Photo: B. A. Huber
Spiders are highly symmetric organisms. There is barely anything in spider morphology to be included in the long list of asymmetries ranging from protozoan cell structures to fiddler crab claws and mammalian hearts. The pholcid genus Metagonia makes a rare exception: the female internal genitalia of several species are asymmetric. This paper describes the first known case of directional asymmetry in spiders, occurring in both males and females of Metagonia mariguitarensis, and tries to answer two intriguing questions: how could the males profit from having two different palps? Why has asymmetry evolved so often in insects but so rarely in spiders?
A few years later I did a large comparative paper in Biol. Rev. in an effort to answer this question (see PDF); a short version of the proposed evolutionary scenario has been published in Genetica (PDF).
Asymmetric structures in the male (above) and female (below) genitalia (from: Huber 2004)
