The hairpencils of the Mediterranean flour moth, Ephestia kuehniella Zeller, are tufts of modified scales between the seventh and eighth abdominal segments of the adult males. The fine structure and development of a hairpencil organule is described. A process of the trichogen cell secretes the modified scale and then withdraws, and the cell invaginates forming a microvillous lumen into which pheromone is probably secreted. This lumen communicates with the outside via rows of pores in the hollow scale. The opening of these pores when the moth emerges from the pupal cuticle may be a result of the drying out of the membrane that covered them. The socket of the hairpencil scale is secreted by a tormogen cell.

The hairpencils of male Lepidoptera differ in position from family to family, but the organules that compose them seem to share a common structure. Epidermal glands and organules are classified in a scheme that takes account of their mode of development.