You've heard the old joke.
Question: "What are the only things that would survive a nuclear bomb?"
Answer: "Cockroaches and a fruit cake. And the cockroaches would starve."
Fruitcake notwithstanding, would cockroaches really survive a nuclear bomb?
Cockroaches do have an advantage over most other animals when it comes to the threat of nuclear annihilation. For one thing, they're really good at hiding themselves deep within tiny cracks and fissures. But hiding under rocks or burrowing in the soil won't be enough to protect them from the impact of a nuclear bomb. Radiation can make its way into those hiding places.
Cockroaches are remarkably tolerant of radiation, though, so don't count them out just yet. Scientists measure radiation exposure in "rems," an objective measure of the specific damage radiation would cause to human tissue. Humans can withstand 5 rems safely. Exposure to just 800 rems would be deadly for us. If you want to kill an American cockroach with radiation, it will take 67,500 rems to do the job. German cockroaches are even more impervious to radiation, requiring between 90,000 and 105,000 before you'll see them on their backs.
That's a lot of radiation, right? It does seem like cockroaches might stand a chance, should we make the unfortunate choice to detonate a nuclear bomb on this planet. In fact, the amount of radiation they can tolerate is within the range of a thermonuclear explosion. But there's more to a nuclear blast than radiation. There's heat.
Should the cockroach happen to be at the center of the nuclear bomb's target, it would find itself cooking at a temperature of well over 10 million degrees Celsius. Even 50 meters away from the epicenter of the blast, temperatures would reach about 10,000 degrees instantly. That's just not survivable, even for a cockroach.