Drosophila bifurca is a species of fruit fly. Males of this species are known to have the longest sperm cells of any organism on Earth, an astonishing 5.8 cm long when uncoiled. The cells are mostly tail, and are delivered to the females in tangled coils. In a male's entire life he can only make a few hundred such cells. The other members of the genus Drosophila also make very few, giant sperm cells, with D. bifurca's being the longest. Such sperm gigantism is thought to have evolved via a Fisherian runaway process, with a genetic link between sperm length and the length of the female seminal receptacle length (sperm-storage organ) combined with an increasing competitive advantage of longer sperm as the seminal receptacle evolves to be longer.
^Lüpold, Stefan; Manier, Mollie K.; Puniamoorthy, Nalini; Schoff, Christopher; Starmer, William T.; Luepold, Shannon H. Buckley; Belote, John M.; Pitnick, Scott (2016-05-26). "How sexual selection can drive the evolution of costly sperm ornamentation". Nature. 533 (7604): 535–538. ISSN1476-4687. PMID27225128. doi:10.1038/nature18005.