Drunken Fruit Flies

Wasps are a major killer of fruit flies. They inject their eggs inside fruit fly larvae, then, when the wasp egg hatches, the wasp larva starts eating the fruit fly lava from the inside!
Scientists at Emory University have found that fruit flies prefer to lay their eggs in an environment  with a "high" concentration of ethanol. The fruit flies have evolved a certain amount of tolerance to this toxic ethanol, but the wasps who inject their eggs inside fruit fly larvae find the ethanol level to be lethal. Furthermore, fruit fly lava that have been infected with wasp larva tend to prefer to eat food with a high ethanol content, this raises their blood alcohol level and helps kill the wasp larva.

The most common natural source of ethanol is rotting fruit. Yeasts on rotting fruit can ferment the fruit sugars, like fructose, to produce ethanol:

C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂

This fermentation reaction takes place in anaerobic environments, that is, environments in which oxygen is not present.
The concentration of ethanol in rotting fruits has been  found to be between 0.04 and 0.72 v/v%. By comparison, the ethanol content in beer is usually between 3 and 6 v/v%, while the ethanol content of wine is between 8 and 11 v/v%.
Volume/volume (or volume) percent is a common way to refer to the concentration of alcoholic solutions. It refers to the volume of solute divided by the volume of solution which is then multiplied by 100, that is:

v/v% = V(solute)/V(solution) x 100

Beer that is 3 v/v% ethanol contains 3 mL of ethanol in every 100 mL of beer.

Wine that is 11 v/v% ethanol contains 11 mL of ethanol in every 100 mL of wine.

This preference for eating rotting fruit containing ethanol displayed by the fruit flies seems to be uncommon. Most animals, including humans, seem to prefer ripe, but not rotting, fruit.

References:

1. B. Z. Kacsoh, Z. R. Lynch, N. T. Mortimer, T. A. Schlenke. Fruit Flies Medicate Offspring After Seeing Parasites. Science, 2013; 339 (6122): 947 DOI: 10.1126/science.1229625
2. Neil F. Milan, Balint Z. Kacsoh, Todd A. Schlenke. Alcohol Consumption as Self-Medication against Blood-Borne Parasites in the Fruit Fly. Current Biology, 2012; 22 (6): 488 DOI: 10.1016/j.cub.2012.01.045

Further Reading:
Fermentation
Carbohydrates
Naming Alcohols
Density

Suggested Study Questions:

1. Calculate the volume of ethanol in a stubby (375 mL) of full strength beer (ethanol concentration 4.8 v/v%).
2. An average standard wine glass has a volume of 150 mL. What volume of ethanol is present in a standard wine glass of white wine with an ethanol concentration of 11.5 v/v%?
3. Port is an example of a fortified wine, that is, a wine that has had an additional distilled beverage like brandy added to it to increase its alcohol content to about 17.5 v/v%. A standard port glass has  a volume of 60 mL. Calculate the volume of ethanol in a standard glass of port.
4. The specific gravity (density) of ethanol is 0.789 g/mL. Calculate the mass of ethanol present in a stubby (375 mL) of
   • full strength beer (5 v/v% ethanol)
   • light beer (2.7 v/v% ethanol)
5. Spirits such as rum and vodka, have an ethanol concentration of approximately 40 v/v%. A standard "nip" is 30 mL. Calculate:
   • the volume of ethanol in a nip of vodka
   • the mass of ethanol in a nip of vodka
6. The alcohol content of Marsala wine is increased by allowing water to evaporate off it. The concentration of ethanol in Marsala wine will reach about 18 v/v%. Calculate:
   • volume of ethanol in a 750 mL bottle of Marsala
   • mass of ethanol in this bottle of Marsala wine
7. A particular type of wine barrel holds 225 L of wine. Calculate the mass of ethanol present if the wine in the barrel is
   • red wine (13 v/v% ethanol)
   • white wine (11.5 v/v% ethanol)
   • champagne (12 v/v% ethanol)