rspb.royalsocietypublishing.org/content/280/1759/20130528.full.html#ref-list-1 This article cites 29 articles, 5 of which can be accessed free

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Reception and learning of electric fields

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via http://rspb.royalsocietypublishing.org.Author for correspondence:Uwe Greggers

e-mail: greggers@neurobiologie.fu-berlin.deKeywords:electric surface charge, mechanoreceptors as

sensors for electric fields, Johnston organ,Received: 27 February 2013

Accepted: 6 March 2013

Subject Areas:Cite this article: Greggers U, Koch G, SchmidtV, Durr A, Floriou-Servou A, Piepenbrock D,

Gopfert MC, Menzel R. 2013 Reception and

learning of electric fields in bees. Proc R Soc B

280: 20130528.1. IntroductionEver since Exners [1] discovery in 1895 that the feathers of birds and the hairs ofmammals store the electric charge created by friction with other materials and byair, the phenomenon of surface charge in animals and humans has been known.Heuschmann [2] added to this the observation that insect cuticle also accumulateselectric charge by friction. In insects, the wax layer of the exocuticle causes a highelectrical resistance between the inner and outer parts of the body, and along thebody surface leading to charge accumulation when insects walk or fly [35]. It isnow clear that surface charge in insects is a biological phenomenon reflecting theirmovements and the movements of their body parts [6]. It is also clear that insectsrespond to electric fields, e.g. by avoidance [7] or by modifying their locomotion[8]. However, it is less clear whether electric fields serve a biological function andwhether they are sensed by the insect itself.

Naturally occurring surface charge is thought to play a role in pollination[911] reviewed by [12], since flowers were found to be usually negativelycharged, whereas the arriving insects appear to carry a positive surfacecharge. The electric charge that accumulates on the cuticle of flying insects dis-charges only partially upon landing because of the high contact resistancebetween legs and ground. Warnke [13] in his early studies assumed a currentflow between the bees body parts and from the cuticle to the ground to explainthe effects of natural surface charge, but this may not be necessary for sensingmovements, because it could be that the cuticular structures of mechanoreceptorsare charged aswell; and therefore, theymay bemoved by the coulomb forces in anelectric field. In fact, electric fields have been used aswell-defined stimuli in phys-iological studies of mechanoreceptors in Drosophila, where electrostaticallyinduced movements of the antennal arista caused by coulomb forces were usedto probe the transduction mechanism in antennal mechanoreceptor cells

& 2013 The Author(s) Published by the Royal Society. All rights reserved.in bees

Uwe Greggers1, Gesche Koch1, Viola Schmidt1, Aron Durr1, Amalia Floriou-Servou1, David Piepenbrock2, Martin C. Gopfert2 and Randolf Menzel1

1Institut fur Biologie, AG Neurobiologie, Freie Universitat Berlin, Konigin-Luise-Strasse 28-30,14195 Berlin, Germany2Department Cellular Neurobiology, Schwann-Schleiden Centre for Molecular Cell Biology,Julia-Lermontowa-Weg 3, 37077 Gottingen, Germany

Honeybees, like other insects, accumulate electric charge in flight, and whentheir body parts are moved or rubbed together. We report that bees emitconstant and modulated electric fields when flying, landing, walking andduring the waggle dance. The electric fields emitted by dancing bees consistof low- and high-frequency components. Both components induce passiveantennal movements in stationary bees according to Coulombs law. Beeslearn both the constant and the modulated electric field components in thecontext of appetitive proboscis extension response conditioning. Using thisparadigm, we identify mechanoreceptors in both joints of the antennae assensors. Other mechanoreceptors on the bee body are potentially involvedbut are less sensitive. Using laser vibrometry, we show that the electricallycharged flagellum is moved by constant and modulated electric fields andmore strongly so if sound and electric fields interact. Recordings fromaxons of the Johnston organ document its sensitivity to electric field stimuli.Our analyses identify electric fields emanating from the surface charge ofbees as stimuli for mechanoreceptors, and as biologically relevant stimuli,which may play a role in social communication.

the calibration of the electrometer for measuring the electriccharge of the wing beat in the electronic supplementary material,

ment fed byto theUi prodrial, figbove noupplem

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on March 27, 2013rspb.royalsocietypublishing.orgDownloaded from [1417]. In these latter studies, voltages of less than or equal to4 V delivered at a distance of 0.3 mm from the arista resulted ineffective forces of less than or equal to 100 pN and displace-ments of the arista of up to ca 2 mm [14]. Given that flyinginsects accumulate surface charges of more than 100 V, electricfields emanating from the insect body may constitute biologi-cally relevant signals. In such a situation, no current needs toflow from or to the insect body. However, an essential require-ment is that the cuticular structures of the mechanoreceptorsare also charged. As one can learn from any physics textbook,electric charge on the surface of an insulator is fixed, thus itwill not move along the surface and it is rather the insulator(e.g. the cuticular structure of a mechanoreceptor) that willbe moved in the gradient of a constant electric field or by amodulated electric field.

Here, we ask what kind and under which conditionshoneybees produce electric fields, how they sense them,whether they respond to them and whether they learnthem. We deal with constant and modulated electric fieldsemanating from stationary or moving cuticle surfaces. Thus,depending on its own movement, the body of a bee is sur-rounded by both static and changing electric fields. Since

(a)

Figure 1. Measuring the electric field of a dancing bee. (a) Measuring arrangerecord the targeted animal. The dancing bee is seen in the middle surrounddancing bee. A silver wire inside the flat sheet of KCl solution is connectedKCl electrode (vertical bar) function as a coupling input capacitor. The voltagethe flat KCL electrode and the animal (see the electronic supplementary matelow-noise instrumentation amplifier has a gain of 100 to shift signal levels afrom the KCL electrode. The input capacitor Ci is 10 nF. (See the electronic swings and other body parts can also rub against each otherduring other body movements (e.g. during wing fanning asobserved in singing drosophilid flies [18]); bees and insectsin general may refresh their electric charge and emanatemodulated electric fields when walking.

To assess whether and how electric fields are produced andreceived in honeybees, we (i) measured the static and modu-lated electric fields of flying, landing and dancing bees,(ii) monitored the movement of the antennal flagellum, and(iii) associated the activity of the mechanosensory neurons ofJohnstons organ (JO). Then we asked whether bees perceiveelectric fields by exposing them to such fields under differenttest conditions. By training the bees to constant and modu-lated electric fields, we examined whether the JOs of theantennae are potentially involved in sensing the electric field.We found that bees emanate modulated electric fields whenmoving their wings and body during the waggle dance.When bees walking stationary on a treadmill were stimulatedwith the modulation patterns of these electric fields in theabsence of any mechanical stimulation they respondedfigure S2. The electric charge carried by bees arriving at the hiveentrance was measured with an electrometer array consisting ofeight electrodes (see the electronic supplementary material,figure S3a). The electrodes picked up the electric field at a distanceof 23 cm, both during the last 12 cm of flight and after landing.In all our charge measuring devices described below, we madeselectively and more strongly to the natural field patterns.Bees were also able to associate constant and modulatedelectric fields with reward. Ablation experiments show thatmechanoreceptors of the antennae are most probably involvedin sensing electric fields.

2. Material and methods(a) Measurement of the electric fields of arriving anddancing beesThe basics of physical conditions for the coupling between chargedinsulators and their effective coulomb forces are described in theelectronic supplementary material, figure S1a,b. We also describe

KCI sensor

wing

UiCs

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or the electric field. The transparent KCl electrode allows us to view and video12 attending bees. Note that the attending bees are positioned around theinput of the AC amplifier. (b) The bees body with moving wings and theuced by the wing beat of the bee depends mainly on the distance betweenures S1 and S2). The capacitor Cs is minimized to avoid input signal loss. Theise from the set-up. The input resistor Ri is set to 100 MV to avoid static loadentary material for the calibration of this recording device.)sure that there was no conductive material close by or betweenthe animal and the detector potentially influencing the signals.Similarly, the dialectic material was selected for low permittivity.

The sensor for measuring the electric fields of dancing beesconsisted of a transparent flat sensor consisting of two roundPlexiglas plates at a distance of 0.5 mm filled with a low concen-tration (0.1 molar) of an electrolyte (KCl). The sensor was locateddirectly above the respective animal in an open observation hiveand also allowed video recording of the dance communicationprocess (figure 1). The conducting fluid between the two trans-parent plates made contact via a ring-like Ag wire (diameter0.5 mm) to the input of an AC amplifier mimicking the averagediameter of the dance rounds. The location of the sensor formeasuring the electric fields of dancing bees was chosen accord-ing to the area used by the dancing bee. Such geometry of the flatelectrode was necessary for detecting the complex spatialtemporal movements of the body parts of the dancing bee. Theconcentration of KCl was kept low (0.1 M) to avoid strong electricfield reflections. The amount of electric field reflection wasmeasured by determining the strength of the electric field infront and behind the sensor and was found to be less than10 per cent for this low KCl concentration. The low-noise AC

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on March 27, 2013rspb.royalsocietypublishing.orgDownloaded from amplifier (AD624, Analog Devices, bandwidth 1 Hz10 kHz)was connected to an AD converter (CED1401, Cambridge Instru-ments). The hive was set up in a Faraday cage. As figure 1 shows,the input voltage divider based on the capacitor between animaland sensor and the input resistor was calibrated with a sine wavegenerator at the wing beat frequency. Further details of the cali-bration of the transmission of power are given in the electronicsupplementary material, figures S1 and S2. The level of poweremitted by a flying bee was found to be at least 1.78 mW at theinput resistor Ri (100 MV). The spatial coverage of the measuringdevice equalled 10 per cent of the full sphere. Note that the totalamount of emitted power from the animal is higher because theelectrode receives only a fraction of the power emanating fromthe animal.

(b) Vibrations of the antennal flagellumTo measure sound and electric field-induced vibrations of theantennal flagellum, bees were affixed ventrum down on top ofa Teflon rod with beeswax (see the electronic supplementarymaterial, figure S4). Prior to taking the measurements, thehead, wings, legs and the base of the antenna were stabilizedwith wax to minimize movement. All measurements werecarried out at room temperature (208C238C) on a vibration iso-lation table (Technical Manufacturing Corporation (Peabody,USA)). The experimental set-up has already been published[19]. In short, stimulus-induced vibrations were measured innon-loading condition at the tip of the antennal flagellumusing a PSV-400 scanning laser Doppler vibrometer with aclose-up unit (7 cm focal length) and a DD-5000 displacementdecoder (Polytec, Waldbronn). The rod holding the bee wasplaced at focal length (7 cm) from the laser Doppler vibrometer,coaxially to the direction of mechanical actuation. The position ofthe laser spot (ca 50 mm diameter) was controlled online duringthe measurements using a coaxial video system of the vibrometer(see the electronic supplementary material, figure S4). Flagellardisplacement amplitudes were determined as Fourier amplitudes(Hanning window, 1 Hz frequency resolution) at the frequencyof stimulation (for details, see [19]).

(c) Neural responses of the Johnston organIn order to measure the neural responses of the mechanoreceptorsin the Johnston organ, an antenna was cut at its base close to thehead capsule and the scapus was pushed over the tip of a shar-pened 200 mm tungsten wire acting as ground electrode. The cutend was quickly sealed and glued to the wire with UHU Bastelk-leber (without solvent, UHU GmbH, Bruhl, Germany) to preventit from drying out and rotating around the wire. Another shar-pened tungsten wire was used as an active electrode and its tipwas gently inserted into the soft cuticle between scapus and ped-icel. The signals were amplified (100) using the head stage of apreamplifier (npi electronic, Tamm, Germany). The high pass ofthe filter was set to 30 Hz and the low pass to 1 kHz. Hum noise(50 Hz) was eliminated by an additional filter (Hum Bug; Digiti-mer, Hertfordshire, UK). Neural activity was sampled at a rate of20 kHz through an analogue-to-digital converter (1401 microMKII; Cambridge Electronic Design, Cambridge, UK) and initialdata analysis was performed by SPIKE2 software (CambridgeElectronic Design), including signal storage. The flagellum wasstimulated by a Styrofoam ball that was friction-charged toabout 100 V by rubbing it with a cotton tissue or with an electrode(an exposed 3 mm wire of a shielded coax cable) at a distance of3 mm to the flagellum. Styrofoam was used because its electriccharge could be measured by mounting it on a moving stickdriven by a loudspeaker or Piezo stepper at a defined distance infront of a calibrated electrode connected to the electrometerdescribed earlier. The detector described in figure 1 was calibratedas explained in the electronic supplementary material, figures S1and S2. Using a micromanipulator, the Styrofoam ball wasslowly moved towards the flagellum (final distance of 3 mm)thus avoiding any mechanical stimulation. For modulated electricfield stimulation, a shielded cablewith an exposed end (3 mm)wasplaced at a distance of 3 mm from the flagellum. The voltagesvaried between 20 and 100 Vpp (Vpp is voltage peak-to-peak)produced by a function generator.

(d) Behavioural analysisBehavioural analyses were performed under two conditions.(i) Animals performing stationary walking on a Styrofoam ball(diameter 8 cm) floating on an air stream were stimulated withelectric and acoustic signals (see the electronic supplementarymaterial, figure S5 andvideo S1). Electric field stimulationwas per-formed via an exposed (3 mm) wire placed at a distance of 3 mmfrom the animals antennae simulating a ratio of more than 4 : 1surface area between wing and flagellum (see the electronic sup-plementary material on capacitive coupling and figure S1). Thewire was connected to a shielded cable reaching voltages between20 and 80 Vpp. Acoustic signals were broadcast via a loudspeaker(Ekulit, Piezo-AL-60P12) driven by an audio amplifier (Pioneer,aux input gain 10). The acoustic signal ranged from a sound par-ticle velocity of 0.0550 mm s21. The movement of the flagellaeinduced by the respective stimulation was recorded by a webcamera and analysed frameby-frame offline.

(ii) For training bees to electric fields, foragers were collectedat the hive entrance, cooled individually on ice and fixed to aplastic tube. After feeding to satiation with 1.25 molar sugar sol-ution, they were stored overnight in a humid chamber. The beeswere conditioned either to a static electric field or to a modulatedelectric field following the procedure developed for olfactorystimuli [20]. In the first case, a static electric field was producedby friction-charging a piece of Styrofoam to about 100 V (chargemeasurement see above) at a distance of 3 mm from the anten-nae. The bees were differentially conditioned by forwardpairing of this electric field (as the conditioned stimulus, CS)for 3 s, and then feeding sucrose solution for 3 s (1 s overlapwith electric field stimulation). Ten minutes later a similarpiece of uncharged Styrofoam was presented without sucrosereward (specifically unpaired stimulus, CS2). Three CS andthree CS2 trials were given to each bee at trial intervals of10 min. A conditioned response (CR) was recorded when theanimal extended its proboscis during the first 2 s of CS orCS2 presentation. Two hundred animals were tested. In thesecond set of experiments, the CS was a modulated electricfield (160 Vpp, 60 Hz produced by a function generator) emittedfrom a probe 3 mm above the bees antenna. Fifty-three animalswere tested. The training trials were always performed with theshortest distance to the stimulating source, and the distancevaried in extinction trials. Four distances were tested (1.0, 0.5,0.75 and 0.6 cm tests in this order, n 36). To test whether learn-ing and responding to the electric field stimulation requiredcharged antennae, we washed the antennae with water to elim-inate any surface charge. To examine whether the antennaewere involved in receiving the electric field, we either coveredthe flagellae and/or the scapus with wax, fixed the scapus tothe head or pedicel to the flagellum with a drop of wax, orremoved the antennae by cutting them at the base of thescapus. Wax was applied because of its high electrical resistance.The wax cover was uncharged when applied. A control of theeffect of wax on the charge of the antennae consisted in con-ditioning animals which managed to free the respective jointsof the antennae by the muscles in the antennae without removingthe wax layer.

Statistical analysis. We used the STATISTICA (Statsoft, Inc. Tulsa,USA) package to evaluate the data. Data presented in figures 57were analysed by ANOVA or ANOVA for repeated measure-ments. Further details are given in the legends to the figures.

time (m

Figure 2. The pattern of electric fields produced by a dancing bee at two temporal rproducted. (a)(greate

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on March 27, 2013rspb.royalsocietypublishing.orgDownloaded from ( figure 1). The high-frequency components of the full electric field pattern(a,b) Give an extended section of the middle part of the pattern as indica(not included in (c)), and (b) the corresponding high-frequency components10up

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3. Results and discussion(a) Bees as a source of electric fieldsElectric charge of bees arriving at the hive entrance results in avoltage of 0450 V as measured by a calibrated electrometerarray at a distance of 23 cm (see the electronic supplementarymaterial, figure S3b). The body charge dissipates very little onlanding (usually less than 5%) and stays high also within thehive because the relative humidity in the hive is usuallylower than outside (U. Greggers 2009, 2010, personal obser-vations: less than 60% even at greater than 90% outside), andfanning behaviour with additional dielectric charging as wellas movement inside the hive leads to an overall accumulationof charge for all bees. Dancing bees not only emit airborne sig-nals (e.g. jet streams) by vibrating the thorax and the wings[21,22] but also electric fields. We measured the electric fieldsemanating from 40 dancing bees in 300 waggle runs and ana-lysed the middle section of each straight run (450 ms) bycomparing high-speed video recordings with the time courseof the electric fields. The furthest side-wise extension of theabdomen together with the extreme position during the rollingbehaviour was located in the high-speed video stream, and thecorresponding reading of the electric signal was determined.

that all three panels have the same ordinate (electric potential in V). The trace inthree to five wing electric field beats (in (b)) can be seen for each outmost bodyto the sides (in (a)), it is in addition lowered in the outmost position compared witheffective distances to the recording electrode, leading to a modulation of the electr700 740

1000s)

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esolutions. The electric field pattern was recorded with the transparent sensored during the straight waggle phase lasting for 1700 ms are shown in (c).The low-frequency components (less than 70 Hz) of electric field patternsr than 70 Hz included in (c)). The time scales for (a,b) are the same. NoteThese analyses allowed us to relate the modulation of the elec-tric field to the movements of the abdomen and thewing beats.A typical example of body movement and electric field duringthe waggle run is shown in figure 2.

Waggling of the body consisted of up-down and side-to-side movements at a basic frequency of 16.5 Hz. Since thebodywas at a higher position (closer to the recording electrode,figure 2, up) when straight and at a lower position (furtheraway from the electrode, figure 2, L) when moved to the sideas judged by video observations from the side, wagglingresulted in a rhythmic rolling and side-to-side movement.The side-to-side movements were synchronized with buzzingof the wings, leading to an average of three to five (range18) electric pulses. These pulses originated from the wingvibrations at 230 Hz (s.e.+5 Hz, n 300) leading to a voltageof typically 200 V (s.e.+60 V, n 300) as measured at adistance of 69 mm. The interval between episodes of wingbuzzing was 29.09 ms (s.e.+1.9 ms, n 300), and the intervalbetween left/low and right/low was 30.26 ms (s.e.+2.14 ms).A comparison with the recordings of the sound signals [22]indicates similar values (typically five wing beat episodes at36 Hz with 250 Hz wing beat frequency).

(b) Electric fields induce vibration of theantennal flagellum

Dancing bees produce air flows and electric fields, both ofwhich induce vibrations of the antennal flagellum. Such

(c) relates to 12 left-right wagglings of the body. Two groups of typicallyposition (left L and right R) of each waggle. As the abdomen is swingingthe middle position (up). This combination of movements results in differentic field signal. (Online version in colour.)

Next we asked whether bees respond behaviourally to modu-

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on March 27, 2013rspb.royalsocietypublishing.orgDownloaded from flagellar vibrations were observed when we oscillated an iso-lated wing placed 2 mm from the bees head with anoscillation frequency of 40 Hz producing both air flow andelectric field. In order to compare antennal vibrations withandwithout charge on the test antenna and/or on the stimulat-ing wing, they were discharged by water. During the first fewseconds, the vibrating wing recharged allowing us to measurethe effect on a charged or discharged antenna either in

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Figure 3. (a) Vibration amplitude of the antennal flagellum as a function ofthe oscillation amplitude of an isolated wing placed at a distance of 2 mmfrom the bees head (oscillation frequency: 40 Hz, n 3 flagella, means+1s.d. See also electronic supplementary material, figure S4). Vibration ampli-tudes were measured as Fourier amplitudes at the frequency of oscillation.Black symbols: wing charged by contact to a piece of charged Styrofoam,grey symbols: wing uncharged so that the flagellum is only driven by airflow. (b) Flagellar vibration amplitude as a function of the voltage amplitudeof a 40 Hz sinusoid fed to an electrostatic probe placed at a distance of 2 mmfrom the bees head. Statistics: uncharged versus charged bodies differsignificantly (d.f. 1, d.f.error 6, F 8.15, p, 0.029, ANOVA forrepeated measures).combination or separately. Laser Doppler vibrometricmeasurements revealed that the flagellum did vibrate inresponse to air flow alone, but the vibration amplitude wasincreased by almost one order of magnitude. Under all ourtest conditions, the wings had an electric charge due to thenature of their movements (figure 3a). An exception is thewing discharged with water in figure 3a.

Electric fields sufficed to induce flagellar vibrations asconfirmed by applying electrostatic force via an electrostaticprobe (figure 3b). Hence, electric fields at frequencies as pro-duced during the waggle dance induce vibrations of theantennal flagellum whose amplitudes exceed those causedby air flows of the beating wing.

(c) Constant electric fields induce neural activity inJohnstons organ

Bees monitor vibrations of their antennal flagellum with JO, amechanosensory chordotonal organ in their second antennalsegment. JOwas found to respond electrically to flagellar displa-cements above some 10100 nm [23], which is well in the rangeof displacement amplitudes induced byoscillations of a chargedwing (figure 3a). To directly test whether electric fields elicitneural JO responses, we recorded compound action potentialsfrom the axons of the JO during stimulation with acoustic,modulated and constant electric fields (figure 4). A sharp onsetof such stimuli induced a phasictonic response. This is notlated electric field signals. We monitored the behaviour of beeswalking on a Styrofoam ball floating on air. After several hoursofwalking on the ball the animals rested, aswitnessed by a lossin muscle tonus and associated downwardmovements of theirantennae. Upon electric field stimulation with 2080 Vpp and700 ms duration, the bees transiently re-lifted their antennaeto the normal position characteristic for the awake state. Afterstimulation, the antennae reassumed their resting positionwithin some 50 s (figure 5a and electronic supplementaryseen in figure 4, because here we show the effects of stimulatingwith a chargedprobewhichwasmoved slowly in order to avoidanymechanical stimulation. Close examination of the flagellumrevealed smallmovements of the flagellum relative to the scapuswhen the charged probe was slowly approached. Stimulationwith modulated electric fields transmitted from an electricprobe induced also in-phase discharges of the JO, and thesedisappeared when the flagellum was not charged (not shown).

(d) Electric fields of different temporal structures evokebehavioural responses in resting bees

1 s

Figure 4. Compound action potentials recorded from the axons of the Johnstonorgan in the pedicel of the honeybee antenna. Two traces with different temporalresolutions are shown. The flagellum was stimulated with a charged Styrofoamball moved slowly close (2 mm) to the flagellum for the time period marked bythe black bar. No mechanical movement stimulus was involved.1 mV

1 msmaterial, figure S5 and video S1).Also acoustic stimuli were applied to resting animals

under similar conditions. The acoustic signal produced soundparticle velocities ranging from 0.05 to 50 mm s21 at 230 Hz.A threshold for antennal responses in live animals was foundat a sound particle velocity of 0.1 mm s21. Note that the passivemovement of the flagellum at threshold (see upper scale infigure 5) was several times larger than the neural responsethreshold (ca 10100 nm) measured by Tsujiuchi et al. [23].

(e) Stationary walking animals respond to biologicallyrelevant electric field patterns

If an electrode emitted greater than 100 Vpp at a distance of3 mm, the animals were aroused and started walking withactively moving antennae for a period of minutes. We usedthis response to test the efficiency of biologically relevant pat-terns of electric fields with different frequencies and temporalstructures as provided by an electrode and compared the

five animals. Each bee was tested for 10 h during which the animal wasexposed to multiple tests consisting of 10 stimulations followed by 10 min

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Figure 5. Flagellar displacement of resting animals to electric field stimuli offour different strengths (black bars). As control the same animals were exposedto similar manipulations, but electric field was switched off in every second trial(open bars). The ordinate at the base gives the flagellar displacement in milli-metres relative to the respective resting position. Four electric field stimulusstrengths of the dance pattern (figure 2) were tested (20, 40, 60 and 80 Vpp,at a distance of 3 mm to the antennae) in six animals in random order. Theaverage amplitude of flagellar displacement as estimated by laser vibrometryis given on the top scale for each stimulus strength. The significantly larger fla-gellar displacement for the two higher stimuli indicates an active componentleading to amplitudes of 1.5 mm that correspond to an angular movementof 508. The arrow marks the flagellar responses to acoustic stimuli inducedby the same dance pattern at 0.1 mm s21 (at a distance of 10 mm). Responsesfor 40, 60 and 80 Vpp are significantly different from all control groups (betweengroups: d.f. 1, d.f.error 76, F 185.82, p, 0.01 ANOVA repeatedmeasures, between stimuli: d.f. 3, d.f.error 228, F 25.25, p, 0.01ANOVA repeated measures), and the responses for 20 Vpp (post hoc comparisonresponses to control animals, which were treated equally butnot stimulated (figure 6).

Walking responses outlasted the stimulation period. There-fore, we analysedwalking activity during the intervals betweenrepetitive stimulations. The most effective electric signals werethe two biologically relevant electric signals, the dance signaland the bees stop signal, also called a beep [24,25]. The stopsignal consisted of short (130 ms) modulated pulses with adominant frequency of 400 Hz repeated at intervals of 1 s.A non-modulated signal with a constant frequency of 400 Hzwas significantly less effective than both biologically relevantsignals. These results indicate that biologically relevant electricfield signals (beep and dance signals) induce long-lastingactivity in bees walking on a floating ball, suggesting that thebees both receive and may even discriminate electric fieldswith different modulation patterns.

( f ) Bees learn to associate constant and modulatedelectric field with reward

We conditioned the proboscis extension response (PER) to elec-tric fields as the conditioned stimulus (CS) by forwardpairing it with the reward (i.e. the unconditioned stimulussucrose solution) and by exposing it to a control stimulus with-out reward (CS2) [20]. Both constant and modulated electricfields were used as CS. The constant electric field was pro-duced by rubbing pieces of Styrofoam against each other (seeabove). The CS2 was a similar uncharged piece of Styrofoam.Modulated electric field emanated from a voltage source pro-viding effective stimulus strength of 160 Vpp (60 Hz). The

C20 versus R20 n.s.).of no stimulation (a total of 60 tests per bee; see electronic supplementarymaterial, figure S5 and video S1). Four different stimuli were used, thoserecorded from a dancing bee (dance, dominant frequency 230 Hz),those recorded from bees following a dancing bee and producing a stopsignal (beep, dominant frequency 400 Hz), and a sine wave of 400 Hzwith the same intensity as that of the stop signal. The controls did notreceive any stimuli during the same sample periods in which the electricfield source was switched off. Significant differences were found for groupcomparisons with the control group and the dance and beep group (betweengroups: d.f. 1, d.f.error 3, F 8.213, p, 0.02 ANOVA repeatedmeasures, between stimuli: d.f. 3, d.f.error 15, F 7.713, p, 0.02ANOVA repeated measures) but not with the 400 Hz group (post hoccomparison 400 Hz group versus control n.s.).stimulus off45

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Figure 6. Electric field induced walking activity in animals in response tobiologically relevant electric field patterns. The electric field strength was40 Vpp at a distance of 3 mm. The ordinate gives the average walking activityinduced by the stimuli as expressed in walking speed (mm min1) forCS2 came from the same inactivated voltage source. Beeslearned to respond with PER to both CS electric field stimuli(figure 7a,b).

In both cases, the CRs increased with the number of for-ward pairings between the respective electric field as the CSand sucrose reward, whereas the response to the unrewardedstimulus (CS2, uncharged Styrofoam or inactivated currentsource) remained unaltered. We also tested whether beesstored the learned electric field stimuli in their memory andfound a significantly higher level of conditioned PER 24 hlater (n 64, d.f. 1, F 24.7, p, 0.01, ANOVA repeatedmeasures). These results demonstrate the bees sensitivity toconstant and modulated electric field and document thatbees are able to associate these cues with reward.

(g) Mechanoreceptors on the antennae are required forresponses to the learned electric field stimulus

We used the CR to test whether sensory receptors of the anten-nae are required for detecting electric field. The flagellae of theantennae carry a large number of olfactory receptors in poreplates, contact chemoreceptors in hair cells andmechanorecep-tors at the base of short hairs. Additional mechanoreceptors arelocated between the joints of the flagellum and the pedicel(JO), and between scapus and head. Bees were conditionedwith fully functioning antennae. Then the flagellae werecovered with wax so that the joints between the flagellumand the scapus or between the scapus and the head werefixed. In one group, the antennae were removed after training(figure 7c). We used wax because we wanted to ensure that

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Figure 7. Response acquisition during classical reward conditioning of con-stant or modulated electric fields. (a) Conditioning of constant electric fieldsemitted from a charged piece of Styrofoam. The response to CS (chargedStyrofoam) increased with the number of training trials (closed bars, abscissa)but did not change for the CS2 (open bars, uncharged Styrofoam).All response values to the CS are significantly different to those toCS2 (between groups: d.f. 1, d.f.error 96, F 133.67, p, 0.01ANOVA repeated measures, between stimuli: d.f. 2, d.f.error 192,F 7.713, p, 0.02 ANOVA repeated measures). (b) Classical reward con-ditioning of a modulated electric field. The response to CS increasedwith the number of training trials (closed bars, abscissa) but did notchange for the CS2 (open bars, source without voltage). The responsevalues for all three trials to the CS are significantly different to those toCS2 (between groups: d.f. 1, d.f.error 52, F 33.67, p, 0.01ANOVA repeated measures, between stimuli: d.f. 2, d.f.error 104,F 5.143, p , 0.001 ANOVA repeated measures). (c) The effect of manipu-lation of the antennae on the CR to a constant electric field as provided bycharged Styrofoam. Group 1: no preparation of the antennae (control group).Group 2: the flagellae were coated with wax. Group 3: the scapus was fixedto the head. Group 4: the antennae were fully covered with wax, but jointsbetween flagellum and pedicel (JO) were free of wax. Group 5: antennaewere fully covered with wax. Group 6: antennae were removed. Group 1 issignificantly different from all other groups: d.f. 5, d.f.error 101,F 74.5, p, 0.01 ANOVA repeated measures).

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on March 27, 2013rspb.royalsocietypublishing.orgDownloaded from 4. ConclusionIt has long been recognized that insects become electrostati-cally charged when flying or walking, or when their bodyparts are rubbed together, but little is known about whetherand how emanating fields of such charge are sensed andpotentially used as meaningful signals. Electric fields canact on mechanoreceptors of their cuticular appendages ifthese appendages are charged, and indeed such fields havebeen used to stimulate insect mechanosensory organs[15,27]. Electric fields induce responses in insects [7,8,28].A biologically relevant property of body surface charge inpollinating insects was traced to the opposite polarity oftheir body surface charge to that of flowers, potentially facil-itating pollen collection and possibly leaving a cue afterelectrical neutralization of the flowers electric field [912].It has also be noted that charged pollen powder sprayed overflowers leads to fertilization of these flowers [29], and biopesti-cides may be more effective if electrically charged [30]. Takentogether these observations indicate that electric surfacecharge is a frequently found property of plants and animalsand may play a role in pollination. However, the question ofwhether honeybees themselves receive electric fields, respondto them, learn them and possibly exploit them in their naturalhabitat, including social context has not been considered so far.

Our findings show that electric fields produced by themovement of the bees wings even at very low amplitudes, asfor example in dancing bees, inducemovements of the antennalflagellum 10 times more strongly than the mechanical stimulusof the wing alone. Similar flagellar movements can be achievedthe electric charge on the antenna is altered as little as possiblewhen interferingwith themovability and surface of the antenna.

The antennae were clearly necessary for sensing thelearned electric field stimulus. Ablating the antennae elimi-nated learned electric field responses (figure 7c, group 6).Immobilizing both joints (flagellumpedicel and scapushead; figure 7c, group 5) strongly reduced learned electricfield responses. Leaving the flagellumpedicel joint free butotherwise covering the antennae fully with wax (figure 7c,group 4) led to an almost equal reduction of learned electricfield responses. Even if only the scapus was fixed to the head(figure 7c, group 3), the learned electric field responses weresignificantly lower than in the untreated control group. If,however, both joints were left untouched but the flagellaand scapus were coated with wax (figure 7c, group 2)response reduction was still significantly lower than in theuntreated control group albeit less marked. Thus, interferingwith the movement of either or both of the joints reduced theelectric field sensitivity most strongly. We thus conclude thatmechanosensory organs associated with both joints of theantennae are most probably responsible for the ability of beesto sense electric field. However, we cannot exclude that othermechanoreceptorsmight be involved in electric field reception.The PER conditioning paradigm is known to work particula-rly effectively with conditioned stimuli that are received viathe antennae (odours, humidity, mechanosensory and thermo-sensory stimuli, [26]). Othermechanosensory receptorsmay beinvolved in electric field sensing in other contexts, e.g. wingmovement, surface probing. Future experiments need toaddress this question to gain a better understanding of theelectro-mechanical sense in insects.

by applying modulated electric fields at strengths well withinthe range of those of the bees body surface charge. Theresponse of bees differs for different modulation patternsindicating that naturally occurring electric stimuli are received.

bepallvereive

and continuous recharging owing to the friction between thebodyparts. Furthermore, fanning and friction betweendifferentbees will also keep surface charge high.

To develop a full picture of this potential information

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electrostatic forces involved in pollen transfer? Plant 19. Albert JT, Nadrowski B, Kamikouchi A, Gopfert MC.acing of. Protoc.

zel R, Fg of prellifera).037/073er WH,als in th

63, 803808. (doi:10.1016/j.elstat.2005.03.075)

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on March 27, 2013rspb.royalsocietypublishing.orgDownloaded from Cell Environ. 5, 125129.10. Fromm J, Hajirezaei M, Wilke I. 1995 The

biochemical response of electrical signaling in thereproductive system of hibiscus plants. Plant Physiol.109, 375384.

11. Vaknin Y, Gan-Mor S, Ronen B, Eisikowitch D.2000 The role of electrostatic forces in pollination.Plant Syst. Evol. 222, 133142. (doi:10.1007/BF00984099)

2006 Mechanical trDrosophila ear. Nat2006.364)

20. Bitterman ME, MenClassical conditioninhoneybees (Apis m107119. (doi:10.1

21. Michelsen A, Kirchnand vibrational signprotein function in the(doi.org/10.1038/nprot.

ietz A, Schafer S. 1983oboscis extension inJ. Comp. Psychol. 97,5-7036.97.2.107)Lindauer M. 1986 Sounde dance language of the

29. Gan-Mor S, Bechar A, Ronen B, Eisikowitch D,Vaknin Y. 2003 Electrostatic pollen applicatordevelopment and tests for almond, kiwi, date,and pistachio: an overview. Appl. Eng. Agric. 19,119124.

30. Gan-Mor S, Matthews GA. 2003 Recentdevelopments in sprayers for application ofbiopesticides: an overview. Biosyst. Eng. 84,119125. (doi:10.1016/S1537-5110(02)00277-5)20202026. (doi:10.1242/jeb.053470)9. Corbet SA, Beament J, Eisikowitch D. 1982 Are

new questions. Hear Res. 273, 713. (doi:10.1016/j.heares.2010.03.092)

Electropellancy behavior of Periplaneta americanato friction charged dieelectric surfaces. J. Electrostat.experiments point to mechanoreceptors inantennae, but other receptors on the beeinvolved in electrosensing via mechanorecthese results establish electric fields as a behstimulus component in honeybees potentiasocial communication. Humidity in the hia problem because of the high temperatuand the fanning behaviour leads to relat

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6. Colin ME, Richard D, Chauzy S. 1991 Measurementof electric charges carried by bees: evidence ofbiological variations. J. Bioelectr. 10, 1732.

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8. Jackson CW, Hunt E, Sharkh S, Newland PL. 2011Static electric fields modify the locomotorybehaviour of cockroaches. J. Exp. Biol. 214,e two joints of theody may also betors. Collectively,viourally relevanty used as a cue inwould not causeinside the hive,ly low humidity

bees and interferefields by dance fo

We thank Peter Knoderike Hagen andgrateful to Anne CThe work was supGemeinnutzige Her

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13. Altmann G, Warnke U. 1973 Registrierung vonTierbewegungen mit Hilfe derKorperoberflachenladungen. Experientia 29, 1044.(doi:10.1007/BF01930455)

14. Albert JT, Nadrowski B, Gopfert MC. 2007Mechanical signatures of transducer gating in theDrosophila ear. Curr. Biol. 17, 10001006. (doi:10.1016/j.cub.2007.05.004)

15. Gopfert MC, Robert D. 2001 Active auditorymechanics in mosquitoes. Proc. R. Soc. Lond. B 268,333339. (doi:10.1098/rspb.2000.1376)

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17. Effertz T, Nadrowski B, Piepenbrock D, Albert JT,Gopfert MC. 2012 Direct gating and mechanicalintegrity of Drosophila auditory transducers requireTRPN1. Nat. Neurosci. 15, 11981200. (doi:10.1038/nn.3175)

18. Nadrowski B, Effertz T, Senthilan PR, Gopfert MC.2011 Antennal hearing in insectsnew findings,selectively with the reception of electricwers.

and Gisela Manz for technical assistance; Frie-rgen Rybak for supporting research. We areney for help with the English manuscript.rted by the Dr Klaus Tschira Stiftung andStiftung.

honeybee, Apis mellifera. Behav. Ecol. Sociobiol. 18,207212. (doi:10.1007/BF00290824)

22. Michelsen A, Towne WF, Kirchner WH, Kryger P.1987 The acoustic near field of a dancing honeybee.J. Comp. Physiol. [A] 161, 633643. (doi:10.1007/BF00605005)

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25. Schlegel T, Visscher PK, Seeley TD. 2012 Beepingand piping: characterization of two mechano-acoustic signals used by honey bees in swarming.Naturwissenschaften 99, 10671071. (doi:10.1007/s00114-012-0990-5)

26. Menzel R, Manz G, Menzel R, Greggers U. 2001Massed and spaced learning in honeybees: the role ofCS, US, the intertrial interval, and the test interval.Learn. Mem. 8, 198208. (doi:10.1101/lm.40001)

27. Albert JT, Nadrowski B, Gopfert MC. 2007 Drosophilamechanotransductionlinking proteins andfunctions. Fly (Austin) 1, 238241.

28. Hunt EP, Jackson CW, Newland PL. 2005have confirmed that bees perceive electric fields. These trainingthBy training bees to static and modulated electric fields, we channel in social communication additional studies will beneeded that monitor and manipulate electric fields of dancing

Reception and learning of electric fields in beesIntroductionMaterial and methods(a) Measurement of the electric fields of arriving and dancing beesVibrations of the antennal flagellumNeural responses of the Johnston organBehavioural analysis

Results and discussionBees as a source of electric fieldsElectric fields induce vibration of the antennal flagellumConstant electric fields induce neural activity in Johnstons organElectric fields of different temporal structures evoke behavioural responses in resting beesStationary walking animals respond to biologically relevant electric field patternsBees learn to associate constant and modulated electric field with rewardMechanoreceptors on the antennae are required for responses to the learned electric field stimulus

ConclusionWe thank Peter Knoll and Gisela Manz for technical assistance; Friederike Hagen and Jrgen Rybak for supporting research. We are grateful to Anne Carney for help with the English manuscript. The work was supported by the Dr Klaus Tschira Stiftung and Gemeinntzige Hertie Stiftung.References