Michael Huemer is Wrong About Insects

Some of his claims about why they don't feel pain are demonstrably empirically false

(Sorry for spamming people about this, but it’s election day in NY-12. I think this is one of the most important elections to vote in, and you should vote for Bores. I’ve made the case for that here. This really really really matters, so don’t forget!)

Michael Huemer is one of my favorite philosophers. His book Dialogues on Ethical Vegetarianism is one of the best books I’ve ever read, and it’s a big part of how I talk about veganism. The book manages to be both hilarious and devastating—decisively refuting all the common arguments against veganism. But there’s one big point in it that I disagree with.

In the book, Huemer argues that insects aren’t sentient. He gives three arguments for this position that came mostly from a paper by Eisemann in 1984. I know the book has been influential at convincing people that insects aren’t sentient, so I thought it would be worth explaining why I don’t buy these arguments. Here’s Huemer’s first argument:

One, they don’t have nociceptors…The kind of nerve cells that sense pain.

This is false. Nociceptors have been found in many different species of insects: flies, moths, bees, and various others. We’ve even found nociceptors in small worms with just a few hundred neurons, so it seems unlikely that insects are too primitive to have nociceptors. The claim Huemer made looked reasonable in 1984, but it turns out that we’ve learned some new things since the Reagan administration.

Second, they have drastically simpler central nervous systems. Like a hundred thousand times simpler.

It’s true that insects have smaller and simpler nervous systems. But small insect nervous systems are enough to sustain the capacity for seeing, problem-solving, memory, and smell. In light of this, it doesn’t seem that unlikely that they could sustain the capacity for pain.

We know that insects’ nervous systems facilitate sophisticated tradeoffs between pain and reward, painkiller response, and conditioned learning. Insect physiological response increases as external damage increases. The most interesting study on insect pain genetically modified insects to have capsaicin receptors which let them taste spicy food. When their food was laced with capsaicin, they stopped eating it—taking rare occasional nibbles and writhing around in agony, with their reaction scaling proportionally with the amount of capsaicin. But when they were given a painkiller, this aversive behavior stopped.

So there’s no dispute about whether insects display a range of behavioral responses characteristic of pain. But insofar as they do, it doesn’t seem that unlikely that they feel pain. If their nervous systems can lead to something that closely resembles pain, why are we so confident that it can’t lead to pain?

An insect with a crushed leg keeps applying the same force to that leg. Insects will keep eating, mating, or whatever they’re doing, even when badly injured – even while another creature is eating them.

This is the most memorable claim in the famous Eisemann 1984 paper. But it’s seriously oversimplified.

1. Insects are very different from humans in a lot of ways. For them, mechanical damage is generally less dangerous given their exoskeletons. They’re also small enough that there isn’t much benefit to getting insects to put less weight on an inured body part. So it isn’t that surprising that they sometimes walk normally even when injured—evolution gives them reason to care a lot less about bodily injury. In response to excessive heat, which is a lot more threatening to creatures with exoskeletons, they behave a lot more like vertebrates in pain.

2. Insects have smaller and simpler brains. It’s harder for them to focus on multiple things. So an insect’s mental life might be a lot more monolithic than a human’s. Thus, an insect mating might simply lack mental room to focus on anything else.

3. Being injured doesn’t always produce aversive behavior. Sometimes soldiers keep fighting after injury without acting like they were hurt. Boxers, when they’re hit in the face, keep fighting normally. Often mammals don’t feel pain in response to damaging stimuli—e.g. naked mole rats don’t react aversively to excessive CO2. So if there isn’t much evolutionary reason for insects to prioritize responses to particular kinds of damaging stimuli, we shouldn’t take their non-response to be particularly evidentially significant. And insects often change their behavior a lot in response to injury: they often stop mating, avoid putting weight on an injured body part, and groom injured areas.

4. Insects heal unusually quickly. Often studies injure insects and then wait to see how it affects their behavior. But by then, their injury might have healed. In addition, insects might experience pain more like fish—fish mostly feel sharp, shooting pain rather than dull aching pain. If so, it makes sense that they wouldn’t feel lasting pain in response to injury.

In short, the evidence against insect pain is that in some cases, simple-brained insects’ brains drown out responses to mating when injured, and they sometimes put weight on injured body parts. But I’m not that shocked, even conditional on insects feeling pain, that insects with their tiny little brains go crazy when mating and don’t notice anything else. Similarly, given that there isn’t much selection against insects putting weight on an injured leg, I’m not that surprised that they sometimes do this.

To be clear, I don’t think it’s any kind of guarantee that insects are conscious. But it’s reasonably plausible. The arguments against insect pain are far from decisive. The sensible position to have at this point is deep agnosticism. Given that there are about a hundred million of them per person, and the arguments for ignoring their interests are bad, insect welfare is a pretty big deal.