THE ULTRASTRUCTURE OF AN INTEGUMENTAL MICROTRICH SENSILLUM IN GAMMARUS SETOSUS (AMPHIPODA)

Abstract

The dorsolateral body surface in Gammarus setosus is equipped with integumental microtrich sensilla whose sockets are oval depressions in the cuticle. Each socket is based on 4 epidermal cells. Cell 1 forms a smooth dome on one side of the rim over the base of the microtrich, and lines the bottom of the depression with tubercles. Cells 2 and 3 flank cell 1 and meet opposite the dome where cell 4 completes the depression. The microtrich has an apical pore under a tapered hood and its surface is decorated by a scaliculus. The 8 microtrich receptor cells are at the base of the epidermis within the basal lamina. Each group is at a distance of 100 um or more from the socket it innervates. The axons associate in bundles and on each side join the tegumentary nerve behind the eyes, pass through the olfactory lobe cell layer into the tritocerebrum. Each microtrich receptor unit receives 8 unbranched dendrites. The neurons and their processes are sheathed by 4 serially arranged auxiliary cells: the thecogen, trichogen, and 2 tormogen, which are surrounded by accessory cells with dense nuclei and cytoplasm. The receptorlymph cavity delimited by the thecogen cell does not expand the receptor unit at the level of the stepwise ciliary connections. The basal bodies of the 9 + 0 cilia are unusual in having 9 doublet microtubules embedded in dense material and 9 peripheral filaments. The cuticle of the microtrich is derived from the processes of the trichogen and 2 thecogen cells which can be identified during ecdysis. In the intermolt the processes are withdrawn from the receptor canal into recesses within the endocuticle below the sensillum. Since all 8 dendritic cilia are structurally similar and receive stimuli via the apical pore, and none is associated with a scolopale or ends in a tubular body, the microtrich sensilla are presumed to be chemoreceptive.