part of major ampullate field sensillum of major ampullate field SPD:0000256 A cuticle strain sensor on the major ampullate field. These receptors are found at the base of the spigots of the major ampullate glands which produce the dragline used by the spider as a safety thread in various behavioral situations. They are likely to provide information on the forces pulling on the dragline and also on its orientation in space. A single sensillum consists of a hole in the cuticle covered by a thin cuticular membrane. It much resembles spider slit sensilla, which are known to detect strains in the exoskeleton. Each sensillum is supplied by two dendrites most likely belonging to two bipolar sensory cells. One of the dendrites ends at the covering membrane and the other more proximally. The sensilla are arranged with their long axes roughly parallel to the circumference of the spigots. External forces, transmitted by the dragline, result in deformation of the central part of the cuticular plate at the base of the spigots and thus in stimulation of the sensilla. Considering their morphology, topography, and electrophysiology, these mechanoreceptors are suggested to be important in the sensory control of dragline release by the spider. spider_anatomy strain sensillum of cuticle