Asymmetric spiders: the Metagonia mariguitarensis group
Published in European Journal of Taxonomy, 2026 - https://doi.org/10.5852/ejt.2025.1026.3117

While genital asymmetry is a common and widespread phenomenon in some arthropods such as insects, it is exceedingly rare in others, such as spiders. Directional asymmetry, where all specimens are same-sided, is particularly scarce. In the New World genus Metagonia, only M. mariguitarensis was previously known to be directionally asymmetric. Here we describe three further species in the genus Metagonia that all share directionally asymmetric male and female genitalia. In three of the four species in this group, right male palps are much bigger than left palps (below left). Intriguingly, however, the sperm containers, i.e. the genital bulbs, show the opposite pattern, with bigger left than right bulbs. It has thus been suggested that M. mariguitarensis males use their palps for different purposes: a predominantly stimulating right palp and a predominantly sperm transferring left palp (Huber 2004). . Left (top) and right (bottom) palps of Metagonia uca, one of the three newly described species. The three males on the right represent three closely related species of the mariguitarensis group; note the large right palps and the barely visible left palps.