Genital behaviour in crane flies (Diptera: Tipuloidea): testing hypotheses for genital evolution

The tendency for the genital morphology of animals to diverge more rapidly than other traits is one of the most pervasive evolutionary patterns in animal form. Current controversy regarding explanations of this pattern stems in part from the difficulty of observing the behaviour of male genitalia during copulation. This limitation is reduced in tipuloid crane flies, because most of the male's elaborate, divergent genital structures remain outside the female during copulation. Observations of genital behaviour during copulation in 45 species in 21 genera and subgenera, the most extensive sample of genital behaviour in any comparable group of animals, show a combination of trends that fits better with the stimulation version of the cryptic female choice hypothesis than with any of the other hypotheses commonly cited to explain rapid divergent genital evolution: sustained, rhythmic male genital movements such as brushing and tapping; frequent female facilitation of stimulation; lack of consistent morphological coevolution between corresponding male and female structures; lack of forceful male manipulations of females; lack of female "defenses" that impede male stimulation; and lack of direct male interference with other males' sperm. Tipuloids are atypical among Diptera in generally lacking rhythmic, forceful genital thrusting and squeezing.