tl;dr: Being able to sense things that are harmful – nociception – developed early in animal evolution. Only much later, when animals became long-lived and limited in reproduction, did the extra, very expensive neural hardware evolve to allow for conscious, subjective experience of suffering. That allowed those long-lived animals to learn in a way that assists them later in their long life.
 

"Pain" is not "Suffering"

Our "pain" relieving medicines mostly work on our system of nociception, not our mental processes that turn those signals into pain (opioids might be an exception). So our "pain" medicines will work on other animals' nociception pathways, too, regardless of if they actually experience subjective pain.

If this isn't enough to break the idea that "pain" and morally-relevant suffering are the same, please see this much deeper OPP explanation, the most honest exploration out there.
 

Why this matters

This is important for several reasons.

The first is that many empathetic people (including many vegans) are inclined to equate "pain" with "suffering" and see "suffering" everywhere. I know a vegan who heard of a robot "escaping" and felt a twinge of moral sympathy for the robot. 

Indeed, many vegans spend more time trying to "prove" that bees are being exploited by the honey industry than they do actually trying to effectively decrease factory farming. (More in Losing My Religions.)

And, of course, some EAs harp on insects in an attempt to "one up" everyone else's expected value.

Both groups are filled with people utterly invested in their position and who will angrily shout down / bombard with endless rants anyone who has doubts. It is like facing people who only watch Fox News.

This is not only a huge waste of time and energy, it makes vegans and EAs look nuts. Sorry to be blunt, but both vegans and EAs have image problems. It isn't enough to be "right." What matters is being effective, and that involves concern for appearances.

Ed Yong on Insects

In addition to the Open Phil report, check out these excerpts from Ed Yong's wonderful An Immense World:

We rarely distinguish between the raw act of sensing and the subjective experiences that ensue. But that’s not because such distinctions don’t exist.

Think about the evolutionary benefits and costs of pain [subjective suffering]. Evolution has pushed the nervous systems of insects toward minimalism and efficiency, cramming as much processing power as possible into small heads and bodies. Any extra mental ability – say, consciousness – requires more neurons, which would sap their already tight energy budget. They should pay that cost only if they reaped an important benefit. And what would they gain from pain?

The evolutionary benefit of nociception [sensing negative stimuli / bodily damage] is abundantly clear. It’s an alarm system that allows animals to detect things that might harm or kill them, and take steps to protect themselves. But the origin of pain, on top of that, is less obvious. What is the adaptive value of suffering? Why should nociception suck? Animals can learn to avoid dangers perfectly well without needing subjective experiences. After all, look at what robots can do.

Engineers have designed robots that can behave as if they're in pain, learn from negative experiences, or avoid artificial discomfort. These behaviors, when performed by animals, have an interpreted as indicators of pain. But robots can perform them without subjective experiences.

Insect nervous systems have evolved to pull off complex behaviors in the simplest possible ways, and robots show us how simple it is possible to be. If we can program them to accomplish all the adaptive actions that pain supposedly enables without also programming them with consciousness, then evolution – a far superior innovator that works over a much longer time frame – would surely have pushed minimalist insect brains in the same direction. For that reason, Adamo thinks it's unlikely that insects (or crustaceans) feel pain. ...

Insects often do alarming things that seem like they should be excruciating. Rather than limping, they'll carry on putting pressure on a crushed limb. Male praying mantises will continue mating with females that are devouring them. Caterpillars will continue munching on a leaf while parasitic wasp larvae eat them from the inside out. Cockroaches will cannibalize their own guts if given a chance.

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For those of us worried about insect suffering, I don’t think it’s so much that we confuse pain and suffering (and there’s some even worse problems that come from ambiguities in what people mean by pain as opposed to suffering. Some use pain to refer to a process for aversively responding to stimulus, but not necessarily conscious the way suffering is, others use it to refer to a conscious experience that is often associated with negative valence, but which doesn’t necessarily rise to “suffering” without this valence), as that the question is just actually really hard. Insects might well not be conscious, the evidence here is quite mixed, some of it depends on interpretation of different phenomena, some of it which types of evidence one prioritizes. I think it is very plausible that no insects suffer, that all species suffer, or that some suffer and some don’t. The philosophers also seem very mixed on this one.

I don’t think this presents a strong counterpoint to your belief that they don’t suffer, but I think it does to your apparent extrapolation from this that the reason EAs and vegans care about insects is because of EV one-up-man-ship. If you shoot up a house for fun but say it’s alright because you think there’s a 60% chance it’s empty, I think any reasonable, non-EV-obsessed person would want a word with you. If I was a bullet biting EV maximizer, I would be worried about electrons or bacteria, not insects.

Of course the real situation is very different in ways other than odds from shooting up a possibly occupied house, it is possible a different concern is with pure aggregation rather than EV fanaticism. Yes perhaps the odds of doing wrong by harming insects are high enough to normally rise to morally meaningful non-fanatical levels, but the amount of harm that would be involved if so is so small per individual, that the only reason to care about insects is how overwhelmingly many of them they are. A comparison to this kind of worry might be if EAs and vegans were obsessing over a pollutant that had a 30% chance of making a billion people have slightly itchier scalps.

I am more sympathetic to this than EV worries, because if it isn’t even clear if something can suffer, then perhaps we should also assume that if it can suffer, that form of suffering is somewhere just over the line into suffering, including the morally meaningful dimensions of it. My concern with this is that I think the morally meaningful aspects of suffering are actually extremely simple as a rule. I can undergo exquisite opera-worthy intellectual angsts, or I can experience brute torture. The latter seems like it usually matters more than the former, despite being much much much simpler, and presumably accessible to much simpler conscious organisms.

So ultimately, I don’t think concerns about insect suffering are comparable to the itchy pollution case. I think it involves some genuinely difficult and important research programs that could easily show us that insect suffering has radical implications (for instance that insect factory farming is morally on par with more familiar forms of factory farming), or that it is completely irrelevant.

Why doesn't the robot argument also similarly count against conscious suffering in birds and nonhuman mammals?

Just dropping a note to say:
1. Ed Yong's 'An Immense World' is indeed wonderful! :)
2. The links in the post need fixing
 

I don't have a lot of context on insects' apparent ignoring extreme injuries, but it's worth noting that they may lack nociceptors in certain parts of their bodies, even if they have them elsewhere or can experience other negative states (e.g. fear). Or certain things may have strong analgesic effects, or they just don't experience pain very intensely relative to other drives. There are alternative possible explanations besides insects being incapable of suffering.

Also, I wouldn't be surprised if those examples aren't very representative.

OTOH, they found no evidence for flexible self-protection in most insects here: https://forum.effectivealtruism.org/posts/yPDXXxdeK9cgCfLwj/short-research-summary-can-insects-feel-pain-a-review-of-the