Pain, by definition, requires a capacity for emotion.
Pain = an unpleasant sensory and emotional experience associated with actual or potential tissue damage or described in terms of such damage.
– International Association for the Study of Pain (IASP)
Pain is more than the stimulation of nerves. In fact, the IASP notes that patients can feel and report pain with no actual physical cause or stimulus. Pain is a subjective and emotional experience. Our response to unpleasant stimuli is influenced by our perceptions and past experiences.
The insect nervous system differs greatly from that of higher order animals. Insects lack the neurological structures that translate a negative stimulus into an emotional experience. We have pain receptors (nocireceptors) that send signals through our spinal cord and to our brain. Within the brain, the thalamus directs these pain signals to different areas for interpretation. The cortex catalogues the source of the pain and compares it to pain we've experienced before. The limbic system controls our emotional response to pain, making us cry or react in anger. Insects don't have these structures, suggesting they don't process physical stimuli emotionally.
We also learn from our pain, and change our behavior to avoid it. If you burn your hand by touching a hot surface, you associate that experience with pain and will avoid making the same mistake in the future. Pain serves an evolutionary purpose in higher order organisms. Insect behavior, in contrast, is largely a function of genetics. Insects are pre-programmed to behave in certain ways. The insect lifespan is short, so the benefits of an individual learning from pain experiences are minimized.
Perhaps the clearest evidence that insects do not feel pain is found in behavioral observations. How do insects respond to injury? An insect with a damaged foot doesn't limp. Insects with crushed abdomens continue to feed and mate. Caterpillars still eat and move about their host plant, even with parasites consuming their bodies. Even a locust being devoured by a praying mantid will behave normally, feeding right up until the moment of death.
Insects and other invertebrates don't experience pain as we do. This doesn't, however, preclude the fact that insects, spiders, and other arthropods are living organisms that deserve humane treatment.
- C. H. Eisemann, W. K. Jorgensen, D. J. Merritt, M. J. Rice, B. W. Cribb, P. D. Webb and M. P. Zalucki (1984) Do insects feel pain? — A biological view. Cellular and Molecular Life Sciences 40: 1420-1423
- "Do Invertebrates Feel Pain?", The Senate Standing Committee on Legal and Constitutional Affairs, The Parliament of Canada Web Site, accessed 26 October 2010.
- How We Feel Pain, Erica Jacques, Guide to Pain on About.com