Elwood agrees this is a useful way to frame the question. "From an evolutionary perspective, the only reason for pain that makes sense to me is that it enables long-term protection," he says. Pain may provide an animal with an additional, and memorable, means of focusing on a source of harm that helps it avoid it in future. If an animal's life span is not long enough to benefit from that - as is the case with most insects - then pain has no use. Similarly, some animals may simply be unable to avoid noxious stimuli in the first place. "Is a barnacle going to benefit from a bad experience?" Elwood says. "I doubt it."
Ultimately, we are up against the problem of consciousness. Like all subjective experience, pain remains private to each individual, leaving us only with educated guesses. But both Elwood and Crook have changed how they treat the invertebrates in their labs. They now use as few animals as possible and keep the potential for suffering to a minimum. And they are pushing others to do the same.
There are signs of change, too: Cephalopods at least now get some protection, in some parts of the world. "We are broadening our understanding of both pain and nociception," Crook says. "How can this not be interesting, even to the skeptics?"