Do Insects Feel Pain?
First written: April 2009; last edited: 26 Nov. 2012
Summary. Do insects suffer? Does it feel like anything to be an injured or distressed insect? This piece aims to shed some light on that question by presenting quotations and references from a variety of sources. My personal conclusion is that we should give some weight to the possibility of insect suffering, especially until more evidence is available. Thus, considering the 10^18 insects that exist at any given time, there is a huge amount of (potential) suffering in nature due to insects alone. We may also want to consider the ways in which humans impact insects, such as through insecticide use, although if insect lives are generally not worth living, insecticides may prevent more suffering than they cause. (Of course, reducing insect habitat permanently would be more humane than simply spraying pesticides.)
Suggestions of Pain
Do insects experience pain? Yes. Well actually, this concept has been disputed, but I think recent evidence suggests that they do experience what is defined as pain.
In any case, abundant evidence indicates that all invertebrates with a brain can experience pain. Like vertebrates, numerous invertebrates produce natural opiates and substance P. These animals include crustaceans (e.g., crabs, lobsters, and shrimps), insects (e.g., fruit flies locusts, and cockroaches), and mollusks (e.g., octopuses, squids, and snails).
Also, crustaceans, insects, and mollusks show less reaction to a noxious stimulus when they receive morphine. For example, morphine reduces the reaction of mantis shrimps to electric shock, praying mantises to electric shock, and land snails to a hot surface.
Field observations showed orb-weaving spiders (Argiope spp.) to undergo leg autotomy if they are stung in a leg by venomous insect prey (Phymata fasciata). The response occurs within seconds, before the venom can take lethal action by spread to the body of the spiders. Autotomy is induced also by honeybee venom and wasp venom, as well as by several venom components (serotonin, histamine, phospholipase A2, melittin) known to be responsible for the pain characteristically elicited by venom injection in humans. The sensing mechanism by which spiders detect injected harmful chemicals such as venoms therefore may be fundamentally similar to the one in humans that is coupled with the perception of pain.
Nociception vs. Pain
Invertebrates, it seems, exhibit nociceptive responses analogous to those shown by vertebrates. They can detect and respond to noxious stimuli, and in some cases, these responses can be modified by opioid substances. However, in humans, at least, there is a distinction to be made between the "registering" of a noxious stimulus and the "experience" of pain. In humans, pain "may be seen as the response of the whole awake conscious organism to noxious stimuli, seated.., at the highest levels in the central nervous system, involving emotional and other psychological components" (Iggo, 1984). Experiments on decorticate mammals have shown that complex, though stereotyped, motor responses to noxious stimuli may occur in the absence of consciousness and, therefore, of pain (Iggo, 1984). Thus, it is possible that invertebrates' responses to noxious stimuli (and modifications of these responses) could be simple reflexes, occurring without the animals being aware of experiencing something unpleasant, that is, without "suffering" something akin to what humans call pain.
[...] What evidence might help in distinguishing between nociceptive "responsiveness" and the perception of pain? [...]
In mammals, responses to painful stimuli often persist beyond a simple reflex withdrawal, so that, for example, the animals may become immobile, limp or "guard" the affected part, show aggression when approached, reduce or stop feeding and drinking, and show decreased sexual activity (Morton and Griffiths, 1985). The animals may also learn in the future to avoid situations similar to the one in which the pain occurred. Such responses, while not proof that the animals have experienced pain, can indicate that something more than a simple nociceptive reflex is involved. Together, they may help the animal to recover from damage caused by the painful event and avoid being harmed in the future.
Would one or all of those neural patterns of injured tissue be the same thing as knowing one had pain? And the answer is, not really. Knowing that you have pain requires something else that occurs after the neural patterns that correspond to the substrate of pain – the nociceptive signals – are displayed in the appropriate areas of the brain stem, thalamus, and cerebral cortex and generate an image of pain, a feeling of pain. [This second process] interrelates neural patterns of tissue damage with the neural patterns that stand for you, such that that yet another neural pattern can arise -- the neural pattern of you knowing, which is just another name for consciousness. [...]
Curiously, if there had been no you, i.e., if you were not conscious and if there had been no self and no knowing relative to hot plates and burning fingers, the wealthy machinery of your self-less brain would still have used the nociceptive neural patterns generated by tissue damage to produce a number of useful responses. For instance, the organism would have been able to withdraw the arm and hand from the source of heat within hundreds of milliseconds of the beginning of tissue damage, a reflex process mediated by the central nervous system. [...] Although all of these responses, simple and not so simple, occur reliably in comparable situations in all conscious human beings, consciousness is not needed at all for the responses to take place. For instance, many of these responses are present even in comatose patients in whom consciousness is suspended. [...]
Tissue damage causes neural patterns on the basis of which your organism is in a state of pain. If you are conscious, those same patterns can also allow you to know you have pain. But whether or not you are conscious, tissue damage and the ensuing sensory patterns also cause the variety of automated responses outlined above, from a simple limb withdrawal to a complicated negative emotion. In short, pain and emotion are not the same thing.
Learning, Memory, and Motivation
A 1986 paper,
The progress achieved over the last 10-15 years in studying a wide variety of forms of learning in simple invertebrate animals is quite striking. There is now no question, for example, that associative learning is a common capacity in several invertebrate species. In fact, the higher-order features of learning seen in some invertebrates (notably bees and Limax) rivals that commonly observed in such star performers in the vertebrate laboratory as pigeons, rats, and rabbits.
[... W]e have reason to hope that the distinction between vertebrate and invertebrate learning and memory is one that will diminish as our understanding of underlying mechanisms increases.
[...] jumping spiders (Portia spp.) plan routes towards their prey ; and hermit crabs (Pagurus berhnardus) show evidence of motivational trade-offs during shell choice . Furthermore, if their brains are implanted with electrodes, garden snails (Helix aspersa) will learn to displace a lever, an action new to their behavioural repertoire, to stimulate those neural regions involved in sexual behaviour . None of these represent concrete evidence of conscious emotion, but they at least suggest that if cephalopods are to now be protected across Europe, then arachnids, decapod crustaceans and gastropods should be too.
"We have shown that the emotional responses of bees to an aversive event are more similar to those of humans than previously thought," said Geraldine Wright of Newcastle University. "Bees stressed by a simulated predator attack exhibit pessimism mirroring that seen in depressed and anxious people." [...]
But, they say, that isn't the same as saying that bees consciously experience emotions in the way that we do. On that point, the jury is still out.
In response to Edge's World Question Center 2005 topic,
What Do You Believe Is True Even Though You Cannot Prove It?, Alun Anderson, Editor-in-Chief at New Scientist, made the following remarks:
Strangely, I believe that cockroaches are conscious. [Moreover, ...] I believe that many quite simple animals are conscious, including more attractive beasts like bees and butterflies.
[... What I mean by consciousness is] the feeling ofseeingthe world and its associations. For the bee, it is the feeling of being a bee. I don't mean that a bee is self-conscious or spends time thinking about itself. But of course the problem of why the bee has its ownfeelingis the same incomprehensiblehard problemof why the activity of our nervous system gives rise to our ownfeelings.
But at least the bee's world is very visual and capable of being imagined. Some creatures live in sensory worlds that are much harder to access. Spiders that hunt at night live in a world dominated by the detection of faint vibration and of the tiniest flows of air that allow them to see fly passing by in pitch darkness. Sensory hairs that cover their body give them a sensitivity to touch far more finely grained than we can possibly feel through our own skin.
[...] And as for the cockroaches, they are a little more human than the spiders. Like the owners of the New York apartments who detest them, they suffer from stress and can die from it, even without injury. They are also hierarchical and know their little territories well. When they are running for it, think twice before crushing out another world.
Many people would pooh-pooh the notion of insects having brains that are in any way comparable to those of primates. But one has to think of the principles underlying how you put a brain together, and those principles are likely to be universal. [...] Attention is a whole-brain phenomenon. A thing is not purely visual, not purely olfactory. It's a binding together of different parts that for us signify one thing. Why couldn't the fly's mechanism [of attention] be directed to a succession of its memories? That, to me, is just a short hop, skip, and a jump away from what might be consciousness.And Christof Koch:
We have literally no idea at what level of brain complexity consciousness stops. Most people say, 'For heaven's sake, a bug isn't conscious.' But how do we know? We're not sure anymore. I don't kill bugs needlessly anymore. [...] Probably what consciousness requires is a sufficiently complicated system with massive feedback. Insects have that. If you look at the mushroom bodies, they're massively parallel and have feedback.
Explaining instinctive behavior in terms of conscious efforts to match neural templates may be more parsimonious than postulating a complete set of specifications for motor actions that will produce the characteristic structure under all probable conditions. Conscious efforts to match a template may be more economical and efficient. [... Of course] it is not necessary to suppose that animals [including insects] are consciously aware of all their neural templates; perhaps only a few are important enough that the animal thinks consciously about them and considers alternative ways of realizing them.
On p. 105, Griffin elaborates an example:
The workers of leaf-cutter ants are tiny creatures, and their entire central nervous system is less than a millimeter in diameter. Even such a miniature brain contains many thousands of neurons, but ants must do many other things besides gathering leaves and tending fungus gardens. Can the genetic instructions stored in such a diminutive central nervous system prescribe all of the detailed motor actions carried out by one of these ants? Or is it more plausible to suppose that their DNA programs the development of simple generalizations such asPage 111 gives an example with spiders:Search for juicy green leavesorNibble away bits of fungus that do not smell right,rather than specifying every flexion and extension of all six appendages?
W. S. Bristowe (1976) describes how orb-weaving spiders sometimes vary their stereotyped behavior in dealing with small insects caught in their webs. If an experimenter holds a struggling fly with forceps close to such a spider, she omits the earlier stages of normal behavior (running along the web to reach the fly) and bites it immediately. If the fly is already dead, she wraps it in silk without biting it first. In constructing their elaborate webs, spiders are often said to follow a rigid series of behavior patterns which are presumably instinctive since a female spinning her first web does so almost perfectly. But she will make some alterations in structure when the surrounding vegetation or the space to be spanned is irregular. Bristowe describes how a spider whose web is ordinarily symmetrical builds a highly asymmetrical web when the opening between leaves makes such a shape appropriate. At the web's hub from which strands of silk radiate out to the surrounding vegetation, the spider ordinarily leaves a hole so she can quickly move from one side of the web to the other when an insect strikes it. In one web this hole, instead of being at the center, was close to one edge of the opening between the leaves of a lilac bush, and the strands formed a semicircle instead of a circle.
Many ethologists dismiss variability in structures such as spider webs as meaninglessnoisein a basically invariant system and deny that a spider could consciously adjust the structure of her web according to the shape of the available opening. But the end results are so efficiently adapted to their function of catching small flying insects that it seems possible that spiders anticipate the likely results of their web spinning.
The moral standing of insects and the ethics of extinction,Professor Jeffrey Lockwood, an entomologist and philosopher, summarizes arguments for self-awareness based on social interaction:
Another theoretical consideration of insect consciousness, is an extension of the work by Humphrey (1978) (who derived his work from that of Jolly (1966)) on the evolution of societies (Griffin 1984). The basic concept states that a critical step in the evolution of animal societies is the establishment of efficient interactions, and these interactions depend on group members' abilities to understand each others' thoughts, intentions, and feelings. Therefore, social insects must correctly judge the frame of mind, as it were, of one another. [...]
Social insects behave so as to meet the communicated needs of the colony. One can construct a system which awkwardly explains social interaction such as food begging and tropholaxis or behaviors such as grooming, without including self-awareness. However, few would argue that social insects, and probably all insects, demonstrate an awareness of outside events; they behave according to environmental conditions and, as discussed earlier, they demonstrate the ability to communicate information about these conditions. Allowing that an insect has awareness of external events but does not have self-awareness is somewhat ridiculous--it is rather implausible to contend that through sensory mechanisms an insect is aware of the environment, other insects, and the needs of conspecifics but through some neural blockage, the same insect is selectively unconscious of sensory input about itself.
'Social' insects and arachnids exhibit forms of complex behaviour that involve cooperation in building a nest, defending against attackers or rearing offspring. [... Some chapters from this book] are as follows:
Post-ovulation parental investment and parental care in cockroaches
The spectrum of eusociality in termites
Maternal care in the Hemiptera: ancestry, alternatives and current adaptive value
Evolution of parental care in the giant water bugs (Heteroptera: Bolostomatidae)
The evolution of sociality in aphids: a clone's eye view
Ecology and evolution of social behaviour among Australian gall thrips
Interactions among males, females and offspring in bark and ambrosia beetles: the significance of living in holes for the evolution of social behaviour
Morphologically 'primitive' ants: comparative review of social characters, and the importance of queen-worker dimorphism
Social conflict and cooperation among founding queens in ants (Hymenoptera: Formicidae)
Social evolution in the lepidoptera: ecological context and communication in larval societies
Sociality and kin selection in Acari
Colonial web-building spiders: balancing the costs and benefits of group living
Here we report that gregarious cockroaches (Blattella germanica) reared in isolation showed (i) stronger exploration-avoidance, (ii) reduced foraging activity, (iii) reduced willingness to interact socially, and (iv) reduced ability to assess mating partner quality than conspecifics reared in groups. We demonstrate the occurrence of a behavioural syndrome induced by social isolation, similar to syndromes described in vertebrates, revealing the importance of social interactions and group-living in this non-eusocial insect species.
Scientists are discovering that these supposedly crude, and creepy automatons are much more sophisticated than we thought.
By unveiling the secret lives of these insects, they are finding out that cockroaches are actually highly social creatures; they recognise members of their own families, with different generations of the same families living together.
Cockroaches do not like to be left alone, and suffer ill health when they are.
And they form closely bonded, egalitarian societies, based on social structures and rules. Communities of cockroaches are even capable of making collective decisions for the greater good. [...]
Cockroaches that do not hang out with one another sufferisolation syndromes. For example, young German and American cockroaches left alone take longer to moult into new larger forms and eventually become adults.
Their later behaviour is also severely affected; young isolated cockroaches find it harder to join a community and mate later in life.
Young cockroaches, it seems, need to be around and in constant physical contact with one another to properly develop.
What about tissue damage?
Do insects feel pain? A biological view(1984), many insects lack nociceptors and so perhaps would not find tissue damage to be aversive. The authors note:
No example is known to us of an insect showing protective behavior towards injured parts, such as by limping after leg injury or declining to feed or mate because of general abdominal injuries. On the contrary, our experience has been that insects will continue with normal activities even after severe injury or removal of body parts.
Nociception has been documented in non-mammalian animals, including fish and a wide range of invertebrates, including leeches, nematode worms, sea slugs, and fruit flies. Although these neurons may have different pathways and relationships to the central nervous system than mammalian nociceptors, nociceptive neurons in non-mammals often fire in response to similar stimuli as mammals, such as high temperature (40 degrees C or more), low pH, capsaicin, and tissue damage.The Wikipedia article on Pain in invertebrates explains more:
The first invertebrate in which a nociceptive cell was identified was the medicinal leech, Hirudo medicinalis, which has the characteristic segmented body of an Annelida, each segment possessing a ganglion containing the T (touch), P (pressure) and N (noxious) cells. Later studies on the responses of leech neurones to mechanical, chemical and thermal stimulation motivated researchers to write "These properties are typical of mammalian polymodal nociceptors".
There have been numerous studies of learning and memory using nociceptors in the sea hare, Aplysia. Many of these have focused on mechanosensory neurons innervating the siphon and having their somata (bulbous end) in the abdominal ganglion (LE cells). These LE cells display increasing discharge to increasing pressures, with maximal activation by crushing or tearing stimuli that cause tissue injury. Therefore, they satisfy accepted definitions of nociceptors. They also show similarities to vertebrate Aδ nociceptors, including a property apparently unique (among primary afferents) to nociceptors — sensitization by noxious stimulation. Either pinching or pinning the siphon decreased the threshold of the LE cells firing and enhanced soma excitability.
Chemical senses include the use of chemoreceptors, related to taste and smell, affecting mating, habitat selection, feeding and parasite-host relationships. [...] Chemoreceptor sensitivity related to smell in some substances, is very high and some insects can detect particular odours that are at low concentrations miles from their original source (Triplehorn & Johnson, 2005).
Mechanical senses provide the insect with information that may direct orientation, general movement, flight from enemies, reproduction and feeding and are elicited from the sense organs that are sensitive to mechanical stimuli such as pressure, touch and vibration (Triplehorn & Johnson, 2005). [...]
Pressure on the body wall or strain gauges are detected by the campiniform sensilla and internal stretch receptors sense muscle distension and digestive system stretching (McGavin 2001; Triplehorn & Johnson, 2005,). [...]
A number of insects have temperature and humidity sensors (McGavin, 2001)
Animal Suffering: An Invertebrate Perspective,Jennifer A. Mather notes:
the physiological systems that control responses to what we call pain mostly are universal across the animal kingdom, and snails often are used as models for such responses. Can we treat them as having sensations that resemble ours without being concerned for their welfare when they do?
Still, it is less easy to take that leap of faith and presume parallels with how you feel when the animal concerned is completely unlike you. Insects, for instance, can walk normally with a couple broken-off legs and survive with apparent unconcern as a parasite is eating them up inside, when presumably we would be in excruciating pain. Does that mean they cannot feel pain? I asked a friend who works with ants what she thought about this apparent inability to feel the pain we do. She said that she spilled a drop of acetone on an ant by accident one day and that it had recoiled and tried to wipe the substance off its abdomen. Maybe it is still pain, just responding to different stimuli. Alternately, maybe it is just an automatic grooming reaction. Because nuclear radiation can kill us without our feeling a thing, humans too do not always respond with pain to possible tissue destruction.
The nervous system and senses of insects appear to be better developed than in crustaceans since an active life on land may be more demanding. With the great diversity of insects, there are great differences in the organization of the central nervous system and senses. In general, insects are equipped with numerous sense organs. The brain is particularly well developed in social insects, and the size of certain neural centers can be correlated with learning capacity. Learning is also known from many solitary species of insects. Insects do not react to damage of their bodies, but may show strong reflexes to constraint. With our present knowledge, it is usually concluded that insects cannot feel pain. Still, doubts have been raised. Among invertebrates, social insects represent a high level of cognition, and their welfare should be considered during handling.
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