system
Given the following post, respond as if you're commenting on Reddit:
human
How much pain do small insects experience when they are smashed and killed by humans?

I know pain is relative which is why i'd like if there is a comparison with a certain kind of pain humans experience.
gpt
Current scientific thinking is that insects don't feel pain. In fact the only invertebrates thought to feel pain are cephalopods - octopus, squid, cuttlefish.

The IASP (International Association for the Study of Pain) define pain as requiring both an automatic physical reaction to noxious stimuli (nociception) AND a component of emotional suffering.

Lots of invertebrates have nociception, but emotional suffering is impossible to prove. We can only infer it by the presence of a complex nervous system in the animal, which insects do not have.

That is not to say that insects definitely do not feel pain. It's not even possible to prove that another human feels pain. There's also the problem that politics in science can affect the outcome. Currently if you want to test on animals, you are limited on what you can do to animals thought to feel pain. So the more animals categorised as such, the less free experimentation can occur.

It makes sense that insects do not feel pain, as their survival strategy is based on having a high number of offspring so that some survive despite low survival rates. What would be the benefit of the additional energy-cost of those insects having the ability to feel pain? Whereas animals which have fewer offspring but put more resources into ensuring the survival of each one, benefit greatly from the ability to feel pain. Expending a little more energy per offspring to keep them out of trouble is a worthwhile investment. Evolution has sorted out these little optimisations long ago.

There's been a lot of research on pain in crustaceans (crabs and lobsters) recently, and if another invertebrate becomes categorised as feeling pain it will probably be them next. Even in these research articles though, there is often confusion as to what pain is, and I've seen many papers conclude that crustations do feel pain, but what they've actually proved is nociception.

TLDR: we can't know for sure, but currently we suspect that insects don't feel pain.