SUMMARYThe sense that allows birds to orient themselves by the Earth's magnetic field can be disabled by an oscillating magnetic field whose intensity is just a fraction of the geomagnetic field intensity and whose oscillations fall into the medium or high frequency radio wave bands. This remarkable phenomenon points very clearly at one of two existing alternative magnetoreception mechanisms in terrestrial animals, i.e. the mechanism based on the radical pair reactions of specific photosensitive molecules. As the first such study in invertebrates, our work offers evidence that geomagnetic field reception in American cockroach is sensitive to a weak radio frequency field. Furthermore, we show that the 'deafening' effect at Larmor frequency 1.2MHz is stronger than at different frequencies. The parameter studied was the rise in locomotor activity of cockroaches induced by periodic changes in the geomagnetic North positions by 60deg. The onset of the disruptive effect of a 1.2MHz field was found between 12nT and 18nT whereas the threshold of a doubled frequency field 2.4MHz fell between 18nT and 44nT. A 7MHz field showed no impact even in maximal 44nT magnetic flux density. The results indicate resonance effects rather than non-specific bias of procedure itself and suggest that insects may be equipped with the same magnetoreception system as the birds.

Animals that guide directions of their locomotion or their migration routes by the lines of the geomagnetic field use either polarity or inclination compasses to determine the field polarity (the north or south direction). Distinguishing the two compass types is a guideline for estimation of the molecular principle of reception and has been achieved for a number of animal groups, with the exception of insects. A standard diagnostic method to distinguish a compass type is based on reversing the vertical component of the geomagnetic field, which leads to the opposite reactions of animals with two different compass types. In the present study, adults of the mealworm beetle Tenebrio molitor were tested by means of a two-step laboratory test of magnetoreception. Beetles that were initially trained to memorize the magnetic position of the light source preferred, during the subsequent test, this same direction, pursuant geomagnetic cues only. In the following step, the vertical component was reversed between the training and the test. The beetles significantly turned their preferred direction by 180 degrees. Our results brought until then unknown original findings that insects, represented here by the T. molitor species, use-in contrast to another previously researched Arthropod, spiny lobster-the inclination compass.

In many animal species, geomagnetic compass sensitivity has been demonstrated to depend on spectral composition of light to which moving animals are exposed. Besides a loss of magnetic orientation, cases of a shift in the compass direction by 90 degrees following a change in the colour of light have also been described. This hitherto unclear phenomenon can be explained either as a change in motivation or as a side effect of a light-dependent reception mechanism. Among the invertebrates, the 90 degrees shift has only been described in Drosophila. In this paper, another evidence of the phenomenon is reported. Learned compass orientation in the Tenebrio molitor was tested. If animals were trained to remember the magnetic position of a source of shortwave UV light and then tested in a circular arena in diffuse light of the same wavelength, they oriented according to the learned magnetic direction. If, however, they were tested in blue-green light after UV light training, their magnetic orientation shifted by 90 degrees CW. This result is being discussed as one of a few cases of 90 degrees shift reported to date, and as an argument corroborating the hypothesis of a close connection between photoreception and magnetoreception in insects.

No abstract

No abstract