Male Asian recluse spiders have the ability to detach their penises to avoid cannibalism while still transferring sperm. (Image credit: ekavieka/Alamy Stock Photo)
Name: Asian recluse spider (Nephilengys malabarensis)
Habitat: South, Southeast and East Asia (including India, Sri Lanka, Philippines, China, Japan and Indonesia)
Diet: moths, beetles, flies, crickets and other small insects.
Why it's interesting: The Asian recluse spider is a unique member of the arachnids. This species has developed an adaptation that allows it to reproduce without the danger of cannibalism from females: it can detach its penis.
This species of spider exhibits marked sexual dimorphism, meaning that males and females are noticeably different in appearance. Females can reach a length of about 0.59 inches (15 millimeters), while males are smaller than 0.20 inches (5 millimeters).
Males are exposed to significant risks during mating due to the aggressive behavior of females, who may kill and eat their partners before or after the mating process. Sex can be so dangerous for males that they have developed the ability to detach their penis so they can leave it and continue transferring sperm while they seek safety.
During the process of “remote copulation,” the male spider's finger – his sperm-transfer organ, of which he has two – may remain inside the female's reproductive tract. The severed finger may remain inside the female and continue pumping sperm into her even after the male has left the area.
In studying this “eunuch phenomenon” among orb-weaving spiders, biologists have found that the longer a severed finger remains in the female's genitals, the more sperm it transfers. And a finger break caused by the female, rather than the male, results in faster sperm transfer.
The detachable penis also serves an important purpose: it acts as a mating plug. Once detached, the embolus—the needle-like structure that delivers sperm—remains inside the female’s reproductive opening, preventing her from mating with other males. This reduces sperm competition and increases the chances of the male’s genes being passed on.
After losing his penis, the male spider becomes more aggressive and defends the female from other males who may try to remove the “finger plug” and fertilize the female.
According to a 2011 study published in the journal Animal Behaviour, the loss of a toe reduces the spider's body mass and increases its endurance, which in turn improves its fighting ability. This finding supports what the researchers call a “no-glove” mating strategy, in which the spiders have no other goals than to protect their potential offspring.
Male spiders have another way to avoid being eaten by females: they sometimes offer one of their legs to the female as a distraction during mating. This act of self-amputation, known as autotomy, reduces the risk of being attacked or eaten during mating. It may also give the male time to escape.
TOPICS amazing animals
Lydia SmithSocial Link NavigationScience Writer
Lydia Smith – journalist
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