In general, pheromones are detected by specialized sensory
cells. In contrast to the situation in most vertebrates,
which possess a specialized sense organ for pheromone
detection, pheromone sensitive cells are integrated into
the Drosophila chemosensory detection systems of olfaction
and taste (Figure 1). The olfactory system of the
adult fly is composed of two pairs of head appendages.
About 1200 olfactory sensory neurons (OSNs) are located
on each third antennal segment (Figure 1a), associated in clusters of 2 to 4 neurons as morphologically distinct types
of olfactory (hair) sensilla (basiconic, trichoid, and coeloconic)
[11]. The maxillary palps (MPs) — considered
secondary olfactory organs—extend as single segmented
bulbs from the two lateral sides of the basic proboscis and
each contain about 120 OSNs pair-wise arranged in 60
basiconic sensilla (Figure 1). OSNs are activated through
the interaction of odor or pheromone ligands with olfactory
receptor (OR) proteins, which are expressed on the
dendritic surface [12]. Each OSN projects a bifurcated
axon that terminates in corresponding glomeruli of the two
antennal lobes (ALs), the first centers of olfactory information
processing. Projection neurons then transmit neuronal
activity from the ALs to the lateral horn and the
mushroom bodies, which are higher brain centers where
olfactory information is further processed and transformed
into behavioral output. It is thought that a small set of
OSNs is dedicated to the perception of pheromones