Does the criterion of sentience exclude insects?

Original text Luciano Carlos Cunha · Volume VI, p. 20 of the PDF edition · English translation

It is sometimes claimed that the criterion of sentience excludes insects from the sphere of moral consideration¹⁵. According to this position, if we should give moral consideration to insects, then we should reject the criterion of sentience and adopt the criterion of biological life in its place.

Someone might put forward this argument because they think that insects are capable of positive experiences (which would come from smelling, hearing, tasting, and so on), but not capable of feeling pain. In that case, however, what is being claimed is that insects are sentient (since they are capable of positive experiences), but not capable of negative experiences. If that were the case, these animals would not be excluded by the criterion of sentience. Sentience concerns each and every experience someone could have, and not only negative ones. Thus, the criterion of sentience also takes into account harms from the deprivation of positive experiences, and not only from the presence of negative experiences.

Although there is doubt in the scientific community about the sentience of some kinds of insects¹⁶, any responsible moral theory prescribes giving the benefit of the doubt where there is reasonable doubt. This is because, if these insects are not sentient and we treat them as if they were, the harm to us is smaller than the harm to them would be if they are sentient and we treat them as if they were not. Therefore, we should treat insects as sentient beings until proven otherwise. In short, it is not insects that need to prove that they are sentient. The burden of proof lies with those who doubt that they are.

Moreover, even if only a small percentage of insects were sentient, the number of insects the criterion of sentience would prescribe considering would already be gigantic. According to some estimates, there are around 10 quintillion insects alive at any given moment¹⁷. Even if only 0.01% of these insects are sentient, that would still be 100 quadrillion individuals. Furthermore, as we will see below, there are strong reasons to think that the percentage of insects that are sentient is not low.

The objection in question can also be criticized for assuming that insects should be considered even if they were not sentient. This does not explain what is wrong with the criterion of sentience (instead, it assumes from the outset that the criterion is wrong; but that is exactly what the argument was supposed to show). Suppose no insect were sentient. Why would we consider them, if there were no one in those bodies, and whatever happened would make no difference to them? The intuition that insects are worthy of moral consideration seems to come, on the contrary, from the possibility that these animals are sentient. And there is good evidence that at least a large number of them are in fact sentient¹⁸. Below we will look at some of this evidence.

Evidence of sentience in insects

The evidence listed below was obtained from studies of insects of certain species. As for insects of other species, however, it is not that they were studied and failed to meet the criteria for sentience: what happens is that they have barely been studied with regard to sentience. Thus, in keeping with the precautionary principle, what we should do is treat all insects as sentient until proven otherwise. This is already standard practice with regard to the sentience of other kinds of animals. For example, the sentience of vertebrates and of invertebrates such as cephalopod mollusks and decapod crustaceans is generalized from studies carried out on animals of certain species to animals of other, related species¹⁹.

There is a growing body of evidence on sentience in insects²⁰. One type of evidence is behavioral. The behavior of these animals is flexible, showing decision-making in contexts of changing circumstances, adaptive behavior, and cognitive generalization. Their behavior also shows learning, integrating different types of information²¹.

There is good physiological evidence as well. Insects have centralized nervous systems with brains. One of the regions of their brains is the protocerebrum. This region includes the

so-called mushroom bodies, which contain between one hundred thousand and one million neurons²². Electrical activity with a response between 20Hz and 30Hz has been recorded in their brains²³.

The mushroom bodies of insects receive and integrate various types of multisensory information. This allows them to learn from previous experiences, remember them, and integrate information²⁴. The mushroom bodies are therefore a specialized center for processing spatial information and organizing movement²⁵.

The map of neural connections is called the connectome. The connectome of some insects is comparable to the fiber tract networks found in primates, displaying so-called small-world organization. This organization combines high connectivity between neighboring regions with shortcut connections to distant regions²⁶.

If what this evidence suggests is correct, then these animals are not mere "living bodies": there is someone in these bodies, very probably with the capacity to suffer and to enjoy. Therefore, according to the criterion of sentience, they should receive the same degree of consideration as any other sentient being.