system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why can some insects survive high radiation levels they wouldn't encounter on Earth and others don't?

For instance take the clichéd cockroach. They can survive substantially higher levels of radiation such as that from a nuclear weapon. Surely in nature they would never encounter such high levels of  radiation, outwith some stellar events. Furthermore why do only certain species have such a great tolerance to radiation whilst others do not.

It seems like genetics are coding for an unnecessary and unlikely exposure to radiation. Thanks!
gpt
They do not have specific adaptations for surviving radiation, but they are pretty tough little bugs that can survive injuries, starvation, etc. Insect biology just handles radiation damage better than mammals and things, mainly because they're much simpler than vertebrates. 

There are arthropods that can survive the vacuum of space, not because they need to live in space bot because they evolved to survive *dessication* (drying out). Tardigrades are the instant ramen of the arthropod world.