About the reproductive organs
Mating organ on male palp (palpal organ); Underside of male showing genital opening (gonopore); Underside of female showing genital opening (epigynum); Epigynum.
Male tetragnathid spider on sperm web absorbing a sperm droplet into its palpal mating organs. Gerhardt, 1928.
Until they are mature it is almost impossible to reliably identify spiders to a particular species. This is because differences in the structure of the mating organs in mature spiders are the most important means of species identification.
In male and female spiders the genital opening (gonopore) is found beneath the front part of the abdomen at the middle of a transverse groove running between the book-lungs.
In all male spiders, the small abdominal gonopore from which sperm is discharged is completely separate from the actual mating organs. These organs are found on the ends of the pedipalps (or palps), a pair of short, leg-like limbs between the jaws and the first leg. In the female and all immature spiders the palps are used as sensory and feeding organs. When young male spiders undergo their final maturation moult, the pedipalps become radically modified. The last segment (tarsus) is transformed into a large, articulated sperm-holding reservoir and mating organ (the bulb) often cradled within a hollow in the tarsus, now called the cymbium.
To get its sperm into these mating organs male spiders deposit a droplet of sperm from the gonopore onto a specially spun sperm web or thread made from unique silk glands near the gonopore. The tips of the palpal mating organs are then placed in the droplet and the sperm is taken up into the sperm reservoir inside the bulb and in readiness for mating.
The sperm reservoir and its duct lie within the tegulum, the broad basal part of the bulb. Several structures are arranged on the tegular surface of araneomorph spider palps, giving them a rather complex appearance. These include the embolus, which carries the opening of the sperm duct at its tip and is inserted into the female's copulatory opening during mating; the conductor which protects the tip of the embolus and helps fix its correct mating position as the tegular-subtegular membranes (haematodochae) expand causing the bulb to twist and push the embolus into the female's copulatory duct; and other structures such as the median apophysis, which perform positioning and bracing functions during mating. Some male spiders also have projections on the palpal segments near the cymbium (tibia and patella) which have similar functions.
In different groups, males may also possess characteristic modifications of the legs, jaws and head areas, all associated with mating. The shape of many of these structures is unique to particular species and provide excellent means of telling them apart.
The female genitalia of most spiders also have unique differences between species This may include the structure of both the external mating opening and the internal ducts and sperm storage sacs. In mygalomorph spiders and some araneomorph spiders (haplogyne spiders) mating and egg laying occur via the same opening, the gonopore.
However, most araneomorph spiders (entelegyne spiders) have separate openings for mating and egg laying. The paired mating openings are placed upon an often chitinous area between the book lungs called the 'genital plaque' or epigynum. They receive the male embolus and sperm during mating and connect via simple or more convoluted ducts with bulb-like sacs (spermathecae). Both the structure of the external epigynum and the internal ducts and spermathecae vary between species and are very useful for the taxonomic identification of spiders.
As they are laid, eggs passing from the female's uterus to the gonopore through the vagina are fertilised via a short fertilisation duct that passes between the spermathecae and the vagina. Because the sperm can be stored and maintained by glandular secretions within the spermathecae, a single mating may provide a female spider with sufficient sperm to fertilise all of its eggs.
