There is a common misconception that spiders are insects. We tend to group them in with other “bugs” and “creepy-crawlies” because they have a lot in common. They are small, they have a similar look, to some degree, and occupy many of the same spaces and purposes.
Spiders are arachnids and have eight legs, while insects have six legs. Spiders have a two-part body, whereas insects have three. Spiders are predators and spin webs to catch their prey. While some insects grow through metamorphosis, spiders do not.
The more we learn about spiders and insects, the more we can appreciate them and their place in the natural world. So, what is the difference between insects and spiders?
Six legs vs. eight legs
First of all, let’s look at what an arachnid is. Arachnids are 8-legged creatures, rather than 6-legged like insects, with the familiar exoskeleton of invertebrates. The hard outer casing on the body protects the soft organs and tissues inside.
This is an excellent way to tell the difference. It isn’t always the easiest as you will find some spiders with seven legs or find they are hidden too much to count every single limb. This distinction means that spiders have more in common with scorpions than insects as they share this number of legs. Still, scorpions are very different in how they operate and the fact that they have pincers and that sting in the tail.
But why do spiders have eight legs? There has to be an evolutionary purpose for having two extra legs. Having eight legs could make it easier to fight and run. Two extra legs can make you faster.
Another reason is that spiders can lose a couple of limbs in those fights or accidents and still get around pretty quickly. Although there are limits, a couple of lost limbs aren’t a massive detriment. You might ask why not have ten legs instead, but eight seems the upper limit for practicality and freedom of movement.
The physiological differences go even further when we look at the body sections. Insect bodies have three main parts: The head, the thorax, and the abdomen. This is most easily identifiable in ant and termite species, where the body’s shape is quite pronounced.
Other insects may have this structure obscured by casings or wings. The spider has two main body parts, the abdomen, and the cephalothorax. The “facial features,” such as the mouth and eyes, are attached to this part rather than a separate head.
Wings and antennae
You will notice that many insects have antennae. These little feelers are essential for feeling their way around and better understanding their surroundings. Some species have very long antennae. There are also some moths where these features are large, furry, and maybe even longer than their bodies.
Spiders don’t need this sort of protective headwear when they are the main predator in their territory. Insects feel disturbances in their antennae as signs of what is around them, such as threats. Spiders, however, wait for vibrations in their webs and the hairs on their bodies. We will talk more about webs later.
Another significant difference is the wings. While it isn’t always evident with some insect species, most have wings. Even ants develop wings when they leave the nest and relocate. Ladybugs and other beetles have wings hidden under a protective casing. Bees, flies, and other pollinators rely on their wings to get from plant to plant. Then, of course, there are the butterflies, one of the most well-loved winged creatures.
There is no reason for spiders to fly. They don’t need to fly away from danger when they can run. Spiders aren’t pollinators and can spend a long time guarding or feeding at their web.
As these creatures mature, these physiological features develop with time, another essential difference between insects and spiders.
Most insects go through the process of metamorphosis. You will be familiar with this if you are interested in butterflies. Butterflies lay eggs, and these hatch out into caterpillars. The caterpillar spends its time fattening up and maturing, sometimes through multiple forms, before it metamorphosizes again inside its chrysalis. This is where the body breaks down into a complex soup of genetic information and reforms into a beautiful winged butterfly.
Spiders don’t go through anything nearly as complex. Adults lay multiple eggs, often in a secure silken nest, and these hatch into spiderlings. These babies will grow and develop into mature adults and eventually mate and produce their own young.
A standard tool in the spider’s arsenal is the web. While there are silkworms that produce silk strands, and other insects that can make their clever materials, few do so in the same way as the spider. These webs are often incredible feats of structural engineering as well. The web’s tensile strength is such that it will hold its shape in the wind and not break when large prey flies into it.
Depending on their purpose, these webs also come in different shapes and sizes. There are intricate patterns that are strong and effective when strung between the stems of plants. This is the stereotypical shape that we see in Halloween decorations. Others will create funnels and designs to trap their prey and make little booby-traps in front of holes. It all depends on their location and target prey.
Also, spiders can use their silk for additional purposes. This silk’s density and strength can create brilliant structures like nests for their young to protect spiderlings from predators. They can also string up their dead prey for later consumption.
Then there is the clever way spiders use web strands to move around. Sailors can find tiny little spiders on their boats in the ocean and wonder how they got there. The wind picks up the silk and carries it – and the attached spider – to a new location. This is essential for spiders to find new territories and expand the gene pool.
Spiders are commonly predatory species. Many spiders feed on prey to survive, typically feeding on flies and other insects. While you might not be comfortable having spiders in your home, safe non-venomous species can provide an excellent service as pest control. Bugs that crawl out of the floor or walls can get caught in the webs, captured, and then parceled in silk for later consumption. This process is fascinating to watch if you get the chance.
This is where we come to some of the most impressive spider species globally – the tarantulas. It is harder to mistake these animals for insects because of their size and appearance. The hairy bodies all give them a furrier look far removed from many insects’ smoother bodies. The name Goliath Birdeater has to be the most intimidating of all spiders. This massive tarantula can indeed kill and eat small birds and small rodents, and lizards.
We also have to consider some highly poisonous spiders, some of which can be dangerous to humans. Spiders, like snakes, have venom in their fangs and use it to paralyze prey. Typically, this isn’t enough to pose a risk to humans, although a spider may bite if provoked. Still, it is vital to know the threat level of spiders in your area.
Some insects can be just as fierce, however. You will find that insects are just as predatory as spiders. They tend to operate differently to compete in the same environment, often for other food. There are deadly assassins in the air, such as dragonflies, and the praying mantis is one of the most intimidating creatures. If you have seen footage of them preying on lizards, you know they aren’t to be messed with.
In short, while spiders look like insects in many ways because of their shared invertebrate traits, there are some significant key differences to consider. This goes beyond the fact that they have two extra legs. Spiders are generally higher up in the food chain, preying on insects, or each other, with that incredible combination of clever webs, patience, speedy responses, and fast-acting venom.
Bryan Harding is a member of the American Society of Mammalogists and a member of the American Birding Association. Bryan is especially fond of mammals and has studied and worked with them around the world. Bryan serves as owner, writer, and publisher of North American Nature.