Sexual Dimorphism

Sexual dimorphism describes the feature of species to show a certain difference in appearance between genders. While some taxa show a huge sexual dimorphism (e.g. Nephila spp., where the females are up to 10x larger than the males), a lot of taxa show a rather minute contrast. The only difference in Oxyopidae are usually a slightly dissimilar morphology and the gender-specific genitals, while markings and coloration is identical.
Male Oxyopidae are a lot thinner than the females, while possessing long, more slender legs. Even with an abundance of food, males never reach the size and weight of females. Especially when females are gravid, their opisthosoma can expand hugely in size.
An infallible trait are the genitalia, they both reliably show gender and maturity in all entelegyne spiders.



The males have their secondary sexual organs located on the distal ends of the pedipalps (Ill. 2.1). They range from simple pipette-like organs in primitive spiders (Liphistomorpha, Mygalomorphae) to highly complex structures with several docking structures that follow an elaborate lock-key-principle in Entelegynae (Oxyopidae (!), Salticidae, Lycosoidea). This makes them irreplaceable in zoological taxonomy to precisely identify species.

The bulbus works indeed like a pipette. Males weave a sperm web and deposit their sperm on the web to resorb the sperm into each bulbus. It can be stored there for months until they finally find a female. The mating rituals differ a lot in complexity and duration but they all end in the male reaching the ventral side of the female to connect the conductor to the epigyne and insert his embolus into the epigyne and deposit the sperm into the Receptaculum seminis (Spermatheca).
Before maturity the bulbus is not fully developed and the structure is not visible. Depending on the final size of it, the tarsal segment of the pedipalp can be bulged up several stadiums before adulthood. In Oxyopidae it is noticeable in the semiadult stadium, two moltings before maturity. In Peucetia it is easily detectable by the huge median apophysis protruding on the ventral surface (Ill. 2.2), in other taxa it is evident by the shine the sclerotized structures emit when shined on with a direct light.

Ill 2.1: Location of bulbus (black structure on the "inner" section)
Ill. 2.2: Bulbus of Peucetia lucasi
Ill. 2.2: Bulbus of Peucetia lucasi


Female araneomorph spiders have their primary organs located on their ventral side, between the book lungs and anterior to the epigastral furrow. It has two parralel openings and ducts below it. The structure of the genitals in female is also highly complex and specific for each species in Entelegynae.
Only in mature females these epigynal plate is fully sclerotized but in contrast to the males, simple structures are already visible in the penultimate stage. They both only have sclerotized genitalia in the adult stage. This sclerotisation works by storing a substance called sclerotin in the Genitalia. Sclerotized structures are always shiny and black, which makes the determination of maturity of females in most Oxyopidae very unchallenging, since they usually have a light base color. In some taxa (especially spiders with a green base color), the epigyne is just weakly sclerotized, which may require the use of magnification devices like magnifying glasses or Macro Cameras to properly check the development of the epigyne.