MORPHOLOGICAL AND PHYSIOLOGICAL-PROPERTIES OF PHEROMONE-TRIGGERED FLIPFLOPPING DESCENDING INTERNEURONS OF THE MALE SILKWORM MOTH, BOMBYX-MORI

1. The morphology of descending interneurons (DNs) which have arborizations in the lateral accessory lobe (LAL) of the protocerebrum, the higher order olfactory center, and have an axon in the ventral nerve cord (VNC), were characterized in the male silkworm moth, Bombyx mori. 2. Two clusters (group I, group II) of DNs which have arborizations mainly in the LALs were morphologically characterized. The axons of these DNs are restricted to the dorsal part of the each connective (Figs. 1-5). 3. Pheromonal responses of the group I and group II DNs were characterized. Flipflopping activity patterns, which have two distinct firing frequencies (high and low) in response to sequential pheromonal stimulation, were usually recorded (Figs. 6-10). 4. Two types of flipflopping activity patterns were classified into those that had an antiphasic relationship (called the 'FF' type) between the left and right connectives and those with a synchronized relationship ('ff' type) (Figs. 8-12). We propose that some group II DNs show 'FF' flipflopping activity patterns (Fig. 10). 5. A state transition was usually elicited by less than 10 ng bombykol, the principal pheromone component. Extra impulses were elicited during constant light stimulation (Fig. 9). 6. Our results suggest that the LAL olfactory pathways might be important for producing flipflopping activity patterns (Fig. 11).