The longest sperm in nature lives in the body of a fruit fly. And we're not talking about proportional size. The sperm of a Drosophila bifurca fruit fly is hands-down the longest of any animal, including humans.
The Drosophila bifurca fly, roughly 3 millimeters (0.11 inch) long, produces sperm measuring about 58 millimeters, or 2.2 inches, long [source: BBC, Leary]. The sperm is tightly coiled and scrunched up in the fly's tiny body. It's so big, it doesn't swim and has to be pushed up to the storage organs by the female's reproductive tract [source: LaFlamme].
The sperm of a human, on the other hand, is about 0.06 millimeters long, or 0.002 inch, less than 1/1000th the length of the Drosophila bifurca sperm [source: 3D Science].
Humans actually have one of the shortest sperm in the animal kingdom, which fits nicely with scientists' hypothesis for the fruit fly's monster ones: The level of "sperm competition" in a species corresponds with the species' sperm length [source: LaFlamme]. In humans, sperm competition is low. In fruit flies, the female stores the sperm from her multiple suitors until it's time to fertilize the egg; in this scenario, the sperm that can barrel through the previously stored sperm has the best chance of being first in line [source: LaFlamme].