There is a common misconception that spiders are insects. We tend to group them in with other “bugs” and “creepy-crawlies” because they appear to 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. This is the hard outer casing on the body that protects the soft organs and tissues inside.
This is a good way to tell the difference. It isn’t always the easiest as you will find some spiders with 7 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 again in the way they operate and the fact they have that sting in the tail along with pincers.
But why do spiders have eight legs? There has to be an evolutionary purpose for having two extra legs. Having 8 legs could make it easier to fight and run. 2 extra legs can make you faster.
Another reason is that spiders can lose a couple of limbs in those fights, or just in accidents over time, and still get around pretty easily. A couple of lost limbs isn’t a massive detriment, although there are limits. You might ask why not have ten legs instead, but eight seems to be the upper limit for practicality and freedom of movement.
The physiological differences go even further when we look at the sections of the body. 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 shape of the body is quite pronounced.
Other insects may have this structure obscured by casings or wings. By comparison, 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
On the subject of facial features, you will notice that a lot of insects have antennae. These little feelers are important for feeling their way around and getting a better idea of their surroundings. Some species have very long antennae. There are also some moth species where these features are large and furry, and maybe even longer than their body.
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 a sign 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 major difference is the wings. While it isn’t always obvious with some insect species, the vast majority do have wings. Even ants develop wings when it is time to 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 aren’t going to 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.
These physiological features all develop with time as these creatures mature, which is another important difference between insects and spiders.
Most insects go through the process of metamorphosis. This is something you will be familiar with if you have an interest 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 essentially 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 out into spiderlings. These babies will grow and develop into mature adults and eventually mate and produce their own young.
A common tool in the spider’s arsenal is the web. While there are silkworms that produce silk strands, and other insects that can produce their own clever materials, few do so in the same way as the spider. These webs are often great 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.
These webs also come in different shapes and sizes, depending on their purpose. 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 other designs to trap their prey and even create little booby-traps in front of burrows. It all depends on their location and target prey.
Also, spiders can use their silk for additional purposes. As mentioned before, 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 that spiders use strands of the web 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. Lots of spiders feed on prey to survive, typically feeding on flies and other insects. While you might not be comfortable with the idea of having spiders in your home, safe non-venomous species can actually provide a great service as pest control. Bugs that crawl out of the floor or walls can get caught in the webs, captured, and then parcelled up 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 that is 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, as well as small rodents and lizards.
We also have to consider that there are some highly poisonous spiders out there, some of which can be dangerous to humans. Spiders have venom in their fangs, a lot like a snake, and will use this to paralyze prey. Typically, this isn’t enough to pose a risk to humans, although a spider may still bite if provoked. Still, it is important to be aware of the threat level of spiders in your area.
Some insects can be just as fierce, however. You will find that there are insects that are just as predatory as spiders. They tend to operate differently, often for other food, to compete in the same environment. There are deadly assassins in the air, such as dragonflies, and the praying mantis is one of the most intimidating creatures around. If you have ever seen footage of them preying on lizards, you know they aren’t to be messed with.
In short, while spiders look a lot like insects in many ways, because of their shared invertebrate traits, there are some important 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.