I don't think it's the size, but the number of insects that pose a threat. In some area's there are no efforts to control the number of insects, therefore insects can count into the millions.

Insects are already a threat to us in many cases, as crop pests and disease carriers. For example malaria, carried by mosquitos, kills about 3 million people a year according to the World Health Organization. That is 8,219 people per day. And, your question is moot on another count, as it asks an impossibility; because of their anatomy, individual insects cannot and will not ever be bigger than us.

IF insects could grow bigger, then with the natural defences they have, they would be a danger - and not just from biting us. There are insects that can spray acid several feet. Most carnivorous insects have some form of toxin - bees, wasps, hornets, etc. There is even a beetle in South America that can shoot fire from its abdomen... Imagine ten-foot tall ants raiding our pantries for the goodies we have in there.. "You stop him!" "No, you stop him!" "I'm not going in there!" Meanwhile Mr. Ant walks out with his mandibles full of our food. Our main defence would be in our technology and gadgets. Even with a thick exoskeleton, they would still be vulnerable to our weapons. Imagine taking down a Bumblebee with a Stinger missile... They would probably still be susceptible to insecticides as well, although you're going to need a damn big can of Mortein to get rid of these guys! -------- Edit: The info on the 'fire shooting beetle' wasn't quite correct - I was working off info I'd read years ago. The beetle does exist, though. It is called a Bombadier Beetle. It uses a chemical reaction within a hollow chamber in its body to propel boiling hot toxic chemicals from its tail. Info on the Bombadier beetle is here: http://www.talkorigins.org/faqs/bombardier.html (one lousy full stop at the end of 'html' made the whole thing not work! Grr! Try it now.) As for venemous carnivorous insects, what about Hornets? Wasps? What about ants with stingers in their abdomens? What about Centipedes? As for acid spraying insects, you've probably never heard of the Australian 'Assassin Bug'. They use an acid/enzyme to digest the innards of their prey within the prey's body, and then suck out the liquid. Several varieties have adapted to be able to shoot streams of their acid up to a foot in distance. This acid can blind a human. For info on Assassin bugs, go here: http://www.ivyhall.district96.k12.il.us/4th/kkhp/1insects/assassin.html If it will end the whinging, I'll take out the spiders. Too bad now you've got nothing to control the populations of giant insects. "etc..."

They'd certainly redefine the phrase "weapons of mass destruction".

Yes, they would become a serious threat, they would eat all of us. Haven't you seen those old horror movies with big bugs? I think I rest my case. Big bugs aren't a good idea if humankind is to survive!

Imagin the movie Starship Troupers. Exept alll on earth. Done.