Is it farther or further?
1852, in reference to microbes, from Latin flagellum "whip, scourge," diminutive of flagrum "whip," from PIE root *bhlag- "to strike."
flagellum fla·gel·lum (flə-jěl'əm)
n. pl. fla·gel·la (-jěl'ə)
A threadlike appendage, especially a whiplike extension of certain cells or organisms that functions as an organ of locomotion.