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What’s the Difference Between Bugs, Insects, and Arachnids?

A butterfly, bee, and moth on a yellow flower
Katho Menden/Shutterstock

“Bugs” and “insects” often seem like catch-all terms for creatures like flies, ants, and spiders. But in truth, bugs, insects, and arachnids are three separate categories for the tiny crawling and flying creatures you might be familiar with. What’s the difference?

Bugs, insects, and arachnids may seem undesirable (at least in your house), but they’re a crucial and valuable part of almost every ecosystem. These small creatures vastly outnumber humans, so you might as well learn about them— they’re all around you! Here’s how to tell them apart.

What Are Bugs?

Bugs are actually a type of insect. So, all bugs are insects, but all insects aren’t bugs. The difference lies in the shape of the mouthparts.

By definition, true bugs have a mouthpart called a proboscis that’s designed for feeding by sucking up fluids. Luckily for us, those fluids often come from plants. However, some of the most notorious types of bugs feed on humans, such as bed bugs.

Things get confusing when you realize that some non-bug insects also have a proboscis, though. For example, bees have a proboscis, but they’re not true bugs.

The difference is that non-bug insects, like bees, have a retractable proboscis. Meanwhile, true bugs have a proboscis that doesn’t retract. There are other defining characteristics of true bugs, but the mouthpart is the main thing that sets them apart from other insects.

In casual language, “bug” can refer to any type of insect or creepy-crawly thing. But if you want to be accurate, “bug” actually means a very specific group of insects: those with non-retractable proboscises.

What Are Insects?

Many of the so-called bugs you’re most familiar with aren’t truly bugs, but insects. Non-bug insects include ants, mosquitoes, and butterflies. However, all true bugs, like aphids and cicadas, are also insects.

Insects are set apart from other animals by their exoskeletons, compound eyes, segmented legs, and antennae, among other features. However, there are also some creatures you may think of as insects that aren’t. For example, millipedes and centipedes aren’t insects: they actually have more in common with shrimp and lobsters.

What Are Arachnids?

We often lump arachnids, such as spiders, in with insects. However, arachnids aren’t bugs or insects at all: they’re in a separate category altogether.

In addition to spiders, ticks and mites are arachnids that are often mistaken for insects. However, other kinds of arachnids, like scorpions, are clearly very different from insects.

What’s the definition of an arachnid? For one, all arachnids have eight legs, while all insects have six legs. Arachnids also lack the antennae that help define insects, and they never have wings. (Probably for the best—imagine if every spider could fly?)

Classifying the Creepy-Crawlies

You might understand the distinction between bugs, insects, and arachnids better when you look at how they fit into biological classification. All three are members of the Animal Kingdom but in different ways.

Within the Animal Kingdom, under the Arthropod phylum, you’ll find the classes of Insects and Arachnids. The Insects class is then divided up into several different orders, one of which is Hemiptera, or true bugs.

So, bugs are a type of insect, and both insects and arachnids are a type of arthropod.

However, if you sometimes mix up these separate categories, you aren’t likely to be judged for it. As with many words, bug has a different meaning in casual conversation than it does in science, so people often use the term “bug” loosely when referring to all kinds of insects and arachnids. Knowing the difference is just a fun piece of knowledge about some of Earth’s most interesting and essential creatures!

Elyse Hauser Elyse Hauser
Elyse Hauser is a Seattle-based writer and editor with a Master's in Writing Studies from Saint Joseph's University. Her work has appeared in publications like Racked, Vine Leaves Literary Journal, and Rum Punch Press. She was awarded a 2017 Writing Between the Vines residency.  Read Full Bio »