Structure of dorsal setae in the two-spotted spider-mite Tetranychus urticae Koch, 1836.

The dorsal propodosomal setae of Tetranychus urticae are of two types: pure mechanoreceptors, and dual chemo/mechanoreceptors. The mechanoreceptors are each innervated by two sensory cells, the dual receptors by three or four sensory cells. A seta of the second propodosomal row has two tormogen cells, one trichogen cell, and one sheath cell, located beneath the base of the hair. Its two sensory cell bodies are located in the excretory duct apparatus of the dorsal silk gland. These setae might aid the mite in controlling the spinning of webs, and may also be involved in regulating the mite s vision. The setal cilia of this species have an 8 + o pattern of microtubules.