Tor J. ALMAAS, Anne K. AMUNDGAARD, Bente BERG, & Hanna MUSTAPARTA
Department of Zoology, University of Trondheim, 7055 Dragvoll, Nonway.
Jan BJAALIE
Department of Anatomy, Faculty of Medicine, University of Oslo, Oslo, Norway.
The male antennae of the heliothine moths possess numerous olfactory sensilla of which the sensilla trichodea type 1 is male specific and houses pheromone responsive receptor neurons (RNs). Pheromone responsive neurons are also located in another type of sensilla, s. trichodea type 2 which has shorter hairs. Some of these type 2 sensilla contain plant odour responsive RNs. Where the axons of the RNs enter the antennal lobe (AL), a functional organization takes place; the pheromone RNs terminating in the macroglomerular complex (MGC) and the plant odour RNs in the ordinary glomeruli. A further functional subdivision of the MGC is shown in several moth species, including two species of heliothines. For instance in Heliothis virescens the information about pheromones is processed in one subunit whereas the information about the interspecific signal is processed in another subunit. In this study we have compared the functional subdivision of the MGC in several heliothine moth species which possess RNs tuned to the same compounds, either acting as pheromones or interspecific signals.
Using 3D computer graphics, we .have reconstructed the MGC on the basis of serial sections of the insect brains. The reconstructions showed MGC structures as surface covered units within the AL. The results confirmed previous reconstructed outlines of the MGC in H. virescens and Helicoverpa zea. Furthermore, the MGC reconstructions of the other heliothine moth species showed some similar and some different characteristics.
Tip-recordings from s. trichodea type 1 combined with a single cell staining technique (cobolt-lysine), were performed to determine the functional type of the RNs and their projections in the MGC. In H. virescens and H. zea, Camera Lucida drawings and 3D reconstructions showed that the RNs tuned to the major pheromone component all projected to the major or central part of the MGC. Like in H. virescens, the RNs tuned to the second essential pheromone component projected into the same subunit in the other species. Furthermore the RNs tuned to the interspecific signal projected to a different subunit.