What is an Insect?

I originally started off writing an article on flight muscles and how exactly an insect stays airborne given the whole “bees break the laws of physics” thing I’ve heard from a few different sources lately, but I don’t want to get too heady too quickly.

Granted, I’d like to get there because I tend to understand things by explaining them, but I don’t want to scare people off just that quick.

We’ll save that to the fifth post or so, aye?

So, let’s make this easy.

This is an insect:


But this is also an insect:


And so is this.


This isn’t.


Nor is this.


Colloquially, I hear all of those things called “insects” or “bugs” on a regular basis. And while there are some insects that are technically considered “bugs”, that doesn’t account for all of them. And it definitely doesn’t account for spiders, millipedes, and the like.

So essentially, there are essentially three things that make an insect an insect:


  1. Antennae

Heads come in many shapes and sizes, but a lot of creatures have heads. Most of you guys probably all have heads, maybe your house even has a “head” (bathroom). But in this context, the most important features of the head for insect determination are the antennae. They come in a wide variety of shapes and sizes:


And they all serve important functions, though variable. Many ants will use them a primary means of chemical communication while many dark-adapted insects will use them to make sure they don’t get cornered by similarly dark adapted predators.


  1. Six Legs

Sounds pretty straightforward, adult insects should have six legs. Sometimes they are all essentially the same and sometimes the front ones are distinctly different from the back legs, as in a Mantis where the front are designed more like the arms in a monster movie.


But it’s not always that easy. Take a typical caterpillar for instance, most people would count up their legs and arrive at somewhere higher than eight or ten and be a little perturbed.

But the trick here is that not all of the legs are actually fully functional legs. There are some called “proto-legs” which are less like legs that bend and move (as we’d think of them) and more like a pirate walking around on a peg leg. They serve to help move and balance, but they are not quite the same as having a leg with normal joints.



  1. Three body segments.

These segments are called the head (the front), the thorax (the middle) and the abdomen (the rear), each with very different shapes, organs, and general styles. The easiest ones to identify are often those of various ants, wasps, beetles, dragonflies and other groups where you can clearly see the distinction between segments.

Again, it’s not always that easy. Things like Stink Bugs, for instance, have a thorax and an abdomen that appear almost fused together. Part of the time that’s purely due to wings covering the spots that would normally be easy to see, and part of the time they just are connected more than others.


But Chris, there’s “more connected” and then there’s “these are literally the exact same segment all attached”. And there is an explanation for that as well. When a thorax or an abdomen is in different sub-segments of it’s own, it is usually given a letter and number designation. For thoraxes with different parts it’s usually T1, T2 or T3. For the abdomen, the story is the same: A1, A2, A3, A4…


It’s kind of like humans, really. One person has longer legs, one has a longer torso, one has a squat neck which makes their head seem to be directly attached at the shoulders, and occasionally their waist is skinny enough to appear almost like another section.

It’s hard to make pure 100% true rules for any group of organisms, especially one that has anywhere from half a million to ten million distinct species floating out there in the world.

And of course this whole process can get more complicated. Some insects have two wings, some have four wings, some have no wings, some have wings that don’t really work, some have antennae that are near impossible to see, some mimic non-insects, and some ball up and make it near impossible to tell for sure.

But hopefully this gave you some introduction to the idea of what makes an insect, an insect. Because if you’re going to become a rich (haha) and famous entomologist some day, you should probably figure out what the heck you’re looking for in the first place.

Happy Insecting!