Light- and electron-microscopic observations on the cornicles of the rose aphid Macrosiphum rosae are presented, and discussed in relation to conflicting interpretations of cornicle structure and function. Lipid filled cornicle secretory cells occupy the lumen of the cornicle and extend into the abdominal cavity. The aphid is readily induced by mechanical stimuli to release cornicle secretory cells from a pore at the tip of the cornicle. The holocrine secretory cells are disrupted and release their lipid contents on leaving the body. They are enclosed within an acellular membranous sac that is apparently identical in structure with the basal lamina of the epidermis. The ultrastructure and anatomical relationships of the cornicle secretory cells suggest that they are oenocytes invaginated with the epidermal basal lamina, and are not anatomically or embryologically related to fat body.