FINE-STRUCTURE OF ANAL PAPILLAE IN LARVAL CHIRONOMIDS, CHIRONOMUS-TENTANS (DIPTERA) WITH REFERENCE TO IONIC AND MACROMOLECULAR TRANSPORT

The ultrastructural features of the anal papillae from fourth-instar l arvae of the midge Chironomus tentans are consistent with their dual f unction of transporting ions into the hemocoel and hemolymph macromole cules towards the cuticle. The thin cuticle of anal papillae exhibits epicuticular grooves containing electron-dense material. In AgNO3-trea ted larvae, silver grains enter the epicuticular grooves to reach the underlying epidermis, which consists of a single layer of flat epithel ial cells with elongate cytoplasmic processes extending into the hemoc oel but no tracheal supply. These cells have large nuclei with well-de veloped nucleoli, numerous mitochondria, free ribosomes, glycogen, ves icles, and microtubules, but rough endoplasmic reticulum and Golgi com plexes are sparse. These cells exhibit extensive infoldings of the api cal 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 pla sma 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 sl its and rest on a delicate basal lamina, resemble the podocyte foot pr ocesses of vertebrate renal Bowman's capsules. In addition, the basal cytoplasm is filled with dense tubular elements, coated vesicles, mult ivesicular bodies, and lysosomes, suggesting receptor-mediated endocyt osis of macromolecules. The association of axons with the epithelial c ells suggests there may be neural control of their functions.