Do Insects Feel Pain?

Scientists, animal rights activists, and biological ethicists have long debated whether or not insects feel pain. There's no easy answer to the question. Since we can't know for certain what insects may or may not feel, there's really no way to know if they feel pain, however, whatever they do experience is very different than what people feel.

Pain Involves Both Senses and Emotion

Prevalent interpretation submits that pain, by definition, requires a capacity for emotion. According to the International Association for the Study of Pain (IASP), "Pain equals an unpleasant sensory and emotional experience associated with actual or potential tissue damage or described in terms of such damage." That means that pain is more than simply the stimulation of nerves. In fact, the IASP notes that some patients feel and report pain with no actual physical cause or stimulus. 

Sensory Response

Pain is both a subjective and emotional experience. Our responses to unpleasant stimuli are influenced by perception and past experiences. Higher-order animals, such as humans, have pain receptors (nociceptors) that send signals through our spinal cord to the brain. Within the brain, the thalamus directs these pain signals to different areas for interpretation. The cortex catalogs the source of the pain and compares it to a pain we've experienced before. The limbic system controls our emotional response to pain, making us cry or react in anger. 

The insect nervous system differs greatly from that of higher-order animals. They lack the neurological structures responsible for translating negative stimuli into emotional experiences and, to this point, no commensurate structures have been found to exist within insect systems.

Cognitive Response

We also learn from the experience of pain, adapting our behaviors to avoid it when possible. For instance, 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 one single individual learning from pain experiences are minimized.

Insects Don't Show Pain Responses

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 as parasites consume their bodies. In fact, a locust being devoured by a praying mantid will behave normally, feeding right up until the moment of death.

While insects and other invertebrates don't experience pain in the same way that higher-order animals do, this doesn't preclude the fact that insects, spiders, and other arthropods are living organisms. Whether or not you believe they deserve humane treatment is a matter of personal ethics, although there's a good chance that if an insect serves a purpose that humans perceive as beneficial, such as the honeybee, or is aesthetically pleasing, like the butterfly—they are much more likely to be treated with kindness and respect—but ants invading your picnic or a spider in your shoes? Not so much.

Sources:

• Eisemann, C. H., Jorgensen, W. K., Merritt, D. J., Rice, M. J., Cribb, B. W., Webb. P. D., and Zalucki, M. P. "Do Insects Feel Pain? — A Biological View." Cellular and Molecular Life Sciences 40: 1420-1423, 1984
• "Do Invertebrates Feel Pain?" The Senate Standing Committee on Legal and Constitutional Affairs, The Parliament of Canada Web Site, accessed 26 October 2010.