ULTRASTRUCTURE OF A TARSAL SENSILLUM OF DELIA-RADICUM L (DIPTERA, ANTHOMYIIDAE) SENSITIVE TO IMPORTANT HOST-PLANT COMPOUNDS

The ultrastructure of a pair of tarsal ''C'' sensilla located on the v entromedial side near the distal margin of the 5th tarsomere of the fe male cabbage root fly, Delia radicum (L.) (Diptera : Anthomyiidae) was investigated by electron microscopy and electrophysiological recordin gs. This ''C'' sensillum is a typical gustatory sensillum, consisting of a uniporous hair-shaft inserted in a specialized socket and innerva ted by 5 sensory neurons (i.e. one mechanosensitive and 4 chemosensiti ve). One of the chemoreceptor cells is sensitive to host-plant compoun ds, stimulating oviposition. Non-host-plant (carrot) leaf extracts and sucrose did not stimulate any of the receptor cells. Direct contacts between sensory cell somata were observed, and the possibility of peri pheral neural interaction at the sensillum level is discussed.