Do insects feel pain?

Many people have this question and it’s one that entomologists talk about frequently.

Humans have nociceptors (also called pain receptors) that can detect uncomfortable stimulus (heat, cold, mechanical etc.) over a certain threshold. This neurological response is called nociception and a similar process has been identified in fruit flies but insects do not have nociceptors. The study I’m referencing poked hot pins in the sides of Drosophila larvae and they would uncharacteristically roll out of the way. So without nociceptors how do they exhibit a nociception response? It was found that a single class of neuron (class IV multidendritic neuron) is sufficient and necessary to cause this nociception behavior. Dr. R. Y. Hwang postulates this behavior evolved to avoid a parasitoid wasp, and the larvae are innately encoded with the directionality in which they roll. So they are not thinking, “Ouch this hurts. I’m rolling out of the way.” They are genetically preprogrammed with this behavior, and given a certain physical stimulus they roll out of the way. Fruit flies are the best-studied insects and are considered a model organism, so it goes to reason if they experience nociception other insects may also. Some activists tout these two studies as proof insects feel pain. I do not. It has always made sense to me that an insect should be able to feel something akin to pain; because it would help them learn to avoid things that could kill them through the unpleasant nature of the experience.

But nociception is not pain. The current definition of pain requires an emotional response. Humans can feel pain without any physical stimulus and are capable of emotions associated with pain; like suffering and terror. Are insects capable of conscious or unconscious experience of emotion? Is consciousness required for emotions? This is where it gets controversial; because how do you quantify if an insect is experiencing an emotion or if insects are conscious? I usually tell people that insects are hardwired with predetermined behavioral responses to external stimuli, but this is a simplification.

From the current literature you cannot definitively say insects have emotion, but there have been many interesting studies on the complexity of insect behavior. Honey bees, ants and other social insects have complex behavior and it has long been known that foraging bees will come back to the hive and dance for other bees to express where resources are. One study even shook bees in tubes and measured their agitated responses. They claim bees are capable of expecting bad outcomes and can exhibit a vertebrate-like emotional state. This was a controversial study, but it raises important questions. Even solitary dragonflies have selective attention to snatch individual prey out of swarms. But do these complex behaviors and cognition elucidate the emotions required to define them as experiencing pain? I’m not sure.

I learned in my undergrad insect biology class that insects can learn but they cannot think. I’ve held on to this as I progress through my graduate degree, but it’s a much more complex issue when you try to define “thinking” and “emotions” and “consciousness”. Some scientists are seriously considering if insects are conscious. On July 7th 2012 The Cambridge Declaration on Consciousness was released. It explains what some of the top scientists in relevant fields think about ‘consciousness’. Consciousness is not a definitive line, but a moving scale.

From The Cambridge Declaration on Consciousness:
We declare the following: “The absence of a neocortex does not appear to preclude an organism from experiencing affective states. Convergent evidence indicates that non-human animals have the neuroanatomical,
neurochemical, and neurophysiological substrates of conscious states along with the capacity to exhibit intentional behaviors. Consequently, the weight of evidence indicates that humans are not unique in possessing the neurological substrates that generate consciousness. Non-human animals, including all mammals and birds, and many other creatures, including octopuses, also possess these neurological substrates.”

So where do insects fall on this scale? Probably further away from mammals and even cephalopods. But how do you KNOW? We don’t. New studies are coming out and it will be exciting to follow them.

As of right now the animal testing regulations draw a line between vertebrates and non-vertebrates (with the exceptions of some cephalopods). I do not think any research to date warrants a discussion of moving the line, but at the same time I do not think we should rule out the possibility that insects are capable of pain, albeit through different neurological pathways.

As Carl Sagan popularized, “Extraordinary claims require extraordinary evidence”. Insect pain and suffering is one of those extraordinary claims and is going to require extraordinary evidence to definitively say one way or another. However, in the meantime I will not expose any insects to undue suffering when I use them in experiments or add to my insect collection.

This was a long answer to a short question, but it is fascinating to think about.

Further reading and sources:
Dragonflies have human-like ‘selective attention’
http://www.eurekalert.org/pub_releases/2012-12/uoa-dhh121812.php

Do bees have feelings?
http://www.scientificamerican.com/article.cfm?id=do-bees-have-feelings

Agitated Honeybees Exhibit Pessimistic Cognitive Biases
http://www.cell.com/current-biology/abstract/S0960-9822%2811%2900544-6

Can invertebrates suffer? Or, how robust is argument-by-analogy?
http://www.scopus.com/record/display.url?eid=2-s2.0-0001460737&origin=inward&txGid=Fefoa6RxV0qF3gMVRibiJw3%3A2

Pain and suffering in invertebrates?
http://www.vliz.be/imis/imis.php?refid=212638

Insect Brains and Animal Intelligence
http://bioteaching.wordpress.com/2010/05/03/insect-brains-and-animal-intelligence/

On the Consciousness of Animals …
http://www.itsokaytobesmart.com/post/30601585978/on-cambridge-declaration-of-consciousness

Consciousness in a Cockroach
http://discovermagazine.com/2007/jan/cockroach-consciousness-neuron-similarity#.UYKKYiv72Do

Unconscious Emotions, Conscious Feelings, and Curricular Challenges
http://education.jhu.edu/PD/newhorizons/Neurosciences/articles/unconscious/

How Pain Works
http://science.howstuffworks.com/life/inside-the-mind/human-brain/pain3.htm

I don’t know who wrote this Wikipedia page, but it really is excellent.
http://en.wikipedia.org/wiki/Pain_in_invertebrates