by Daniel P. Haeusser — Reg­u­lar read­ers of Small Things Con­sid­ered may re­call our men­tion of the mag­ne­to­tac­tic bac­te­ria, or­gan­isms that can sense the Earth's mag­netic field to ori­ent them­selves and dis­tin­guish up from down. The mag­ne­to­tac­tic bac­te­ria vary greatly in mor­phol­ogy, me­tab­o­lism, and phy­logeny, but they all share a lit­tle tool that per­mits the mag­netic sens­ing un­der­ly­ing their co­or­di­nated move­ment (mag­ne­to­taxis).