The morphology of spider sensilla i. mechanoreceptors

Abstract

ABSTRACT. The fine structure of Chemosensitive hair sensilla is described for a web spider ( Araneus diadematus ) and for a wolf spider ( Lycosa punctulata ). Chemosensitive hair sensilla occur on the distal segments of all extremities. Their morphological characteristics are: (I) a steeply inserted, socketed hair shaft; (2) three different lumina (C,, C2 C,) within the hair shaft; (3) a blunt tip with a large (0.5 μ), subterminal pore opening; (4) a multiple innervation of typically 21 neurons per sensillum: 19 chemoreceptive dendrites enter the hair lumen and 2 mechanoreceptive dendrites terminate with tubular bodies at the hair base. The 19 chemoreceptive dendrites traverse the lumen C, without branching and are exposed to the outside through the large pore at the tip. Concluding from these structural properties, as well as from distribution and position on the legs and palps, these sensilla are considered to function as contact chemoreceptors. Another supposed chemoreceptor in spiders, the so-called tarsal organ , was studied in Araneus diadematus . The tarsal organ is a cuticular capsule on the dorsal side of each tarsus which encloses 6-7 sensilla. Most sensilla contain 3 unbranched dendrites which pierce the cuticular lining of the capsule and thus communicate with the environment. It is possible that the tarsal organ functions as a hygro and/or olfactory receptor as was claimed by Blumenthal (1935), but better physiological evidence is needed.