develops from prothoracic coxal bristle on hairy island FBbt:00004259 VFB:FBbt_00004259 A relatively long bristle located on a small island of hairs (trichomes) in an otherwise bare region of cuticle on the lateral side of the costa at the distal end, adjacent to its joint with the trochanter. fly_anatomy.ontology T1 BH- microchaeta prothoracic leg mechanosensory bristle prothoracic coxal bristle presumptive prothoracic coxal bristle on hairy island