Structure and function of tarsal sensilla in the spider Araneus diadematus

The tarsal hairs of the spider Araneus diadematus (Clerck) were examined by light and electron microscopy and for surface structures with a scanning electron microscope. Three different types of hairs could be found: (1) straight, sharp‐pointed hairs, (2) serrated bristles, (3) curved, blunt‐tipped hairs. The third type of hair shows all typical features of chemosensitive hairs, such as multiple innervation and an open tip, and was not included in the present study. The firs two types share the same histological characteristics of having the dendrite of one bipolar neuron attached proximally to the hair base. Also, most of the fine structural details, such as the presence of an extracellular space, a scolopale and a tubular body, correspond to tactile hairs as described in insects. However, in Araneus diadematus the nerve terminal has a three‐lobed structure and seems to be fastened to the socket cuticle which is not known from other arthropods. The five or six serrated bristles at the tip of the tarsus, together with the middle hook, grasp the thread. This relationship was suggested by Nielsen ('31) and was verified by means of the scanning electron microscope. The importance of this mechanism for grasping the thread was demonstrated in ablation experiments with quantitative evaluation of changes in web geometry.