Tyranids are not insects. There. This post should be over by now. Unfortunately the Internet is so full of stupidity that I’m going to have to explain myself. That, and I like the sound of my own typing. True, Tyranids have an exoskeleton and (generally) six legs. Tyranids also live in swarms, and have a “hive conciousness”. There are other similarities between insects and Tyranids. All of this is purely circumstantial. Not all insects live in hives (far from it, in fact), and those that do certainly have no “overmind” to guide them. The hive is a result of thousands of individual instincts that strive towards a common goal. The Tyranid Hivemind is in essence a god; a warp power; a soul that can actively guide the behaviour of the very creatures that are its source of power. I’ve never heard of any insect that does that.
As for physical similarities, Tyranids have features that are more akin to mammals, reptiles or even crustaceans. Their eyes are not facetted; Tyranids have tails of one kind or another; they have internal skeletons; at least some “higher” Tyranids have a brain instead of nerve cortexes – unlike you. Yeah, I’m speaking to you, the American 33-year-old in your parent’s basement. A Tyranid body is full of redundant systems, some similar to a variety of Terran animals, and some not. Jes Goodwin has also listed dinosaurs as one of the main inspirations to the Tyranids. For more information on this, I refer to the excellent treatise on Tyranid physiology in the 3:rd edition Codex: Tyranids. (BONUS: Here’s a motivational poster for you ‘Nid players to bring out when the opponents start calling you a bug.)
Of course, it is highly likely (by that I mean a certainty) that the feeling the Tyranids were meant to invoke was inspired by the “insectoid” aliens in Starship Troopers and Ender’s Game. There’s also a movie called Alien that you might not be old enough to watch, but I promise you it’s good. It has a non-insectoid monster awfully like a Genestealer. Also, the dinosaurs I mentioned above. To sum up, Tyranids are not insects. They’re bloody Tyranids.