Do fruit flies feel pain?

Implicitly, we are often assuming a ladder of consciousness where humans stand at the top and rocks lie at the bottom. The higher an organism is positioned, the more we care about how much it suffers. Directly below humans are the great apes (“human monkeys” in German). In the European Union, it is mostly illegal to experiment on them and lots of people would want them to have a limited set of rights. Climbing down the ladder, we soon reach other large mammals, such as elephants, dogs and pigs. It is generally okay to kill them in large shacks built for that purpose, even though people get upset when somebody butchers a dog on his front lawn. Descending a few steps, we now reach a spot, where it is unlawful to torture mice for no reason, but setting a glue trap to slowly starve them to death is totally fine (only recently some jurisdictions have decided to ban this practice). Below that, ethics stop. The idea that insects or snails have even a modicum of consciousness almost never comes up. But is this sensible? This essay will explore how much we know about insect suffering taking the common fruit fly as an example because this animal is extremely well documented in the literature and most people interact with it frequently. Among my primary sources are these two excellent articles by Shelley Adamo [1] [2].

Arguments from biology

It is well established, that insects experience nociception, a term that describes the process of coding harmful stimuli into nerve signals. For example, even fruit fly larvae try to roll away when being exposed to heat [3]. Some areas in insect brains show similarities to reward circuits in mammals. It is unclear how insects process the nociceptive information, however, it is known, that two brain areas, called the “mushroom bodies” and the “central complex” play important roles. Sadly, there exists only a poor understanding of how these two areas process the incoming stimuli. The biggest difference to human brains is the fact that these areas only have very few output neurons and are not connected to each other. It is generally assumed, that a complex network is required in order to create a subjective feeling of pain in humans and there are doubts if the sparse connections in the insect brain suffice for that.

Arguments from evolution

Because the neural circuitry of fruit flies is so foreign to us, any comparison to mammal brains is difficult. Maybe a top-down approach could give us hints about whether it could have made sense for evolution to produce pain in insects?

Pain can be helpful to create long-lasting changes in behavior. Being burned by a hot stove builds memories that disincentive you from touching it again. To test, whether fruit flies can learn from pain, Australian scientists cut off the right middle leg of several flies and monitored how their response to heat changed. Normally, these animals try to escape from surfaces 42°C or hotter but are not too bothered by temperatures below that. The injured flies, however, fled more readily from 38°C hot surfaces. This could show that they remember the injury and try to protect themselves better afterwards. Fruit flies can also show a behavior called “learned helplessness”. To quote a German study [4]:

Here we investigate learned helplessness in Drosophila, showing that this behavioral state consists of a cognitive and a modulatory, possibly mood-like, component. A fly, getting heated as soon as it stops walking, reliably resumes walking to escape the heat. If, in contrast, the fly is not in control of the heat, it learns that its behavior has no effect and quits responding. In this state, the fly walks slowly and takes longer and more frequent rests, as if it were ‘‘depressed.’’

Arguments from robotics

It is easy to prove that nociception does not require any emotional processing of pain. Everybody with access to some electronics and a bit of programming knowledge can create a program that outputs “Ouch that hurts, please stop!”, once a button is pressed. Only recently, the arrival of reinforcement learning, which are algorithms that allow the computer to learn by trial and error shows that machines are able to remember past mistakes and act accordingly.

Arguments from authority

Because I am only a layman, here are some opinions of professional philosophers and scientists on the subject. Most of them are taken from Brian Tomasik's excellent collection [5].

Shelley Adamo, the author of the two cited articles (both of which I highly recommend if you want to learn more on the subject):

We can never know with certainty whether insects feel pain, but given our current understanding of insect behaviour, neurobiology, and evolution, the likelihood is low. The main issue is not even the small number of neurons in insects (several orders of magnitude less than in humans; Herculano-Houzel 2012). The more important difference is the lack of connections between relevant brain areas. If the subjective experience of pain is produced by a network composed of brain regions that integrate sensory information processing, emotions, cognition, and memory, then it does not appear that insects have their relevant areas wired up in this way.

Peter Singer [5]:

Insects are a very large and diverse category of beings. Honeybees have about a million neurons, which isn’t many compared to our roughly 20 billion neocortical neurons, let alone the 37 billion recently found in the neocortex of a pilot whale. But it is still enough to be capable of performing and interpreting the famous “waggle dance” that conveys information about the direction and distance of flowers, water, or potential nest sites. Caterpillars, as far as we know, have no such abilities. But they may still be conscious enough to suffer as they starve. […]

In the West, we tend to smile at Jain monks who sweep ants from their paths to avoid treading on them. We should, instead, admire the monks for carrying compassion to its logical conclusion. http://animalstudies.org.au/archives/5998

Entomologist Jeffrey Lockwood [6]:

So given that we can’t be sure whether insects experience pain, how should we treat these creatures? When I was teaching insect anatomy and physiology I insisted that the students anesthetized insects before conducting experiments that we would expect to inflict pain on a mouse […].

Conclusion

I am confused by the argument set forth by Adamo, because it seems to assume that because the “mushroom bodies” and the central complex are not very well connected and they are well connected in mammals, we can deduce that fruit flies probably don’t feel pain even though we are not sure what these two organs actually do. It seems unconvincing to me that animals which could experience “learned helplessness” are unlikely to also feel pain. I am equally unconvinced by the arguments from robotics. Computer algorithms are artificially created by humans and I don’t see why nature should also be able and willing to create nociception without some form of consciousness. The fact that evolution created no mammals or birds without consciousness [7] leads me to think that there are some strong incentives for this that we have not yet understood.

All in all, I am on the fence about pain in fruit flies. The arguments based on biology seem pretty strong and I don’t have the necessary expertise to properly examine them. On the other hand, the complex behaviors and pain responses shown by insects make me doubt the assumption that they are just unconscious robots.

Before writing this essay, I thought that farming insects for food could produce cheap protein for humans while evading the various moral issues concerning modern factory farming. Now, I am not so sure.

Sources

[1]Adamo, Shelley. (2016). Do insects feel pain? A question at the intersection of animal behaviour, philosophy and robotics. Animal Behaviour. 118. 75-79. 10.1016/j.anbehav.2016.05.005.

[2] Adamo, S. (2019). Is it pain if it does not hurt? On the unlikelihood of insect pain. The Canadian Entomologist,151(6), 685-695. doi:10.4039/tce.2019.49

[3] Hoyer, N., Petersen, M., Tenedini, F. M. and Soba, P. (2018). Assaying Mechanonociceptive Behavior in Drosophila Larvae. Bio-protocol 8(4): e2736. DOI: 10.21769/BioProtoc.2736.

[4] Yang Z, Bertolucci F, Wolf R, Heisenberg M. Flies cope with uncontrollable stress by learned helplessness. Curr Biol. 2013 May 6;23(9):799-803. doi: 10.1016/j.cub.2013.03.054. Epub 2013 Apr 18. PMID: 23602474.

[5] http://animalstudies.org.au/archives/5998

[6] https://reducing-suffering.org/do-bugs-feel-pain/

[7] The Cambridge Declaration of Consciousness available at https://fcmconference.org/img/CambridgeDeclarationOnConsciousness.pdf