In A Nutshell
Emotional and physical pain allow us to learn from our experiences and modify future behavior. Nocireceptors, responsible for the sensation of pain, are not present in insects—whose lifespans are too short for pain to be useful. Since insects lack nocireceptors, they cannot experience pain.
The Whole Bushel
Pain is a complex emotion. It’s our brain’s response to negative stimuli, from a heart-wrenching break-up to a hot stove scalding a hand. This response is delineated into an experience that encourages us to avoid pain in the future—the negative consequences of pain allow us to learn from past behavior.
Pain is useful to higher-order animals with longer lifespans and was likely a contributing factor to the development of those lengthier lives. The ability to learn from a negative experience ensures a greater chance of avoiding threatening stimuli in the future.
It makes sense, then, for insects, whose lifespans are comparatively short, to lack nocireceptors, which are the receptors directly responsible for pain detection. Nocireceptors send signals to the brain that trigger a painful response. As insects lack nocireceptors, or any equivalent, they cannot experience pain (or at least our version of pain). This conclusion extends to all arthropods, which also includes arachnids, crustaceans, and myriapods.
Observations of insects seem to corroborate this idea. A caterpillar or grasshopper will attempt to continue its routine even while being eaten alive or tortured by a toddler. Insects with damaged bodies attempt to operate as usual—no limping or crying.
Since many insects live for only a few days, they have no need to learn from a painful experience. They’ll be dead soon anyway. Insects are bred for pre-programmed robotic actions; they can glean few benefits from learned behavior or emotions of any kind.
But some scientists continue to debate this issue. Though they lack nocireceptors and show a general indifference toward painful stimuli, arthropods still may have a capacity for experiencing pain of some kind—though it is almost certainly not the same type of pain we experience.
For example, an experiment at Queen’s University Belfast examined the response of crabs (crustaceous arthropods) to electric shocks. The crabs demonstrated an aversion to the shock. While some researchers argued that the crabs’ negative response to the shocks was automatic, others suggested that perhaps crabs—and all crustaceans—actually do experience pain.
A similar experiment was conducted by Stanford University involving fruit flies. The flies were exposed to heat probes, to which they showed some measure of aversion. The researchers discovered that a sensory neuron was responsible for the evasion, but were divided over whether the flies were experiencing pain or were responding to the heat probe with preprogrammed reflexes.
These experiments indicate that arthropods may experience some type of basic pain, but a definitive conclusion may never be possible. However, without nocireceptors and the capacity for emotional behavior, insects cannot experience pain the way other animals do.