The way I understand it, most small insects don't really "suffer" pain, although they might feel it. But with how simple their nervous systems are, they're more like machines than anything. When an ant loses a leg, for example, it doesn't sit there and writhe in pain like a human would. It asses the damage, searches for the source of the damage, and if the source is a threat, it runs away from it.

It's the difference between, "Aagh my leg aaah!!," and, "Limb no. 4 out of service. evaluating threat level... calculating next best move." ...so we think.

Of course, no way to know for sure. But larger arthropods seem to "suffer" more. Stabbing a millipede will make it writhe and squirm (not that I've done that on purpose... blame little kids for that). Pet tarantulas also seem to suffer mental stress - a tarantula that's constantly startled, threatened, or lightly injured will have a shorter lifespan, even when physically they're fine.

Then again, I've seen videos of beetles that have lost almost all their organs that keep walking like nothing happened, or wild spiders with 4 legs missing that still live normal lives. It's really hard to say for sure.