The morphology and function of the trifoliate organ of five species are studied. Two examples are given where the ultrastructure of this organ assists the interpretation of speciation. The movement of the trifoliate organ during the courtship of Atherigona laevigata and its position during copulation is briefly described. A simple (A. seccata) and a complex (A. conigera) organ are described in detail, with emphasis on structure and function of the sensilla, and surface microsculpture. The A. hancocki-group organs are described and possible functions are suggested for these unusual structures. A distinct population is separated from the A. lineata species complex. A group of three species A. steeleae, A. secrecauda and A. matilei are confirmed to have a close, natural relationship.
