Fine structure of antennal sensilla basiconica and their detection of plant volatiles in the eucalyptus woodborer, Phoracantha semipunctata Fabricius (Coleoptera : Cerambycidae)

The ultrastructure and distribution pattern of two types of basiconic sensilla (I and II) on the antennal flagellum of both sexes of Phoracantha semipunctata (Coleoptera: Cerambycidae) was investigated by scanning and transmission electron microscope. Both types are thin-walled multiporous sensilla and occur mostly along the anterior border of the Fl(1)-Fl(6) flagellomeres, while on the distal flagellomeres (Fl(7)-Fl(9)) they are more evenly distributed on both surfaces. Clusters of sensilla basiconica II are found on the distal half of the anterior border of the Fl(1)-Fl(6) flagellomeres. Sensilla basiconica I have one bipolar sensory cell with a branched distal dendrite, whereas the sensilla basiconica II contain two bipolar sensory cells with branched distal dendrites. No sexual dimorphism was found in the fine structure and distribution pattern of both types of sensilla basiconica. Responses from single sensory cells to host and non-host plant odors were examined, using gas chromatography linked with electrophysiological recordings. Most cells associated with each sensillum type were narrowly tuned, each specialized for the detection of one or two chemically related compounds. No clear functional distinction between the two morphological types of sensilla was found, although the few cells that responded specifically to non-host volatiles were associated with sensilla basiconica II.