It matters whether insects can feel pain, in what ways, and to what degree. Yet, scientists often assume that these animals can’t feel pain at all, based on an overly confident reading of outdated research.

In this paper, the authors respond to a review from 1984 that’s come to be treated as particularly authoritative on the topic of insect pain. They document how many of the review’s claims have been contradicted by more recent research, as well as how some of its arguments shouldn’t give us the kind of confidence that many who still cite it tend to assume.

The 1984 review essentially makes two key arguments:

1. Insect biology should lead us to suspect that they can’t experience pain; and
2. Insect behavior indicates that they don’t experience pain.

However, with the benefit of the last 40 years of research, the picture looks much more complicated on both fronts.

The Insect Biology Argument

The 1984 review claims that insects don’t have nociceptors, which are the sensory cells that detect harmful stimuli like heat. This has turned out to be untrue: modern research has found a variety of nociceptors in insects. Nociception isn’t equivalent to pain — it’s more like the reflex response to a burn, separate from the pain that comes soon after. But the systems are tightly connected in many cases, so the presence of nociceptors weakens the claim against insect pain.

The 1984 review also argues that even though insects have opioid receptors (a key part of many vertebrate pain systems), this shouldn’t be taken as strong evidence that they feel pain, as we don’t know they’re performing the same function. Interestingly, research since then has found that insects actually don’t have opioid receptors. Instead, they have a number of other neurochemical systems that seem to be performing similar pain-relief functions.

It isn’t certain whether these neurochemical systems mean that insects experience pain, but much modern research strongly suggests they do. For instance, there’s evidence that non-steroidal anti-inflammatory drugs, analgesics which are known to affect pain in mammals, also work for insects like fruit flies.

Another argument raised in the 1984 review is that insects don’t have large or complicated-enough brains to feel pain. However, brain size varies greatly between species and isn’t an obvious predictor of animals’ abilities. Pinning specific functions down to particular areas, structures, or neuron counts has proven difficult. Furthermore, some insects actually have brains larger than some vertebrates, so this argument rests on shaky ground.

The Insect Behavior Argument

The argument from insect behavior in the 1984 review mostly focuses on the ways that insects respond to being injured, which can be vastly different from how vertebrates respond. Insects often won’t behave as if they’re hurt in any obvious way even when suffering extreme physical injuries. Many take this as evidence that they don’t experience anything pain-like or subjectively unpleasant about the injury.

There are a few problems with this argument. First, insects have very different bodies to vertebrates. One of the most obvious ways vertebrates respond to injury is by altering their movement to reduce pressure on the injured area — like limping, for example. But insects have exoskeletons, and the distribution of forces around their bodies as they move is a far cry from our own pulley-and-lever systems. Thus, it isn’t clear whether limping is something that would mitigate any pain an insect might be feeling, so a lack of limping shouldn’t make us confident that the insect isn’t feeling pain.

The second problem is that this argument wasn’t based on much systematic study of how insects respond to injury, but rather on anecdotal reports. These naturally tend toward the extreme and memorable, such as a locust continuing to eat while being eaten by a mantis. Much research since, as well as some that was available at the time, has shown insects responding in much more intuitively pain-like ways to injuries like burning and surface cuts. They, too, stop eating, tend to their wounds, and protect affected body parts. It may be that a focus on bizarre and grotesque-seeming responses to mechanical injuries, where the body type differences make it hardest for us to guess at what a pain-minimizing response might even look like, has unduly influenced thinking on behavioral arguments against insect pain.

There is another, more subtle behavioral argument that insects are so rigid and instinctive in their behaviors that it wouldn’t make evolutionary sense for them to experience pain. Pain is useful for vertebrates because we learn from it and adapt our behaviors to avoid it. The claim made back in the 1980s was that because insects mostly just act on basic instinct, not learned behavior, there would be no evolutionary reason for them to experience pain.

It was unclear at the time whether insects really were “pre-programmed,” or whether they could learn and adapt like most vertebrates. However, it’s now clear that, at least for a large number of insects, behaviors can be and are learned. Just take bumblebees, for example, who demonstrate social learning and make pain/reward tradeoffs. This undermines the evolutionary case against insect pain.

A Call For Caution

Intriguingly, despite being very skeptical of the evidence at the time for insect pain, the 1984 review stressed that the picture is uncertain and advised a precautionary approach. However, this has often been forgotten in the review’s use as a scientific authority against insect pain in the years since.

In this paper, the authors urge even more caution, given that much more evidence in favor of insect pain has come to light since 1984, many of the arguments made at the time were somewhat questionable even then, and how hard it is to generalize conclusions across the million-plus insect species. They argue that we should adopt significantly more humane practices in the treatment of insects in the laboratory. Many easy and cost-effective improvements could be made — like actually euthanizing insects at the end of experiments rather than just letting them starve to death — but are often ignored based on mistaken confidence in an outdated scientific authority.