The question of whether spiders are asexual stems from a misunderstanding of their complex mating habits. Spiders are arachnids belonging to the order Araneae, and the vast majority of the over 50,000 known species reproduce sexually, requiring both a male and a female. The idea that they might be asexual is largely incorrect, though a tiny number of exceptions do exist.
How Spiders Typically Reproduce
Spider reproduction is a physically indirect process that relies on specialized anatomy in both sexes. Before locating a female, the male spider must prepare his sperm for transfer. He begins by spinning a small sheet of silk known as a sperm web, onto which he deposits a drop of sperm from his abdominal genital opening.
The male then draws the sperm from this web into his pedipalps, the two short, leg-like appendages near his mouth. This action, called sperm induction, prepares the pedipalps to function as copulatory organs. The pedipalp structure contains an elaborate bulb and duct system to store and inject the sperm.
The female possesses a hardened external genital plate on the underside of her abdomen known as the epigynum. This structure features the openings through which the male must transfer his sperm. The epigynum acts like a lock, with the male’s pedipalp bulb serving as a species-specific key, ensuring reproductive compatibility.
During copulation, the male inserts a pedipalp into one of the female’s openings, often alternating between the left and right pedipalps in some species. The sperm is then transferred into the female’s internal storage sacs, called spermathecae. The female stores the sperm until she is ready to lay eggs, at which point the eggs are fertilized as they pass through the oviduct.
Specialized Mating Behaviors
The physical mechanics of sperm transfer are preceded by a series of behaviors that are often complex and carry significant risk for the male. Courtship rituals are necessary for the male to signal his identity and intent to the female, who might otherwise perceive him as prey. These signals are highly species-specific, ensuring that the female recognizes a potential mate.
Courtship signals are highly species-specific. For web-building species, the male performs a delicate tapping or plucking pattern on the female’s web. Jumping spiders, which rely on keen eyesight, engage in elaborate visual displays involving specific dances and leg movements. These intricate movements must be performed correctly to prevent the female from attacking the male.
Chemical signaling also plays a significant role in bringing the sexes together, with females releasing pheromones to attract males from a distance. The male’s approach is often cautious, driven by the danger of sexual cannibalism, where the female attacks and consumes her mate. While this behavior is not universal, it is dramatic in species like the Australian redback spider.
In some species, such as the Australian redback spider, the male actively assists in his own consumption. He performs a somersault during the act to place his abdomen over the female’s fangs. This self-sacrifice can increase the male’s paternity success by prolonging copulation.
Other males, such as the European nursery-web spider, offer a nuptial gift. This is typically a fly wrapped in silk, used to distract the female during mating.
Parthenogenesis: The Asexual Exception
While the vast majority of spiders reproduce sexually, rare instances of asexual reproduction, known as parthenogenesis, do exist. Parthenogenesis is a biological process where an embryo develops from an unfertilized egg, producing offspring without genetic contribution from a male. This is an exception and not the standard reproductive strategy for the order Araneae.
The few known cases of parthenogenesis in spiders have primarily been observed in laboratory settings or in isolated populations where finding a mate is difficult. Genera such as Triaeris and Dysdera have shown this capability. The goblin spider Triaeris stenaspis, for example, is thought to reproduce exclusively this way in some populations, as males have never been found in the wild.
In laboratory studies of T. stenaspis, isolated females laid fertile eggs that developed into female offspring. The offspring produced through this process are typically clones of the mother, which limits genetic diversity. This mode of reproduction is an adaptation allowing for survival and population growth in environments where males are scarce or absent.