Antennal lobe projection patterns of olfactory receptor neurons involved in sex pheromone detection in Spodoptera littoralis (Lepidoptera: Noctuidae).

Pheromone-specific receptor neurons in male and female cotton leafworms, Spodoptera littoralis (Boisd.) (Lepidoptera: Noctuidae) were typed physiologically and traced into the antennal lobe using cobalt lysine as a marker. In male S. littoralis, the macroglomerular complex (MGC), which is responsible for the initial integration of information concerning sex pheromone components, contains three glomerular compartments as revealed in a morphological study. No such specialized structure was seen in the female. In the male, olfactory receptor neurons that responded selectively to stimulation with the major sex pheromone component, (Z)9, (E)11-tetradecadienyl acetate (Z9E11-14:OAc), had arborizations that were restricted to a large glomerulus of the MGC (designated a), situated near the antennal nerve entrance into the antennal lobe. Neurons that were stimulated by (Z)9,(E)12-tetradecadienyl acetate (Z9E12-14:OAc), a second pheromone component, had arborizations in a lateral, smaller glomerulus of the MGC (designated c), while receptor neurons specifically tuned to a behavioural antagonist, (Z)-9-tetradecenol (Z9-14:OH), projected to a medial glomerulus of the MGC (b). In the female, receptor neurons tuned to the major pheromone component projected to a glomerulus situated at the entrance of the antennal nerve. This glomerulus did, however, not have the size or the structure of the male MGC. A second neuron housed in the same sensillum projected its axon to an ordinary glomerulus situated medially in the antennal lobe.