Given the similarities between us and other animals, it should make more sense to have to disprove that they don't feel pain, rather than having to prove that they do, i.e., that they must feel pain should be the default assumption in the absence of data.
But we don't know that insects absolutely couldn't experience pain, in the same way that we're certain a brick couldn't (unless one is a panpsychist). At least they have a nervous system.
It may be more complicated, but do we know the minimum complexity required of a nervous system to support 'suffering'? If not, surely we are obligated to give even the simplest nervous systems the benefit of the doubt.
