You may be wondering how many legs does a spider have, spiders have eight legs and belong to the arachnids group. Far too many legs if you ask me.
There are millions of different species on the Earth and much more that have gone extinct. Among many of them, there are many that are bizarre and strange because circumstances have allowed these strange traits and characteristics to persist and survive until now. Centipedes, which are arthropods, have a lot of legs because their bodies are segmented into similar pieces because of the process of metamerization. It is thought that they have these legs because of common ancestors with crustaceans and insects, which also have segmented bodies.
The naked mole rat is the first mammal to display hive-like behaviors like ants or bees, called eusociality. This significant divergence from other mammal species has allowed them to be successful borrowers and continue to today. Humans are also a very bizarre species because of our sentience. As far as we know there are many intelligent creatures, but none with the level of conscientious and awareness as humans.
Spiders are a combination of bizarre and scary because they have become the center of many folktales and stories that use their many legs or eyes as centerpieces to tell these stories. Their eyes and legs have allowed spiders to spread across the globe and become successful predators that can survive through a lot of trials and tribulations.
To understand why spiders are successful, we only need to look at how evolution shaped them and made them successful.
What Are Spiders
Spiders are arthropods that belong to the class Arachnida, which is composed of joint-legged invertebrates that include spiders, scorpions, ticks, mites, solifuges, and harvestmen. They are all characterized by having eight legs. In some species, the front two legs have taken on sensory functions.
Spiders make up the largest order (Araneae) in the class as there are over 45,000 different species across the globe. They are found in all corners of the Earth except Antarctica because it is too cold for them at the moment. Majority of the world is familiar with them to a certain extent because they are commonly found in most homes and areas that humans inhabit.
They are an old order as fossil records estimate that true spiders appeared about 380 million years ago. Researchers believe that spiders evolved from extinct species of Trigonotarbids, which had many spider-like characteristics.
Almost all spiders are carnivores predators that capture and consume prey. There is only a single known herbivore species, Bagheera kiplingi, that consumes parts of leaves for nutrients as well as nectar. They are not strict herbivores because they are known to also engage in cannibalistic behavior as well as steal the larvae of ants.
Spiders are famous for their ability to create silk, which they use to create homes and traps. This ability comes from spinnerets, which are glands that produce the silk material. Some spiders have more or fewer spinnerets, based on how they use them than other spiders. They are very strong and durable for the needs of a spider as they are effective at capturing unsuspecting preys and holding them until the spider is ready for a meal.
Spiders exist in a range of sizes and colors based on how they evolved in particular regions. The smallest spider, Patu digua, can be as small as .37mm (.015 inches) while the largest spiders, tarantulas, can be as big as 90mm (3.5 in) and have a leg spans of about 250 mm (9.8 in). They are all generally colored to match the environment because they want to camouflage themselves.
Why They Have So Many Legs
Besides their silk, spiders are known for how many eyes and legs they have. Now, because they belong to the Arachnida order, they all have eight legs, which they inherited from their trigonotarbid ancestors. Each of their legs is composed of seven parts that extend and function through a hydraulic system, which they also inherited from their ancestors.
Like other arthropods, their front legs have evolved to have sensory functions because they lack antennas. Again, this is a feature that has existed since trigonotarbids. It seems that the legs and their functions existed for hundreds of millions of years because they allowed the spider to be highly versatile and survive many threats. Spiders are capable of using their legs to climb many different surfaces and angles, giving them a large range to explore for food, mates, and safety.
Recently, researchers found that spiders have more legs than they needed. Scientists from the University of Nancy 1 in France took two groups of Zygiella x-notata spiders, ones with all their legs and another group where they missed one or two of their legs. They found that all the spiders were still able to build similar webs as well as hunt effectively. This means that spiders could afford to lose some of their legs and still survive, which makes them more effective when facing other predators. They also found that there was a limit to the loss because there are very few spiders with only five legs. In the lab, those five-legged spiders produced webs that were inferior to the spiders with more legs.
Why They Have So Many Eyes
Spiders have four pairs of eyes that they use for many functions. The central pair of eyes are the primary eyes of the spider. They are used for the detection of light and formation of images, allowing the spiders to see and pick out details in their setting. This is in contrast to some other arthropods, which are capable of detecting light and not forming images.
The other pairs of eyes on the spiders are thought to have come from the compound eyes of their ancestors. These other pairs are used for peripheral detection as well as increasing their ability to see at night.
Recently, researchers examined the eyes of spiders to understand why they had so many eyes. The researchers examined the pair of eyes next to the primary ones, called the anterior lateral eyes (ALE), and found that they were crucial to the detection of danger and anything approaching the spider. They had three groups of spiders: ones with their primary eyes blinded, ones with the ALE blinded, and one with no eyes blinded.
They found that the group with their primary eyes blinded and the group with no eyes blinded reacted defensively to an approaching stimulus. They found that the group with the ALE blinded did not react defensively to the stimuli like the other groups. This means that even if a spider sees a danger approaching with its primary eyes if the ALE does not detect it then it will not react. This highlights the importance of these secondary eyes to the function and survival of the spiders.
These traits developed as the eyes diverged from the compound eyes of the ancestors and became highly specialized to assist the spider in surviving in open environments because of their large range of movement.