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First published online February 15, 2008
Journal of Experimental Biology 211, 686-698 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.013938

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The sound field generated by tethered stingless bees (Melipona scutellaris): inferences on its potential as a recruitment mechanism inside the hive

Michael Hrncir^1,2,*, Dirk Louis P. Schorkopf², Veronika M. Schmidt², Ronaldo Zucchi¹ and Friedrich G. Barth²

¹ Department of Biology, University of São Paulo, FFCLRP, Av. Bandeirantes 3900, 14040-901 Ribeirão Preto, SP, Brazil
² Department of Neurobiology and Cognition Research, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria

View larger version (12K): [in this window] [in a new window]   Fig. 1. Airborne sound (sound pressure, mPa, and air particle velocity, mm s–1) generated by sling-tethered stingless bees was measured using a MicroflownTM USP-probe. (A) We measured particle velocity in the horizontal plane around the bee as well as above the vibrating individual. In the horizontal plane, the microphone to measure sound pressure and the airflow sensors (S) to measure air particle velocity either parallel to the substrate or perpendicular to it were kept at a constant distance of 5 mm above the plane acrylic plate used as substrate (15x15 cm2). (B) Sound pressure and air particle velocity were picked up at 24 different measurement points in the horizontal plane around the vibrating bee. The different directions of the measurement points relative to the long axis of the bee were labelled (i-vi). Inset: USP probe positions above the bee's head (He), thorax (Tx) and wingtips (Wt); only measurement points (filled circles) at 5 mm distance are shown.

View larger version (22K): [in this window] [in a new window]   Fig. 2. Simultaneously measured thorax vibrations (laser vibrometer) and air particle velocity (USP-probe). In the case shown here, the USP-probe was 5 mm away from the bee at direction iii (see Fig. 1). (A) Velocity amplitude (VA), pulse duration (PD), and pulse sequence (PS). (B) A single pulse illustrating the close similarity between the thorax vibrations and the air particle oscillations. (C) Frequency power spectra of the pulses shown in B; MF, main frequency component.

View larger version (29K): [in this window] [in a new window]   Fig. 3. Typical examples of airborne sound picked up with the USP-probe 5 mm in front of the head (left panels) and 5 mm behind the wingtips/abdomen (right panels). (A) horizontally and (B) vertically oriented air particle oscillations, and (C) sound pressure induced by the thorax vibrations before (upper rows) and after wing ablation (lower rows).

View larger version (28K): [in this window] [in a new window]   Fig. 4. Vertically oriented air particle velocity amplitude VA (p-p) above a vibrating bee. (A) Mean values ±1 s.d. (N=11) above head, thorax and wingtips/abdomen, before (filled circles) and after wing removal (open circles). Circles are slightly displaced horizontally for better visibility. Asterisk indicates significant difference (paired t-test; Pcorr.0.025) between intact and wingless bees. (B–D) Ranges above the vibrating bee in which air particle velocities had the same mean amplitudes (extrapolated from hyperbolic decay functions, see Appendix 2). Different colours indicate mean velocity amplitudes between 2 mm s–1 and 40 mm s–1 as explained by the logarithmic colour scale. (B) Intact individuals, (C) wingless individuals, (D) portion of particle velocity generated solely by wings (see text for details). Values for directions in front and behind of the bee were taken from vertically oriented particle oscillations in Fig. 7. Shaded area above bee marks the 1 mm range that cannot be accurately described by decay functions (see Appendix 1). Because the airflow sensors were positioned at least 5 mm above the substrate, no values are given for the region below 5 mm (shaded area).

View larger version (34K): [in this window] [in a new window]   Fig. 5. Vertically oriented air particle velocity amplitude VA (p-p) around a vibrating bee. (A) Mean values ±1 s.d. (N=12) picked up 5, 10, 15 and 20 mm from the vibrating bees and at measurement points in different directions relative to the long axis of the bee (i–vi; see Fig. 1B); values before (filled circles) and after wing removal (open circles). Circles are slightly displaced horizontally for better visibility. Asterisks indicate significant difference (paired t-test; Pcorr.0.025) between intact and wingless bees. (B–D) Ranges around the vibrating bee with particle velocities of the same mean amplitudes (extrapolated from hyperbolic decay functions, see Appendix 2). Colour scale as in Fig. 4. (B) Intact individuals, (C) wingless individuals, (D) particle velocity generated only by wings (see text for details). Shaded area around bee: marks the 1 mm range that cannot be accurately described by decay functions (see Appendix 1).

View larger version (37K): [in this window] [in a new window]   Fig. 6. Horizontally oriented particle velocity amplitude VA (p-p) around a vibrating bee. (A) Mean values ±1 s.d. (N=12) at distances of 5, 10, 15 and 20 mm from the vibrating bees at measurement points in different directions relative to the long axis of the bee (i–vi; see Fig. 1B); values before (filled circles) and after wing removal (open circles). Circles are slightly displaced horizontally for better visibility. There were no significant changes of values after wing ablation (paired t-test; P>Pcorr, 0.025). (B–D) Ranges around the vibrating bee where particle velocities had the same mean amplitudes (extrapolated from hyperbolic decay functions, see Appendix 2). Colour scale as in Fig. 4. (B) Intact individuals, (C) wingless individuals, (D) portion of particle velocity generated solely by wings (see text for details). Shaded area around bee marks the 1 mm range that cannot be accurately described by decay functions (see Appendix 1).

View larger version (11K): [in this window] [in a new window]   Fig. 7. Sound pressure (p-p) around a vibrating bee. (A) Mean values ±1 s.d. (N=12) measured at distances of 5, 10, 15 and 20 mm from the vibrating bees and at measurement points in different directions relative to the long axis of the bee (i–vi; see Fig. 1B); values before (filled circles) and after wing removal (open circles). Circles are slightly displaced horizontally for better visibility. Sound pressures generated by a bee before and after wing removal do not differ significantly between intact and wingless bees (paired t-test; P>Pcorr, 0.025).

View larger version (18K): [in this window] [in a new window]   Fig. 8. Simultaneously measured thorax vibrations and unidirectional air movements. A jet airflow was only to be expected behind the vibrating bees (see text). The airflow recordings made 5 mm behind the wingtips (direction vi relative to the long axis of the bee, see Fig. 1), and those made 5 mm laterally (direction iii relative to the long axis of the bee; see Fig. 1) did not differ. Scaling for the airflow was chosen in accordance with the velocity amplitude of the air jet described in honey bees, 150 mm s–1 (Michelsen, 2003). Insets: amplified vibratory pulses showing air particle oscillations along with the thorax vibrations.

View larger version (39K): [in this window] [in a new window]   Fig. 9. Distribution of hive bees (H) around a vibrating forager (F) during trophallactic food transfer measured within a circle of 2 cm radius around the centre of the forager's thorax. Food receivers (R) were not included in the analysis. The closest position between the heads of hive bees (midpoint indicated by white dot) and the foragers served as a measure for the distance. (B,C) Distribution of 128 hive bees attending 20 trophallactic interactions (six different foragers). Different colours represent different regions around vibrating foragers; the borderlines between different regions correspond to directions (i–vi) given in Fig. 1B.

View larger version (7K): [in this window] [in a new window]   Fig. A1. Theoretical and measured radiation of sound from the thorax of a bee. Line graphs represent particle velocity (VA) as a function of the distance from the thorax, which was assumed to be a monopole source (continuous line) or a dipole source (broken line). Open circles, means (± 1 s.d.) of the particle velocities determined experimentally above the thorax in the present study. Hyperbolic decay functions (hyperbolic decay 1, red line; hyperbolic decay 2, blue line) were calculated from the measured values through regression analysis.

View larger version (12K): [in this window] [in a new window]   Fig. A2. Extrapolation of air particle velocities close to a bee from exponential decay functions. Horizontally oriented particle velocity in front of a bee (A) and behind a bee (B); vertically oriented particle velocity behind an intact individual (C) and behind a vibrating bee without wings (D). Measurements were made at 1, 3, 5 and 10 mm distance to vibrating individuals of M. seminigra (open diamonds; A, N=2, single values shown; C, N=5), and at 1 and 2 mm distance to vibrating M. scutellaris (open circles; B, N=5; C, N=5; D, N=4). The values (means ± 1 s.d.) are superimposed onto respective measurements in M. scutellaris (filled circles; N=12) and the resulting exponential decay functions (hyperbolic decay 1, red lines; hyperbolic decay 2, blue lines). Some symbols are slightly displaced horizontally for better visibility.