Antennal contact chemosensilla in Psylliodes chrysocephala responding to cruciferous allelochemicals

Sensilla chaetica, which protrude above all other sensilla on the antenna of Psylliodes chrysocephala L., the cabbage stem flea beetle, were investigated ultrastructurally and found to be innervated by five to six sensory neurones. A dendrite from one of these neurones terminates in a tubular body at the shaft base, whereas dendrites from the others run unbranched to a pore at the shaft tip. Such a structure typifies a sensillum with a combined gustatory/mechanosensory function. Electrophysiological recordings using the tip-recording technique confirmed that this sensillum contains one mechanosensory cell and several chemosensory cells. The chemosensory cells were responsive to host plant chemicals. Sensilla chaetica were also found to be responsive to glucosinolates. One of the sensilla chaetica emerges from a domed area of cuticle on antennomere six. This was found to be relatively less responsive to the chemical stimuli tested and more responsive to mechanical stimulation. It is suggested that the sensilla chaetica are contact chemosensilla, that respond to chemicals present in plant surface waxes when P. chrysocephala antennates a leaf.