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Title: Magnetic anisotropy and organization of nanoparticles in heads and antennae of neotropical leaf-cutter ants, Atta colombica

Author
item ALVES, ODIVALDO - State University Of North Fluminense
item Srygley, Robert
item RIVEROS, ANDRE - Pontifica University
item BARBOSA, MARCIA - Brazilian Center For Physics Research
item ESQUIVEL, DARCI - Brazilian Center For Physics Research
item WAJNBERG, ELIANE - Brazilian Center For Physics Research

Submitted to: Journal of Physics D
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/10/2014
Publication Date: 10/3/2014
Citation: Alves, O.C., Srygley, R.B., Riveros, A.J., Barbosa, M.A., Esquivel, D.M., Wajnberg, E. 2014. Magnetic anisotropy and organization of nanoparticles in heads and antennae of neotropical leaf-cutter ants, Atta colombica. Journal of Physics D. 47(43):435401. DOI: 10.1088/0022-3727/47/43/435401.

Interpretive Summary: Leaf-cutter ants are an important nutrient recycler of tropical forests and a major pest of tropical and sub-tropical agriculture. Previously we have shown that leaf-cutter ants use a magnetic compass to update the direction of their path-integrated home vector. We have also shown that contact with soil is necessary for leaf-cutter ants to use magnetic cues to orient. However the location and composition of the magnetic compass remain unknown. Using ferromagnetic resonance, we investigated whether magnetic nanoparticles exist in the head, thorax, abdomen, and antennae. In addition, we tested for asymmetries in their magnetic resonance, or anisotropies by rotating the specimen in the applied magnetic field. Anisotropy of the magnetic nanoparticles is a feature that could be used to sense the directionality of an external magnetic field, such as the Earth’s dipole field. Spectra from both the head and antennae were anisotropic, whereas signals from the thorax and abdomen were isotropic. Even so the spectra of the heads and antennae were different, and they probably represent two very different magnetic systems. If both systems are involved in magnetoreception, then they might be used to sense different aspects of the magnetic field (polarity, inclination, or intensity) or they might be integrated to reinforce the same sensory information from the magnetic field. Knowledge of how insects orient can be used to improve our ability to predict and disrupt insect movement between agricultural patches.

Technical Abstract: Oriented magnetic nanoparticles have been suggested as a good candidate for a magnetic sensor in ants. Behavioral evidence for a magnetic compass in Neotropical leafcutter ants, Atta colombica (Formicidae: Attini), motivated a study of the arrangement of magnetic particles in the ants’ four major body parts by measuring the angular dependence of the ferromagnetic resonance (FMR) spectra at room temperature. Spectra of the thoraces and those of the abdomens showed no significant angular dependence, while those of the antennae and those of the heads exhibited a periodic dependence relative to the magnetic field. Fitting of the angular dependence of the resonant field resulted in an unexpected magnetic anisotropy with uniaxial symmetry. High values of the first order anisotropy constant were observed for the magnetic material in antennae (-2.9 x105 erg/cm3) and heads (-1 x106 erg/cm3) as compared to body parts of other social insects. In addition, the magnitude of the anisotropy in the heads was comparable to that observed in magnetite nanoparticles of 4-5 nm diameter. For the antennae, the mean angle of the particles’ easy magnetization axis (EA) was estimated to be 41º relative to the straightened antenna’s long axis. For the heads, EA was approximately 60o relative to the head’s axis running from midway between the spines to the clypeus.