There are a lot of giant insects and arachnids in fantasy and sci-fi. The understanding of why this is impossible on a world with Earth-like conditions is pretty universal: ineffectiveness of blood-circulation and breathing systems as well as exoskeleton becoming more of a hinderance and disadvantage at bigger sizes.
But what about a creature/taxon that would look like an insect? Doesn't have to be an Invertebrate per se but must:
- be a macro-sized lifeform(let's say a range between a Sparrow and a Blue Whale);
- have very "insect-like" features - enough for both uneducated peasant and an early Naturalist to classify it as an insect.
On a planet with conditions similar to Earth's:
- Why and how would such creatures evolve?
- What obstacles and how they'd have to overcome?
- Could some of them form Eusociality and how they would interact with human-like species?
For some of the more "recent" examples of such creature look for "Kikimore" and "Endrega" from the Witcher franchise.