Is a Spider an Insect?

We tend to think any tiny creature with a bunch of legs is a bug or an insect. For most of us, spiders fall into that category. However, is a spider truly an insect? If not, why not? Keep reading to learn more about spiders and insects and find out how to tell the difference between the two.

How Are Insects and Spiders Related?

Spiders and insects have a common ancestor since they are both arthropods. This means that they both have jointed legs and exoskeletons. However, they do not belong to the same class. Insects are classified as Insecta, the largest group of arthropods. There are over a million described insect species, and scientists believe there are millions more still undiscovered.

Spiders are classified as Arachnida. There are over 100,000 described arachnid species, with estimates of many more yet to be discovered. Arachnids don’t just include spiders, though. Only around 50,000 species are spiders. Ticks, scorpions, mites, and harvestmen, or daddy long legs, are also arachnids.

Insects vs. Arachnids

Bodies

Both insects and arachnids have segmented bodies. However, insects have three body segments, while arachnids only have two. Insect bodies consist of the head, thorax, and abdomen. Arachnids have a cephalothorax and an abdomen.

Insect bodies change shape as they go from a larva phase to an adult phase. However, arachnids do not undergo metamorphosis. Arachnids remain in the same shape and molt as they grow and mature.

Legs

Insects have six legs, or three pairs of legs. Insect legs are segmented and connected by flexible joints. The number of segments can vary by species and the stage of development.

Arachnids have eight legs, or four pairs of legs. Unlike insects, arachnids have a type of hydraulic system that controls leg movement. Increasing thoracic blood pressure allows the arachnid to extend its legs outward.

Antennae

Insects have pairs of antennae that they use to investigate their surroundings. They are located toward the front of the insect’s head and consist of different segments. Antennae are similar to human senses, in that they are sensitive to touch, smell, and sometimes sound.

Arachnids, however, have no antennae. Instead, they typically have fine hairs on their bodies that are sensitive to touch. Arachnids also have sensory slits on their exoskeletons, covered with a fine membrane. These slits help arachnids sense vibrations. Some researchers believe they may also aid in hearing.

Wings

Most insects have wings, but there are no arachnids with wings. Flying insects usually have two pairs of wings controlled by muscles inside their bodies. These wings consist of veins enclosed in thin membranes.

Arachnids may not have wings, but some can fly in a non-traditional sense. Spiders in the genus Selenops can hang glide and even steer in mid-air. Others use a technique called “ballooning,” where they spin trails of silk that catch the wind and carry them along for the ride.

So, Is a Spider an Insect?

Spiders are arachnids. Since arachnids are not insects, a spider is not an insect. Spiders have two segments in their exoskeleton, while insects have three. Spiders also have eight legs, but insects only have six. Spiders have no antennae and no wings, but insects have both. So although spiders and insects are both part of the phylum Arthropoda, they are very different creatures.