The ultrastructural features of the anal papillae from fourth-instar larvae of the midge Chironomus tentans are consistent with their dual function of transporting ions into the hemocoel and hemolymph macromolecules towards the cuticle. The thin cuticle of anal papillae exhibits epicuticular grooves containing electron-dense material. In AgNO3-treated larvae, silver grains enter the epicuticular grooves to reach the underlying epidermis, which consists of a single layer of flat epithelial cells with elongate cytoplasmic processes extending into the hemocoel but no tracheal supply. These cells have large nuclei with well-developed nucleoli, numerous mitochondria, free ribosomes, glycogen, vesicles, and microtubules, but rough endoplasmic reticulum and Golgi complexes are sparse. These cells exhibit extensive infoldings of the apical plasma membrane (facing the cuticle) associated with mitochondria. Highly folded lateral membranes are linked by desmosomes and septate junctions and enclose intercellular channels and spaces. The basal plasma membrane (facing the hemocoel) is invaginated to form a labyrinth of channels that anastomose freely and extend deep into the cytoplasm. The slender basal cytoplasmic portions and pedicels, which enclose slits and rest on a delicate basal lamina, resemble the podocyte foot processes of vertebrate renal Bowman's capsules. In addition, the basal cytoplasm is filled with dense tubular elements, coated vesicles, multivesicular bodies, and lysosomes, suggesting receptor-mediated endocytosis of macromolecules. The association of axons with the epithelial cells suggests there may be neural control of their functions. Aquatic insects, like other animals living in fresh water, are faced with the problem of maintaining the osmotic pressure and ionic composition of body fluids higher than that of the surrounding medium. To overcome this problem, larvae of aquatic insects such as Aedes, Chironomus, and Helodes have developed anal papillae that absorb sodium and chloride ions into the hemolymph (Wigglesworth 1933a,b, 1938; Koch 1938; Ramsay 1953; Treherne 1954, 1965; Stobbart 1960, 1967; Stobbart & Shaw 1974; Wright 1975). In addition, the anal organ in semiaquatic larvae of Drosophila (Gloor & Chen 1950; Quintart 1960), the ventral tube in adults of Podura aquatica (Noble-Nesbitt 1963), and the labium in adult corixids (see Jarial et al. 1969; Komnick & Schmitz 1977) are important sites for the exchange of inorganic ions with the external medium. Ultrastructural studies on the anal papillae/organ of aquatic dipteran larvae (Copeland 1964; Sohal & Copeland 1966; Eichelberg et al. 1972; Meredith & Phillips 1973a,b; Credland 1976; Edwards & Harrison 1983; Garret & Bradley 1984; Jarial 1987) have revealed a strong correlation between cell structure and function in the form of multiple infoldings of the apical and basal plasma membranes. These membrane infoldings and their close association with mitochondria provide increased surface area and energy, respectively, for active uptake and transport of ions from the environment. But the potential role of these structures in the transport of hemolymph macromolecules to the cuticle has received little attention. The aim of the present study was to elucidate the fine structure of the anal papillae of full-grown larvae of the midge Chironomus tentans and to correlate it with their function of transporting ions from the external medium to the hemolymph, as well as their potential role in transporting hemolymph macromolecules to the cuticle of the integument.