Figure 1. How hunger influences the attractiveness of food odors in Drosophila.

Vinegar (or acetic acid) is the ultimate product of the fermentation process in fruit, which is why fruit flies are attracted to vinegar odor. However, both low and high concentrations of vinegar odor leave flies indifferent (left). This is because low concentrations indicate that the fruit is just-ripe (green banana), whereas high concentrations mean that it is rotten (brown banana). Hungry flies behave differently because the low levels of insulin caused by starvation trigger two distinct neuropeptide signaling systems that reshape their olfactory responses (right). In hungry flies, the receptor for short neuropeptide F (called sNPFR) is upregulated in a subset of olfactory neurons. This helps the transmission of signals within the DM1 glomerulus, which increases the sensitivity to low concentrations of attractive food odors. In parallel, elevated Tachykinin signaling (through the DTKR receptor) inhibits the transmission of signals within the DM5 glomerulus. This decreases the avoidance of normally unpleasant smells (such as high concentrations of vinegar). Together these effects allow the pursuit of less-than-optimal food sources (depicted by the green arrows pointing toward the just-ripe and rotten bananas). DM1 and DM5 are specific glomeruli found in the antennal lobe (AL) of the fly brain and their color intensity represents the strength of their activation in fed vs hungry flies.