Interesting experiments referenced. I think I my personal assumption was that fish could maybe feel pain, and insects probably couldn't. But now one of those is answered.
I saw something (can't find source right now) that insects don't feel pain. The key observation was that insects with injured legs don't favor that leg at all. The evolutionary explanation is that pain is a survival mechanism to avoid further damaging an already-damaged body part; if the organism doesn't live long enough for its body parts to heal, it doesn't benefit from feeling pain.
I asked myself this a while back when I was feeding my tarantulas their weekly crickets and from my brief searches it seems like there isn't a general consensus on if inverts do or do not feel pain, but most people lean towards no. One big indicator for this was the fact that various insects have been observed to not react or notice at all when they are being eaten. I've anecdotal observed this myself sometimes where crickets do not seem to realize they are currently being eaten by a giant tarantula.
I saw something (can't find source right now) that insects don't feel pain. The key observation was that insects with injured legs don't favor that leg at all. The evolutionary explanation is that pain is a survival mechanism to avoid further damaging an already-damaged body part; if the organism doesn't live long enough for its body parts to heal, it doesn't benefit from feeling pain.