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First published online February 20, 2004
Journal of Experimental Biology 207, 1241-1248 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.00874

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Magnetic orientation in the mealworm beetle Tenebrio and the effect of light

Martin Vácha^* and Helena Soukopová

Comparative Physiology Department, Faculty of Science, Masaryk University, Kotláská 2, 611 37, Brno, Czech Republic

View larger version (22K): [in a new window]   Fig. 1. Design of (A) training and (B) testing set-up. (A) For training, beetles were kept in constant humidity for 24 h, being exposed to a directional light emanating from one of four geographic directions. The light in training came from a frosted light bulb and passed through a diffuser of white paper. (B) For testing, beetles were placed in the centre of the arena under a plastic Petri dish. They were released by raising the Petri dish after a delay of 2 min, and their movement was observed by a camera located underneath the arena. Both training and testing took place in the same location in the laboratory.

View larger version (15K): [in a new window]   Fig. 2. Top view of a coil system and magnetic vectors. Both training and testing took place in the centre of a four-coil system that produced magnetic fields that differed in alignment by 90°. The two fields had a horizontal intensity of 20 µT and an inclination of 65°. The black light shield was placed between the observer and the test arena. N1 and N2 are the directions of the experimental fields; N is the direction of the local natural field.

View larger version (33K): [in a new window]   Fig. 3. 5% relative humidity (RH) – random check controls (A–C) and tests (D–F). All bearings of untrained beetles are randomly distributed, both in the magnetic field used in training (A,B) and in the magnetic field rotated 90° anticlockwise (C) (P>0.05, Rayleigh test; n.s. = not significant). Each dot represents one beetle (N=120). mN = magnetic north during testing. The line starting at the centre of each circle is the mean vector bearing; the radius of the circle corresponds to a mean vector length (r)=1.0; the inner broken circle gives the 5% significance level. During tests, unimodal orientation towards the trained magnetic position of light was expected. Beetles did not exhibit a consistent direction of orientation relative to the magnetic field in darkness (D). By contrast, in the light, beetles oriented in the trained magnetic direction (E). In addition, when the magnetic field was rotated 90° anticlockwise, the orientation in the light was shifted accordingly (F). All distributions after training differ from each other significantly (double-headed arrow indicates Watson's U2 test). Filled triangles, trained topographic direction; open triangles, trained magnetic direction. Lines on either side of the mean vector indicate the 95% confidence interval for the mean vector bearing. The Rayleigh test probability level is indicated in each circle.

View larger version (23K): [in a new window]   Fig. 4. 75% relative humidity (RH) – random check controls (A,B) and tests (C,D). Unimodal orientation away from the trained magnetic position of light was expected. As in the earlier experiment, controls failed to exhibit a consistent direction of orientation relative to the magnetic field in the dark (A) or in the light (B). Again, beetles trained to the directional light source and tested in the 90° rotated field were not oriented in the dark (C) but oriented in the expected magnetic direction in the light (D).

View larger version (23K): [in a new window]   Fig. 5. 95% relative humidity (RH) – random check controls (A,B) and tests (C,D). Unimodal orientation towards the trained magnetic position of light was expected. As before, random check controls are randomly distributed (A,B). Beetles did not prefer any direction in darkness (C) but oriented in the expected magnetic direction in the light (D).

View larger version (24K): [in a new window]   Fig. 6. 50% relative humidity (RH) – random check controls (A,B) and tests (C–F). Bimodal orientation was expected. Untrained beetles' bearings are randomly distributed (A,B). In darkness (C), females showed no orientation but (E) males were bimodally distributed (P<0.05, Rayleigh test on doubled angles) and differed significantly from females (sexes could not be pooled together). However, the 95% confidence interval for the males' mean axis of orientation did not include the trained magnetic axis. In light (D), females oriented unimodally away from the magnetic bearing of light and (F) males oriented bimodally in the expected direction – confidence interval for the mean axis of orientation included the trained magnetic axis. N=60.

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