how can some insects survive a nuclear explosion?

Answer 1

The simpler an animal is, the less likely it is for a critical mutation or injury to occur that will kill it. Having fewer parts, they simply require less machinery to keep them alive. A good analogy would be a pocket calculate vs. a PC. We have no end of trouble keeping the latter properly nourished with updates and medicated with antivirus software, but when was the last time your calculator picked up a virus? Also, having an exoskeleton and being small help when it comes to surviving the overpressure of the blast. An air shock can turn your organs to mush, leaving your skeleton intact, but it would have to crush the exoskeleton first to hurt a beetle. Just being small helps because the overpressure is more uniformly distributed over the bug's surface.