AbstractMales ofUlomyia fuliginosapossess two pairs of eversible mesothoracic appendages. A detailed investigation of the anterior appendages, especially of the hypertrophied cup‐shaped cells, every one which is covered by a cuticular knob, is presented in this paper. The most striking feature of these cells is the apical plasma membrane, which is deeply invaginated into the cell, intensively folded and surrounded by cytoplasmic zone tightly packed with mitochondria. The intracellular equipment with organelles resembles that of pheromonesecreting cells of insects. Morphological evidence of neurosecretornotor control of the cells' activity is given.—Females are not prepared to copulate when the anterior mesothoracic appendages have been put out of use. As a result of the findings in this study the cup‐shaped cells are presumed to be concerned with the secretion of an aphrodisiac subs