Exaggerated genitalia and other reproductive structures under sexual selection are common in males, and easily explained by sexual selection. Females, on the other hand are (like males) under selection to minimize costs associated with choice and not under selection to impress or convince males. Therefore, exaggerated female genitalia are rare. The spider shown above is obviously an exception. Apparently, these females have resorted to a costly mechanism of female choice, and their genitalia are indirect evidence for the benefit females must derive from choosing among males.
Photo: Mesabolivar cuarassu, female; B. A. Huber