To expose worms to controlled and homogeneous earth-strength magnetic fields we constructed a triple magnetic Merritt coil system (Merritt et al., 1983). (A) Each system creates a magnetic field along the x (i), y (ii), and z (iii) directions and consists of four 1-m2 squares, each arranged orthogonal to the other two. The system generates magnetic and electric fields. To prevent electric fields from affecting our experiments we built a Faraday cage around the experimental volume (iv). Dedicated DC power supplies for each coil (v) allowed us to control the orientation and the magnitude of the net magnetic field within the coil system. Assay plates (vi) were then placed inside the coil system for testing. We empirically calibrated the field within the coil system with the aid of a milligausmeter (vii) from AlphaLab Inc. (Utah, USA). (B) In each magnetic coil system experiment, the north direction of the imposed magnetic field is signified by the 0° on the top of the circular plot. Directly beneath this, and inside the circular plot, the strain's genetic background or geographic origin is indicated. The solid circular histograms represent the heading of the tested populations in a circle where the radius equals 10% of the entire population. Well-fed animals are represented by the black contour, while starved worms are represented by the grey contour. Circular plots had 18 bins (20° each). Similarly, the black and grey arrows represent the mean heading vector for the well-fed and starved populations respectively. The length of the vector is 0 if the population of animals migrated at random, and it is 1 if all animals migrate to a single point. The brown and green dashed curves indicate the heading that would result in (respectively) downward or upward translation at the original isolation site of each strain.