The labial glands in Prorhinotermes simplex consist of secretory cells organized into acini, water sacs and the ducts connecting the gland parts to the basis of the labium. Acini are composed of central and parietal cells. Central cells type I contain predominantly lucent vacuoles and are involved probably in hydroquinone production. They are lacking in soldiers. Type II central cells produce vacuoles of proteinaceous content which are of the same electron density (type IIa) or present in more shades (type IIb). Type IIa cells are present in all older individuals, whereas type IIb are lacking in soldiers and neotenics. Type III cells represent a specific stage of type I cells development, but they are definite functional secretory cells in soldiers. Acini of first instar larvae contain undifferentiated cells which differentiate into type I cells during the second instar. Specific larval central cells start to change into type II cells during first instar. The central cells of presoldiers show a transition from the pseudergate into the soldier situation. The parietal cells keep a uniform structure throughout the whole ontogeny. Only one type of cells form the water sacs in all castes. The cells are very flat with scarce organelles. The water sac cells produce lipid-like secretion, small lucent vacuoles and bunches of angulated vacuoles. The water sac probably functions as water storage organ only. Ontogenetical changes in water sac development are small. The acinar ducts originate inside the acinus where they are formed by flat cells with rare organelles. At the acinus border, cells equipped with mitochondria, microvilli and basal invaginations appear. The water sac ducts are formed by flat cells with rare organelles. Acinar ducts outside the acinus and water sac ducts are equipped with taenidiuam.