Spiders are arthropods from the class Arachnida that have 8 legs and fangs that can often inject venom. They are found in all continents except Antarctica, and have established permanent colonies in every habitat except sea and air. As of 2019, approximately 120 families and more than 48,200 species of spiders have been identified. Spiders differ from insects in that they lack antennae and have an extra pair of legs. Most spiders are harmless or inflict non-lethal bites to humans, but some species such as recluse spiders, wandering spiders, and widow spiders contain venom toxic enough to kill an adult human. Most spiders are carnivorous and feed by preying on smaller spiders and insects.
Legs and Locomotion
Like other members of the class Arachnida, spiders have four pairs of limbs, for a total of eight legs. Each leg is divided into seven parts, and spiders use hydraulic pressure to move and extend them. The only extensor muscles in a spider’s leg are located in the hip joint. Consequently, a punctured cephalothorax means that the spider cannot move its legs, which then curl up.
Spiders are capable of generating pressure up to eight times their resting level, and can jump up to 50 times their length by quickly increasing the blood pressure in the third and fourth pair of legs. Most species of spiders use their front pair of legs for sensory functions since they lack antennae. Additionally, the limbs have evolved over millions of years and have allowed the spider to survive certain threats. They are capable of scaling various surfaces at steep angles, enabling them to access food, safety, and mates. Biologists previously believed that eight legs gave spiders a mobility advantage over other organisms, but recent studies suggest a higher number of legs can hinder movement, and make it difficult for spiders to move quickly.
Advantages of Having 8 Legs
Research has also shown that spiders with six or seven limbs can move as efficiently as those with eight, but create inferior webs. In addition to walking, spiders also use their limbs to attack and pin their prey, while delivering venom from their fangs. Some researchers believe that the evolution of eight legs is not related to mobility, but is a tactic used to evade attackers. Some spiders use their limbs as a defensive mechanism by raising and spreading their front legs to counter-attack or intimidate predators.