The casual ignorance in this thread is astonishing.
I am very wary of any suggesting that insects/crustaceans/molluscs can't feel pain.
First of all.. define pain. Is my pain like your pain, how much pain am I in right now if I tell you its a 7 out of 10 and I am taking Fentanyl via a patch daily - have you ANY idea? Most of you won't, some of you might, a tiny bit. Would you know to look at me sitting here typing this - no, not unless you know me very very well.
And we're the same species (I assume).
Pain serves a function - don't touch that/don't let that touch you/move away/stop doing that.
To cause avoidance, pain must be in some way aversive (how do I extrapolate that the quadrants of learning theory apply to insects - see Ramirez, training butterflies via positive reinforcement. if one quadrant applies, potentially, they all apply). - otherwise why WOULD you avoid it?
It is a very simple system - that hurts, don't do it. That limb is falling off, don't jump on it.
Why would there be another system to tell something 'don't do that', it would be MORE complex, require more memory, more logic, more innate behaviour than a simple 'ow, fuck, i wont do that'.
Now do they feel pain the same way we do, no, highly unlikely. Is it AS painful as it would be for us if the same thing happened to us... who the fuck knows... does it make them miserable, depressed, angry and ranting on internet forums like it does us.. again doubtful.. but who knows.
But again - we don't KNOW if we each experience pain the same way as one another, are my toothaches as vile as yours, are my burning joints as agonising as his, who knows.
Not that long ago, we thought lobsters and fish didn't experience pain, within MY lifetime, there have been vets trained and in practice who genuinely believe dogs didn't suffer certain types of pain or that it was acceptable to ignore animal pain.
We commonly view insects as being less advanced, less developed than we are, and thats really showing your ignorance - these are highly specialised creatures, they are INCREDIBLE.. at what they do.
A flys compound eye can see a wider field of vision than yours, processes light better, processes multiple images far FAR faster and sees colours you do not.
Some of the jumping spiders see so well they can recognise faces, not just 'thats a human face' but 'thats a human face I know' vs 'that is a human face I do not know'.
The mantis shrimp can see colours nothing else can, they have the most complex eye on the planet, they have 12 to 16 types of photoreceptor (you btw, have just 3) and can TUNE their eyes colour sensitivity long wave-length to adapt to the environment.
These creatures are incredible and HIGHLY specialised - we need lots of them, they pollinate, consume each other, recycle dead things and they are the bottom of the food chain, feeding many many species.
Without bees and wasps and flies and spiders and shrimp and krill and so on... we'd be overloaded with dead bodies of all manner of species, have no birds, no whales, no bats, amphibians, many reptiles, and fish would be lost...
Without any insects, the planet is fucked.
Without us - it will succeed - we're a lumbering generalising non-specialist, easily adaptable species - we take a lot and we give fuck all back.
Without us.. the world would revert to a lush green planet very rapidly indeed, we're about the only species you could wipe out without any negative effect at all.
So yes when I hear people killing insects for fun and think thats just great, teach that to their kids too... it does make me feel sick. You don't know how stupid you are.