Is that thing crawling outside your window a bug or an insect? While you may think they’re one in the same, the science behind their difference in terminology can get a bit tricky.
Over time as a matter of convenience, the term “bug” has been used interchangeably with the term “insect” to describe any manner of a tiny creature with more than two legs, wings, and large eyes. While this broad usage cuts down on confusion when describing these creatures in a general sense, it can get complicated when trying to make a scientific classification.
Simply put, all bugs are insects, but not all insects are bugs. True bugs belong to the order Hemiptera and exhibit wings and a slender, straw-like mouth called a proboscis it uses to feed. These bugs belong in their own order because of their distinctive body parts and fall under the higher class of insects (Insecta).
The semantics in the usage is where the confusion lies and why it’s simpler just to use “bug” and “insect” interchangeably. The best way to determine whether you’re calling the small creature you see by its proper classification is to look at the mouthparts. If it has a proboscis that functions to suck up its source of food, is beak-like, rigid and placed under its head when not in use, it’s a bug. If it doesn’t, it’s another kind of insect, arachnid, or something entirely different.
“True Bugs” Arizona State University