Insect Or Arthropod: What’s The Difference And The Similarity?

A lot of people call any crawly creature insects. One reason is probably that many people don’t like these creatures: They are strange-looking and when a creature has more than 4 legs, it must be an insect, right? Unfortunately, many arthropods are mistaken for being insects. But, there is a clear distinction between insects and arthropods. If you look at some of the basic characteristics, you will soon learn the difference between these creatures.

Arthropods are creatures with jointed legs and a chitinous exoskeleton, and include insects as well as non-insects such as spiders, millipedes, centipedes, and isopods. Insects are a subdivision of arthropods that are characterised by a three-part body, antennae, three pairs of legs and many of them have wings.

Different naming is often used interchangeably and incorrectly. In this article, we will discover what the difference is, what they have in common (why they are mistaken for insects) and list the creatures that are commonly called insects but are actually not.

The major differences between arthropods and insects

To most of us, the term ‘insects’ represents all arthropods. Nothing is further from the truth. Actually, it is the other way around.

All insects are arthropods, but not all arthropods are insects.

Generally, we lump all of these organisms together and name them from insects, bugs, arthropods and invertebrates. However, although we think almost all creatures are insects, most of them are technically not insects at all.

Even I was surprised by some of the creatures that they weren’t insects. But we can distinguish insects very easily from the other arthropods by looking at three characteristics:

  1. Insects have a clear body plan of head, thorax and abdomen
  2. Insects have antennae (although some very small)
  3. Insects have three pair of legs (so six in total)
  4. Insects (well, many of them) have two pair of wings
Taxonomic relation between different categories and groups of invertebrates. Invertebrates make the majority of species. There a quite a few species of them that are kept as pets.

There are several differences between insects and other arthropods. Here we discuss the major ones:

  • Body segmentation — As we mentioned before, insects have a distinctive three-part body segmentation consisting of the head, thorax and abdomen. Other artropods have often a different body segmentation, ranging from only two segments to many more segments in centipedes and millipedes. That’s already one reason why spiders, millipedes and isopods are not considered insects.
  • Number and location of legs — All insects have six legs, or three pairs of legs. It is one of the most important characteristics of insects. Therefore insects and insect-like creatures are also called hexapods (six-legged organisms). These six legs are all connected to the thorax segment of the body. Other arthropods have other leg arrangements. Some have many more legs like millipedes and centipedes, and legs are attached to multiple body segments.
  • Antannae — Insects have only one pair of antanae and are used for a variety of purposes including olfactory communication and signalling, taste, mating or as a compass.
  • Wings — Unique to insects are the wings. They are the only invertebrates that are able to fly. Generally insects have two pair of wings. They have developed muscles that contract multiple times on a single nerve impulse and therefore can lift off.
  • Eyes — Generally insects have dichoptic arrangement of the eyes, meaning they have two eyes on either side of the head. Often times they are compound eyes, but insects generally have bad eye sight. Of course, there are some exceptions like praying mantises, which have pretty good eye sight.
  • Metamorphosis — Metamorphosis is often seen in insects. Generally, with complete metamorphosis insects go through four distinctive life stages: egg, larva, pupa and adult (imago). Some have an incomplete metamorphosis, where the larva are miniature versions of the adult form. Many other arthropods don’t have metamorphical stages like insects, although there are some exceptions.

Why do people think arthropods are insects: The similarities

So, why do people think all many arthropods are insects?

There are similarities between arthropods and insects. No wonder, though. Insects belong to the larger group of arthropods and that means they all have an exoskeleton. Although the composition differs between insects and arthropods, they look quite the same.

Besides that, almost all arthropods have six or more legs. Terrestrial multi-legged creatures are quickly called insects, but insects have always six legs (by definition to be an insect). However, I have to make a small side note that some insects (like some arthropods) have legs that have developed for other purposes. If you think of the front legs of praying mantises for example. They are enlarged and are used to catch and grasp prey (but are still used to move around).

Arthropods use more than four legs as a necessity for the stability of the strange body plan and to move the body around. Think about it. Their legs are attached outside the centre of gravity. If they only had four legs, as soon as they raise one they will fall over on that side.

A safe name to use for all these strange creatures is invertebrate. It is estimated that 97% of the animal species are invertebrates, so you are almost never wrong when you call these crawlers invertebrates.

And what about the difference between bugs and insects

Another name that is often used is ‘bugs’. Heck, we even used that for this website! Although the boundaries around insects are quite clear, that of bugs is not that clear at all. Many insect creatures are called bugs, but is this correct?

First, when we use the technical definition of bugs, all bugs are insects, but not all insects are bugs. Because even in science the name bug is overused, we like to speak about ‘true bugs‘ and bugs in general. There are a few characteristics that distinguish bugs from other insects. Most true bugs have a straw-shaped mouth to drink juice from plants or blood from animals. They have long segmented antennae and have wings that are though and dark where they are connected to the body and translucent at the ends.

Examples of true bugs are assassin bugs, bed bugs, stink bugs, and cicadas. To make it more confusing, there are some insects with ‘bug’ in their name that aren’t actually true bugs. For example ladybugs and June bugs belong to the family of beetles, and not true bugs.

Regularly the term bug is randomly used to refer to all small living creatures and most of them are indeed insects, but also arachnids, myriapods and crustaceans are sometimes called bugs.

List of creatures that are non-insect arthropods

Now, what creatures are arthropods but often mistaken for insects. In short: Arachnids (spiders, scorpions) Myriapods (millipedes and centipedes), isopods and crabs are not classified as insects, but many people call them insects. I have listed them here below.

Spiders

Spiders, including tarantulas, whip spiders, ticks and mites (and others), are not insects. These creatures belong to the class Arachnids. They have a body plan that differs from insects. They have two body segments — a cephalothorax and an abdomen — and eight legs. Still smaller spiders are called insects or bugs, which by now you know is not correct. But they all belong to the arthropods.

Scorpions

Scorpions, including whip scorpions, also belong to the class of Arachnids. They have the same body plan as spiders and also eight legs, but differ in their tail and pincher-like pedipalps. They are most known for their long tail that ends in a bulb-like sting. All scorpions are venomous, but only a few are considered dangerous for us humans.

Millipedes

Millipedes are often called insects but actually belong to the myriapods (class Diplopoda) — meaning creatures with many legs. These worm-like animals have many body segments and how to pairs of legs on every body segment. Some millipedes have a more flat body shape. Most millipedes have more than 60 legs, although there are some with more than 500 legs. Millipedes are perfect forest cleaners and live on decaying organic materials.

Centipedes

Centipedes belong just like millipedes to the myriapods (but to the class Chilopoda). They have a more flattened body and differ from millipedes by having only one pair of legs per body segment. Unlike millipedes, centipedes are hunters and eat prey animals which in turn are often arthropods themselves (insects, like crickets, beetles and grasshoppers). Where millipedes are rather harmless, centipedes have a poisonous bite and care should be taken when handling a centipede.

Isopods

Terrestrial isopods, also called sowbugs, pillbugs, or roly-poly, belong to the class of Crustaceans. These are very common creatures on the forest floor and most of them think they are insects (and are also called bugs many times). These crawling creatures are small and live on decaying organic matter. They are just like the millipedes the clean-up crew on this planet. Because of their size and way of life, it is not strange that they are often mistaken as insects, but now you know that is incorrect.

Crabs

Crabs (and also shrimps, lobsters and crayfish) belong to the group of Crustaceans. They have many appendages, but typically have only 5 pairs that are used as legs. The first of its legs usually have claws. Often these creatures are wrongly called water insects. But they all belong to the phylum arthropods.

Several non-insect arthropods (source)

And what about snails and worms…

Well, snails, slugs and worms are neither insects nor arthropods. Snails and slugs belong to the molluscs, a group of animals that don’t have legs and don’t have a spine. So they still belong to the larger group of invertebrates. Most molluscs live in water, like clamps and mussels, but (many) snails and slugs in this group live on land.

The most popular known and also kept as pets are the giant African land snails. These can become real giants, where Archachatina marginata ovum is one of the largest of them all.

Worms belong to another phylum called Annelids and are just like arthropods a group of invertebrates. There are many species that belong to this category but are not insects nor arthropods (which may be quite logical because they don’t have an exoskeleton).

Much more to learn!

There is much more to learn about bugs. Check out the suggested reading below for more interesting articles. Want to learn more about keeping bugs? Start your journey here to learn about these amazing pets.

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