No different than pain receptors, you mean.
> Plants have no brains or analog at all
No brain, yes. But what of "no analog" if the qualifier for suffering is deliberately clouded in ambiguity?
The only reason the brain matters qua suffering is owing to the capacity for consciousness. Plants have no brain, well, sardines have no cerebral cortex.
If you're going to trivialize the cerebral cortex based on whim/feeling, you can do the same for the whole brain. By extension it makes as much sense to attribute the possibility of consciousness to insects as it does to plants, because you're throwing the particulars of the brain out the window. Some "magical unknown thing 'X'" is giving them consciousness - but there's no reason "magical unknown thing 'X'" has to be specific to a brain.
Not the best argument. I don't really mind my pain receptors going off if the signal doesn't make it to my spine. Do you?
The animals I mentioned do, even if very simple ones. If you really want to question this bivalves are a better example, as I've seen many debates in vegan circles about them.