From the 1954 movie Them!
Holy f***! That was a giant ant! Nuclear radiation caused ants to grow into giant ants terrorizing the country! If the special effects weren’t so cheesy, this would be a terrifying movie.
I mean, real giant insects are bad enough (click here for examples). If I ever found a bug larger than my own hand in my apartment, I’d run screaming through the streets. Now imagine an ant as tall as a human coming straight for you with the many parts of its mandible working hungrily. You’d run screaming through the streets too (don’t lie; you know you would).
And if this giant ant caught you… have you ever seen an ant cut through a blade of grass? Imagine it doing that to your arm!
Thank God this can’t actually happen. Not because of the radiation part; radiation can cause mutations, and I suppose it could cause an animal to grow larger than normal. But ants… in fact all insects… are limited by their passive respiratory system. Instead of drawing air into the body, like we do with our lungs, ants depend on little holes called spiracles. Oxygen passes through these spiracles, entering a series of hollow tubes that deliver it to the body cells.
Spiracles work fine so long as an ant is small, but giant ants like the ones in that movie wouldn’t get enough air circulating through their bodies. They’d suffocate. Thus, the human race is saved!
P.S.: You might be wondering what would happen if you raised the oxygen content of the air. According to this article from Science Daily, some insect species would grow larger (no radiation required) and some would not.