Functional organization of the macroglomerular complex related to behaviourally expressed olfactory redundancy in male cabbage looper moths

Abstract. The neurophysiological bases for behaviourally expressed olfactory redundancy in the sex pheromone communication system of the cabbage looper moth, Trichoplusia ni (Hübner) (Lepidoptera: Noctuidae), were examined by coupling the cut‐sensillum extracellular recording technique with a highly specific neuronal marking method for moth peripheral receptors. In seventy‐two antennal sensilla, axonal pathways of cobalt‐stained neurones could be traced into the male‐specific macroglomerular complex in the antennal lobe. In T. ni males this comprises five glomeruli, two of which are subdivided into morphologically, and in some instances functionally identifiable, regions. Axonal arborizations of forty‐eight neurones (single stainings) showed high fidelity (98%) for containment within a specific glomerulus or glomerular subdivision, and the neuropil targeted seemed to be related to the specificity of a neurone to a particular female‐emitted sex pheromone component (27‐12:Ac, Z7‐14:Ac, Z9‐14:Ac, 12:Ac, 11–12:Ac, Z5‐12:Ac), or to a behavioural antagonist (Z7‐12:OH). Double (twenty‐one) and multiple stainings (three) showed axons projecting into two or more glomeruli, respectively, with 100% fidelity for the component‐specific glomerulus or glomerular subdivision to be targeted. We suggest that the potential for a single minor component to cross‐stimulate two or more neurones within a sensillum may enable partial blends to continue to provide sensory input into all of the pheromone‐processing glomeruli of the complex. Our interpretation is that redundancy occurs at the receptor level on neighbouring dendrites, and thus allows various four‐component partial blends to evoke full pheromone‐mediated behaviour.