Induced Magnetization in the Monarch Butterfly, Danaus Plexippus (Insecta, Lepidoptera)

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Abstract
Eighteen preserved monarch butterflies, Danaus plexippus L., were analysed for their magnetic characteristics using a superconducting cryogenic magnetometer. These dead butterflies were either non-magnetic or possessed a natural romanent magnetization (NRM) indistinguishable from the background noise of the magnetometer (2 × 10−8 e.m.u. or less). Using a 500-gauss magnet, each specimen was induced with a magnetization which had a mean value of 1.27 × 10−6 e.m.u., indicating the presence of magnetic material in the monarch body. Cutting the specimens into pieces, re-applying a strong field, and remeasuring revealed that most (about two-thirds or more) of the magnetization is concentrated in the head-thorax rather than the abdomen in contrast to the opposite situation reported for honey bees. The direction of the induced NRM, particularly in the horizontal plane, closely tracked the direction of the applied field. The intensities of induced magnetization and tracking of the applied field are similar to those reported for honey bees. There is no significant difference in the intensity or alignment of induced magnetization between males and females.