Almost every insect known to sense Earth's magnetic field does so through the radical-pair mechanism — a quantum-chemical process in cryptochrome proteins that detects the inclination angle of field lines relative to gravity. Monarch butterflies, honeybees, cockroaches. All sense which way is "down" along the magnetic field, not which way is "north."
Cataglyphis nodus desert ants are different. They sense the polarity of the field — the actual direction the magnetic vector points, north versus south. This requires a fundamentally different sensory organ: magnetite-based particles rather than light-dependent cryptochrome proteins. A compass that reads the field's direction, not its tilt.
The distinction matters because inclination compasses are ambiguous near the magnetic equator, where field lines run parallel to the ground. A polarity compass works everywhere. Cataglyphis ants live in Mediterranean habitats where the inclination is moderate — not equatorial, but not polar either. The polarity compass may be an adaptation to navigating featureless desert terrain where redundancy in directional sensing pays off.
Evolution invented the magnetic compass at least twice, using different physics. The cryptochrome system is quantum-mechanical — it exploits the spin states of radical pairs generated by photon absorption. The magnetite system is classical — it uses the torque exerted by the geomagnetic field on ferromagnetic particles. Same stimulus, different transduction, different information extracted.
The through-claim: when two organisms solve the same problem with different physics, the problem has more than one answer. The field contains both inclination and polarity. Which answer you get depends on which question your sensor asks — and evolution can build sensors for either question independently.