Do invertebrates, such as insects, feel pain? There is quite a bit of debate among the scientific community on this topic.
While there have been some experimental results, there is still considerable discourse on the exact definition of “pain” and whether insects do in fact feel it.
In this article we’ll give you a deep dive in answering this exact question.
Be sure to read til the end as there are some interesting surprises in there!
What is Pain?
First, we need to discuss some of the critical elements that contribute to this discussion.
There are two main “components” of pain.
First, there is nociception, which refer to a set of reflex actions whereby a living organism moves either its whole body, or a part of its body, away from a noxious stimulus that has been introduced into its environment.
As an example, a soft tissue injury or touching a scalding hot iron can produce a muscle reaction whereby the animal withdraws from the pain or reacts to the pain.
This physiological reaction is connected to the Central Nervous System (CNS) and subsequently conveyed via neurons to the brain.
These reflexes are often correlated with pain, which is a different phenomenon.
To be precise, pain refers to an emotional (psychological) stimulus that originates in our brains.
The conflation of nociception and pain is often triggered when we “humanize” the problem by drawing analogs to the way we ourselves feel pain. This may be a false equivalence.
As an example, an amoeba will move away from noxious stimuli and an insect may writhe “in pain”.
But that visible reflex has been generated through millions of years of evolution to protect the species from dangerous environments.
Thus, shying away from noxious stimuli due to nociception is not what the strict scientific definition of pain.
Acute Pain vs. Chronic Pain
Scientists often distinguish between nociception and pain as follows:
- Nociception is termed as “acute pain” since it’s driven by a direct physical stimulus.
- Pain is sometimes referred to as “chronic pain” to distinguish it from nociception. It can have two components:
- Inflammatory pain, and
- Neuropathic pain.
The latter is the result of overactive nerves – the result being that the organism experiences shooting electrical pain in its body.
In common usage, the two terms are frequently used to differentiate between the immediacy and/or the duration of the pain – a loose interpretation that can be reconciled for humans
(with our complex brains, nervous systems and powers of expression) but which causes confusion when trying to be precise with insects and their “pain”.
The Key Statement
Scientific studies often cite the fact that while acute pain perception (nociception) developed over 500 million years ago. As neurobiologist Dr. Clifford Wolff of the Harvard Medical School remarks:
“From an evolutionary point of view, even single cell organisms like an amoeba need to be able to detect their environment and react to it by moving away from any danger.
So, the drive to detect potential harm has been there right at the beginning of the formation of living organisms.
Obviously as higher species with complex nervous systems evolved, it’s become much more complicated, involving sensations, mood, memory and movement.
Nevertheless, pain is sufficiently important—80 percent of our peripheral sensory neurons are pain fighters activated by pinch, pinprick, excessive heat or cold and other noxious stimuli.”
While nociception (covering the afore mentioned 80% of our neurons) is well documented, almost nothing is known about the molecular origin of chronic pain.
This leads to one of the fundamental issues we face when we “humanize” the issue by correlating motor responses to stimuli by insects with the origin of the pain, namely …
Physical Differences are Relevant in Terms of How Stimuli are Conveyed
There are many differences between our neural networks and those of insects, both with respect to the neurons (number and type) and where they are located.
Organisms have evolved to be efficient at what they do. An insect, by definition, needs simpler functions carried out by their neural networks than what we do.
Also, as we discuss in a later section, insects do not have their principal neurons (that could produce actual “pain”) concentrated in their heads.
This creates some interesting situations where the analogy driven comparisons may become hard to sustain.
More about this later. First, let’s discuss some recent discoveries with seemingly definitive answers for at least one species of insects.
Recent Discoveries that Insects can Feel Chronic Pain
Scientists had studied how fruit flies felt nociception in the early 2000’s. However, they were careful to call the sensation “akin to pain”, as opposed to pain itself in the scientific sense.
In July 2019, a study published in the journal Science Advances (https://advances.sciencemag.org/content/5/7/eaaw4099) took their experiments to a deeper level and concluded that fruit flies that sustain injuries can, in fact, have their nervous systems affected to the point where they feel chronic pain after those injuries have healed.
Without going into too many gruesome details, the study involved experiments such as cutting off the legs of fruit flies, letting them heal and then tracking behavior over the rest of their lives.
As senior researcher Greg Neely, from the University of Sydney, put it: “After the animal is hurt once badly, they are hypersensitive and try to protect themselves for the rest of their lives. That’s kind of cool and intuitive.”
One of the common forms of behavior alteration was that injured (but healed) flies seemed to have highly sensitive nerves.
For example, they would leave the room far more often in the event of a drop in temperature than other flies who had not been injured.
In digging deeper, Neely and his team found that there was in fact a cellular basis to this. The previously injured flies had developed “central pain sensitization”.
They developed a hypersensitivity to the surroundings, which in turn produced a painful reaction when exposed to common stimuli (e.g., the aforementioned drop in temperature) that may cause discomfort but would not cause pain in the normal course of events.
As Neely goes on to explain: “The fly is receiving ‘pain’ messages from its body that then go through sensory neurons to the ventral nerve cord, the fly’s version of our spinal cord. In this nerve cord are inhibitory neurons that act like a ‘gate’ to allow or block pain perception based on the context.
After the injury, the injured nerve dumps all its cargo in the nerve cord and kills all the brakes, forever. Then the rest of the animal doesn’t have brakes on its ‘pain’. The ‘pain’ threshold changes and now they are hypervigilant.”
By this measure, then, it seems that when you swat a fly and hit it a glancing blow, it will “suffer” for the rest of its life. Does this settle the debate about insects feeling pain, then?
Not quite. Let’s look at other scientific opinion to the contrary.
Back to the Physical Differences between Us and Insects
Dr. Shelley Adamo, an internationally renowned expert in the field of ecoimmunology and comparative psychoneuroimmunology, published a study in the Canadian Entomologist in late 2019, shortly after the article in Science Advances was published.
Dr. Adamo argues that its difficult to determine whether nociception turns to chronic pain in insects.
Insects process acute pain (nociception) in two areas of their brains that are responsible for higher order functions – the central complex and the mushroom bodies.
Unlike in higher animals or humans, there is limited evidence that a coordinated pain network – that would integrate these two areas with each other – exists.
In the absence of such coordination, it is difficult to propose that insects have the capability to experience significant chronic pain like we do.
What About Modest Pain?
Dr. Adamo does not rule out the possibility of some pain being shared through a poorly integrated neural circuit, but she presents a logical argument that even a modest pain response is likely not present in insects.
She goes back to evolution and some of the observed behavior from recent experiments involving artificial intelligence.
She points out that relatively simple circuitry can be drawn up to “teach” adaptive behavior, without subjective experience ever coming into play.
Dr. Adamo notes that since even a modest sharing of subjective experience such as pain would require some neuronal investments.
Based on insect behavior, neurobiology, and evolution, she concludes that such a neuronal investment would not really “benefit” the insect.
Her strong conclusion is that insects are unlikely to feel even modest amounts of chronic pain. It has less to do with the fact they have far fewer neurons (several orders of magnitude lower) than humans, but …
“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.”
A Move Towards Clarity … or More Confusion?
Having considered two opposed views, both supported by science, a few things seem clear:
- Nociception is common in insects – they definitely react to adverse stimuli.
neuronal structures are simpler and not well connected (as with humans), so its
possible that they do not feel chronic pain.
- Studies in robotics, for example, have proved that simple circuits can create adaptive behavior without accompanying subjective experiences.
- However, the fruit flies experiment seems to suggest that they do.
A Philosophical Take – Playing What If and Why Not Games
An interesting philosophical take on the matter has also been presented, combining known facts with logic to posit the possibility of pain perceptions in insects.
A good (if a bit dated) synopsis was presented by Jeffrey Lockwood in an Oxford University Press blog in 2011.
Some of Lockwood’s hypotheses have since been further examined and categorized in ways that distinguish nociception from chronic pain. But some are still relevant.
To start with, says Lockwood: “… insects can hear, smell, taste, and feel. Many of our pains arise from pressure, shock, heat and other stimuli administered at high levels—and insects most assuredly respond to these bodily sensations.”
Next: “At least some invertebrates possess endorphins and enkephalins. These chemicals are opioids (think opium) produced by the body to alleviate pain and stress.
So the presence of these in insects suggests that they might experience pleasure/pain.”
Lastly, he points out that insects do not stuff all their neurons into their head like we do. As evidence, scientists have found that headless cockroaches can exhibit learning behavior.
The combination of all these phenomena, per Lockwood, is that it may not be prudent to disavow the notion that insects can feel pain.
Enter the Exoskeleton?
An interesting finding reported by Lockwood was from a late 20th century insect physiologist, Vincent Wigglesworth, who observed that insects do not feel cuticular pain.
This can translate to a simple conclusion – if our skin gets pierced, we feel pain both during and after the event, so both acute and chronic pain.
If an insect’s exoskeleton gets pierced or a leg gets injured, they don’t feel the same level of pain as we do.
But do we know this? The fruit fly experiment seems to prove otherwise, especially with regard to the injured leg part.
The Final Verdict
The jury still seems out if we are looking for a definitive answer on whether the neuronal structure of insects are capable of delivering a pain experience like humans feel.
We do know that they react to adverse stimuli, and there has been some research results that do seem to indicate the possibility of chronic pain.
Perhaps the best approach to adopt is philosophical. Going back to Lockwood, he picks the example of a lab animal, like a mouse or a frog, that we do know is capable of feeling pain the way we define it.
He suggests that if conducting lab experiments involving an insect, anesthetize the subject if you would have done so with a mouse.
Why take the risk of inflicting pain if there is even a small possibility that they could feel it?
Then, of course, you have the situation where you may squash a bug or call the exterminator, but that’s a different story …
Alright guys, that’s it for this article, if you want to learn more than check out some of our other hand selected articles that you might find interesting!
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