The last two questions we tackled revolved around the cognitive abilities (communication, and personality) of arthropods, specifically insects and spiders. These are really tough questions to tackle because a lot of people, even (especially?) those who genuinely like insects and spiders, want to humanize them as a way to identify with them.
I think it’s a good thing to want to identify with these animals, so long as we’re making proper comparisons. Sometimes, however, this very human impulse can get us into trouble…like this Twitter post by the Featured Creature twitter account.
— Featured Creature (@ftcreature) July 22, 2013
This image is typically shared as an example of maternal care in jumping spiders. Although maternal care does exist in this group, it doesn’t typically involve the mother carrying them to a new location in the manner shown.
Assuming the picture is real (see Chris M. Buddle’s comment), the activity pictured here is most likely cannibalism. Instead of being an endearing picture, this is more of an example of how our desire to identify with insects and spiders can cloud our image of how they behave. This desire has a name, it’s called ‘anthropomorphism’…which means to imbue human characteristics or motivation onto non-human organisms.
I wanted to open our discussion of insect pain up with that story, because it’s a very important point to make. Insects react to the world differently than we do, and when it comes to cognition it’s very difficult to separate our motivations from theirs.
Separating Pain and Sentience
The first thing I think we need to do is to define ‘pain’, because it’s not as simple as avoiding injury. During the deepest stages of sleep, people still toss and turn to avoid putting themselves into potentially damaging positions…even if they don’t remember the experience. We can also react to stimuli without feeling it, using a reflex arc. It’s possible to avoid injury without experiencing pain as we understand it.
The people who study pain in humans have a pretty useful definition of pain, which differentiates between experiencing pain and avoiding injury (which is termed nociception). You can read the full definition here, but I’ll remove the relevant snippets:
An unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage.
The inability to communicate verbally does not negate the possibility that an individual is experiencing pain and is in need of appropriate pain-relieving treatment. Pain is always subjective. Each individual learns the application of the word through experiences related to injury in early life.
This definition avoids tying pain to the stimulus. Activity induced in the nociceptor and nociceptive pathways by a noxious stimulus is not pain, which is always a psychological state, even though we may well appreciate that pain most often has a proximate physical cause.
Although it might seem to be a little nitpicky, we really should separate pain and nociception. After all…bacteria can avoid toxic substances just as well as animals can.
Insects and Injuries…what do we know?
We know that insects can avoid injuries, and sense most potential sources of damage. The best studied nociceptor is the Drosophila nociceptor TRPA1, which is pretty closely related a gene in mammals which has an identical function. It senses pressure and heat, and reacts to the chemicals which make peppers hot (capscaicin) and mustard spicy (isothiocyanate). I’m not sure how widespread this protein is, because there have been gains and losses in some nociceptors. It’s also found in beetles, so painless appears to be somewhat widespread.
Different animals use different receptors to sense different things. Drosophila is repelled by the taste of wasabi, but crayfish are not. Additionally, crayfish do not react to the type of burn that dry ice produces when you get a wart burnt off at the doctor’s office. They do, however, respond to heat. It’s very likely that crayfish have TRP receptors, but it seems they use different receptors than fruit flies and humans use.
Pain and Emotion
Pain is ultimately an emotional response, so the question of whether insects feel pain as we would understand it really depends on whether they feel emotion. This is where scientists run into problems with the whole ‘do insects feel pain’ question.
There’s a rather obvious philosophical issue which needs to be mentioned, The Problem of Other Minds. I’m not going to spend much time on this, because we can avoid that issue by comparing the behavior of injured insects to injured mammals. Mammals, being animals we *know* feel pain, are the positive control here.
It’s important to note how Drosophila defends itself against parasitic wasps. It does this by rolling over them, which makes it impossible for the wasps to get enough leverage to pierce the larva’s exoskeleton.
Mammals have a diverse response to pain, often avoiding use of the injured area and responding to different types of pain differently. Drosophila, on the other hand reacts to many different insults by rolling towards the thing that hurts.
If you poke the larvae with a needle, stick them with a soldering iron, bathe them in blue light, or put them in hot water…it’s always the same response they have to the wasp ovipositor.
That being said, we can make some further observations about how insects learn to avoid bad situations. Drosophila can be conditioned in a manner to the famous Pavlov experiments. The gene White, which we talked about when we spoke about personality, appears to regulate how extreme the fly experienced a situation. Some White mutations, in addition to regulating some aspects of the fly personality, appears to make flies overestimate how bad the experience was. They learn to avoid stimulus associated with shocks quicker, and un-learn the association slower than flies without the mutation. This only happens with bad memories, though. If you reward (rather than punish) the fly, the White mutants perform pretty similarly to non-mutants.
Interestingly, the human equivalent of White is involved in panic disorders in humans.
Bringing it all together
With this topic there’s a million different ways we could go with this, and there’s just not enough room on this blog to go through all of the required material in detail. While researching this article, I found a diagram which sums up this topic rather nicely:
Drosophila does a lot of things in this graph which indicate the presence of pain. It…
- can sense when it’s injured, using equipment similar to mammals.
- can learn to avoid bad things, and seek out good things
- can differentiate degrees of ‘good’ and ‘bad’ experiences.
However, fruit flies don’t seem to…
- Have appropriate responses to different degrees of injury
- Employ mitigating behaviors, such as rubbing, limping, or guarding the injury.
So it’s likely that Drosophila doesn’t feel pain…because it doesn’t match the positive control, right?
Well, the tobacco hornworm can also do all these things…but it has responses which are more mammal-like than Drosophila.
Noxious stimuli to anterior or posterior segments can evoke a transient withdrawal (cocking) that precedes a strike towards the source of stimulation and may function to maximize the velocity of the strike. More intense noxious stimuli evoke faster, larger strikes and may also elicit thrashing, which consists of large, cyclic, side-to-side movements that are not directed at any target. These are sometimes also associated with low-amplitude quivering cycles. Striking and thrashing sequences elicited by obvious wounding are sometimes followed by grooming-like behavior.
There’s also some evidence of self-medication in insects, which usually involves ingesting substances that have a negative effect on their development. So they’ll pay a cost to access some sort of mitigating measure, when it comes to parasites or illness. Drosophila happens to be one of these insects.
The Bottom Line: Do Insects Feel Pain?
This article, for me, was one of those really weird articles where I think my opinions changed while doing research for the article. The typical position of entomologists is that insects feeling pain is pretty unlikely. In the past, I’ve argued a position stronger than the one that Bova argues in his piece…one which was based upon the information similar to what Bova received during his undergraduate training.
However it now appears to me that a lot of the pathways insects use to not only sense injury, but interpret these experiences, and respond to that injury are a lot more similar to my pathways than I had originally realized. The paper on hornworm responses to pain was something I was unaware of, and have never had the opportunity to observe in person. The grooming behavior is what interests me, and it doesn’t seem to have been addressed further in the literature.
So based on all of this new information, my response has gone from outright skepticism to something more akin to:
Insects are a very diverse group, with biology that can vary widely between all groups. I would not expect every insect to interpret the world the same way, because they do not all have the same level of complexity.
Among insects, most of the pieces of the evidence required to say that insects feel pain appear in some groups to some extent. However, they do not appear in all groups to the extent which would result in a definitive answer. It would not surprise me to learn that some insects, particularly some of the social insects, would posses all the pieces of evidence.
While we can’t say for sure with our current state of knowledge, it seems that the field of insect nociception may be heading in that direction.
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