Dessi,
How do ethical vegans determine which individuals have biographical life and which don't?
I think for most ethical vegans, having a brain is a good starting point. Certainly all vertebrates and maybe a few invertebrates like cephalopods have at least a rudimentary ability to feel pain -- in that they don't just react to painful stimuli, they
experience it.
Why are ants out and worms in?
I don't think worms are really at a greater or lesser mental level than ants.
I seem to remember an interesting blurb on a species of burrowing wasp in a book by Richard Dawkins. This wasp, on capturing food, brings the food to its burrow, sets it on the ground, enters the burrow for a moment, comes back out, fetches the food, and re-enters for snackums. If, while the wasp in the burrow for the first time, a researcher moves the food a few inches, the wasp will fetch the food, set it on the ground, enter the burrow, come back out, bring the food back in -- its a very mechanical operation, so much that a researcher can move the bit of food 40 or 50 times or until he gets bored, and the wasp performs the same steps. Its like moving the dial back on a clothes washer, the machine doesn't care that its washed the same clothes, it just goes through cycle all over again.
Really interesting stuff. Much insect behavior seems to be mechanical in this way.
Regarding whether insects feel pain, they certainly don't behave as if they do. According to
this and
this, "An insect with a damaged foot doesn't limp. Insects with crushed abdomens continue to feed and mate. Caterpillars still eat and move about their host plant, even with parasites consuming their bodies. Even a locust being devoured by a praying mantid will behave normally, feeding right up until the moment of death." This is pretty consistent with the scientific consensus on the subject.
I don't know enough about insects to speak with any level of authority on the subject, but I imagine that they react in a similar way as humans who reflexively pull their hands away from hot stoves before having knowledge of the painful stimuli.
Perhaps they feel pain, so a more interesting question would be whether they have the corresponding emotional response of
suffering that vertebrates have?