Arrangement of output information from the 3 macroglomerular units in the heliothine moth Helicoverpa assulta: morphological and physiological features of male-specific projection neurons.

Helicoverpa assulta is exceptional among heliothine species studied so far as concerns composition of the pheromone blend. Previous reports have accordingly pointed out distinct characteristics in the male-specific olfactory pathway of this species, peripherally by an unusual distribution of 2 sensillum categories and centrally by a particular anatomical arrangement of the male-specific glomeruli constituting the macroglomerular complex (MGC). In order to determine the physiological tuning of the 3 MGC units in this species, we have characterized male-specific antennal-lobe projection neurons morphologically and physiologically by use of the intracellular recording and staining technique combined with confocal microscopy. The results show 2 projection neuron types of equal numbers, one that responds to the primary pheromone component, cis-9-hexadecenal, and arborizes in the cumulus and one that responds to the interspecific signal, cis-9-tetradecenal, and arborizes in the dorsomedial unit. A third type responded to the secondary pheromone component, cis-11-hexadecenal, and innervated the smaller ventral unit. The results complement previous findings from tracing of physiologically identified receptor neurons and determine for the first time the functional specificity of each glomerulus in the MGC of H. assulta. The results are particularly interesting because heliothine moths are attractive objects for comparative studies addressing questions concerning divergence of male-specific olfactory characteristics related to speciation.