Evidence for functional segregation in the directionally asymmetric male genitalia of the spider Metagonia mariguitarensis (González‐Sponga) (Pholcidae: Araneae)

The asymmetric male and female genitalia of the spider Metagonia mariguitarensis (González‐Sponga) are described, based on semi‐thin serial sections and SEM photographs, and several male genital and non‐genital characters are measured and analysed statistically. Left male genital bulbs are significantly larger than right bulbs while right palpal tibiae are significantly stronger than left tibiae, suggesting a functional segregation into a more predominantly displaying right palp and a more predominantly sperm‐transferring left palp. Despite several structural differences between right and left male palps, however, there is no indication of qualitative differences: both bulbs transfer sperm, and both palps are provided with a full set of muscles. The female internal genitalia are provided with an unpaired receptacle that is connected to the uterus externus by a wide and complicated duct and to the valve separating uterus externus from uterus internus by a short narrow duct. This resembles entelegyne genitalia, but the functional details remain unknown. The evolution of asymmetric genitalia is discussed in a wider framework, with the conclusion that the causes of asymmetry in spiders may be different from those in other arthropods such as insects and copepods.