I felt sorry for this 7-legged jumping spider I found outside of my house. Although it seemed like it was able to get around without any trouble, losing a leg must hinder it somehow, I speculated. Yet the loss of legs in the spider world is not uncommon. In fact, it is actually done on purpose to help spiders survive.
Spiders usually lose legs in order to escape predators. The legs are long and easy to grab. When predators try to use them as a gripping point, the spider will snap its own leg off on purpose to escape! This is called “autotomy”, which means self-amputation. Many animals use this behavior to discard their own appendages when they are grabbed by a predator. A good example is when a bird grabs a lizard by the tail, only to have the tail snap clear off in its beak!
Of course, if a person loss a limb, they risk bleeding to death. However, spider’s are designed to lose their legs. Spider’s legs will readily fracture just near the spider’s body at the coxa-trochanteral joint, leaving a hollow “socket”. The wound near the body of the spider heals quickly to prevent death due to blood.
If a spider’s leg is injured, it will readily shed it. This is most likely because a spider would be slowed down by a lame leg. Notably, orb-weaving spiders are known to lose a leg if it is stung by a venomous insect, such as a wasp. This is so that the venom does not flow to the heart of the arachnid. A similar technique is employed when a human amputates a leg to avoid the spread of infection.
Interestingly, autotomy is a voluntary action. This means the spider can “decide” if it wants to lose a leg, like a person can decide to move an arm. There doesn’t have to be an external pulling force on the leg in order for a spider to lose it. That being said, a spider doesn’t just shed a leg for no reason. One will only do so if it helps it to survive.
With 7 legs, spiders may be able to get around, but what they do is affected. Spiders with less than 8 legs can not forage for food, move around, or find mates as easily. However, if a spider has to choose between death and living with less legs, it will chose the second one.