Do insects feel pain

It is a question raised sometime ago by Eisemann and six other scientists (Eisemann et al. 1984), and also a question that PJ raised himself (Jolivet, 1999, 2002). Really the question is rather perplexing and has been discussed by Berembaum (2000) and earlier by Wiggleworth (1980). It is very difficult to answer satisfactorily, because, if a reptile brain is so difficult to understand, the invertebrate brain, being much smaller and complex, seems to belong to another world. Darwin, who had recognized the complexity of invertebrate brains, used to say that the ant brain gave him a headache. The complexity of the eye and its evolution was also a headache for the patriarch of Downes. The ants did not exist during the early Cambrian, and they appeared during the Jurassic, but, much earlier in the history of the organic world, trilobites and other invertebrates already had very complex eyes and a complex nervous system. They had predators and enemies and probably even they felt pain. Not only ants have sophisticated eyes, they also have a brain capable of memory, learning, and complex adaptations required for strict instinctive behaviour. If we say so for the ants, what can we say about the bee, the earwig, the beetle, the cockroach and so many insects and invertebrates, capable of extremely complex behaviour, parental care and "intelligent" discrimination in labyrinths? Contrary to what Fabre and Bergson believed, the insect behaviour is not all instinctive. Learning also has a place in their behaviour. The question of pain among the insects induces ethical implications for the biologists, who often manipulate live insect material. Even very simple mobile organisms, such as bacteria, show evident avoidance and escape responses, when subjected to traumatic stimuli, such as heat, chemicals, electricity, pressure, etc. (Berg, 1975). The problem becomes more complex with higher organisms, and seems proportional to the degree of complexity (Gould and Gould, 1982). We can feel sympathy for a dog or a cat in distress, but we feel little compassion for a worm or a snail facing a traumatic situation. Perhaps our mental response, on seeing a suffering organism and realization of its pain, are proportionate to our evolutionary closeness with the organism.

Pain is subjective. The reality of pain, apart the imagination of Descartes, who crucified his dog to show that it was a machine, is deduced from the physiological and behavioural responses, shown by the organism, such as flexor reflexes, blood pressure increase, vocalization (screams), and hasty respiration (tachypnoea). Sensation of pain in an animal is deduced from the observations of reactions similar to those shown by a suffering human being. According to Wiggleworth, stimuli, such as high temperature and electric shocks, are perceived by the insects, as if they are feeling pain, while other manipulations do not seem to affect them much. Among mammals, pain induces reactions such as withdrawal, protraction, aggression, learnt avoidance etc. Mammals, especially, man, have limited preprogramming of their behaviour patterns, and they learn from pain and pleasure experiences (Elzack, 1973).

In contrast, most insect behaviour patterns are to a large extent preprogrammed (instinctive). There is, however, a capacity for learning (including avoidance) in both intact and decapitated insects. Some of the receptors present among the vertebrates seem missing among the insects, but we cannot imagine that reflex avoidance of a traumatic situation can exist without the implication of a sensation of pain. The discovery of receptors of opioid peptides among the invertebrates could implicate that these animals feel pain, but that is not fully convincing (Eisemann et al, 1984). Insects go on with their activities after a traumatism or the removal of essential parts of their body such as the head or the legs. Hyperexcit-ability, ataxia and convulsions, shown by insects after insecticide poisoning, sound production, secretion of alarm pheromones after an attack can be considered as an instinctive response, and not as a reaction to pain.

In conclusion, it seems impossible to say at present if insects really suffer pain like us or only by reflex respond to external stimuli. It is evident that the problem can be totally different with mollusks, like Cephalopoda, with a very complex nervous system. Eisemann et al. (1984) advise, as also Wiggleworth (1980), that, in the state of uncertainty, it is necessary to anaesthetize the insects before start of traumatizing laboratory manipulations. The method not only facilitates the manipulations, but also avoids pain to animals, physiology of which is only imperfectly understood.

Berenbaum (2000) recounts her experience in Cornell with a dissected cockroach, emptied of its visceral mass, beheaded and legless, and still trying to swim after experiments on the water in the dissection dish. She too does not know if the insects feel pain, but she also advises to anaesthetize them before all experiments. Pain is probably universal among all the living beings, since the origin of life, but we do not know to what extent lower form of life have consciousness to feel it.


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Eisemann, C. H., Jorgensen, W K., Merritt, D. J., Rice, M. J., Cribb, B. W, Webb, P. D. and Zalucki, M. P. 1984. Do Insects feel pain? A biological view Experientia, Bassel 40: 164-167. Elzack, R. 1973. The Puzzle of Pain. Basic Books, New York. Gould, J. L. and Gould, C. G. 1982. The insect mind: physics or metaphysics? In: Griffin, D.R. (ed.) Animal Mind-Human Mind. Springer-Verlag, Berlin-Heidelberg-New York: 269-297. Jolivet, P. 1999. Les Insectes ont-ils des droits? Ou le Standing moral des Insectes.

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