Peter Godfrey-Smith discusses this topic at length in his recent-ish book Metazoa and uses observations of insects and other invertebrates to argue, compellingly in my opinion, that sentience and cognitive abilities are not intrinsically linked, but rather depend on the specific organism's evolutionary/reproductive strategies.
Most insects do not exhibit the classic signs of pain responses (the new findings discussed in this New Yorker article, notwithstanding). For example insects generally don't groom or guard an amputated limb. This puzzling (lack of) response can be explained as being aligned with their reproductive strategy: they reach breeding age quickly and die soon after. Thus, it's better not to waste energy avoiding limb loss for a future that won't happen.
Despite this lack of sentience, insects can be quite intelligent and learn complex cues and behaviors. Other invertebrates that look superficially like insects, prawns for example, have quite different life cycles and lifespans and often do exhibit signs of pain / sentience.
Basically pain/sentience emerge when there is a reason for the organism to protect the body from damage, and does not evolve (or is subsequently lost!) when there are more important short term goals. One wonders, for example, whether salmon experience pain when they fling their bodies up rivers, over and onto rocks, damaging them horribly in the process: all for purposes of spawning.
Most insects do not exhibit the classic signs of pain responses (the new findings discussed in this New Yorker article, notwithstanding). For example insects generally don't groom or guard an amputated limb. This puzzling (lack of) response can be explained as being aligned with their reproductive strategy: they reach breeding age quickly and die soon after. Thus, it's better not to waste energy avoiding limb loss for a future that won't happen.
Despite this lack of sentience, insects can be quite intelligent and learn complex cues and behaviors. Other invertebrates that look superficially like insects, prawns for example, have quite different life cycles and lifespans and often do exhibit signs of pain / sentience.
Basically pain/sentience emerge when there is a reason for the organism to protect the body from damage, and does not evolve (or is subsequently lost!) when there are more important short term goals. One wonders, for example, whether salmon experience pain when they fling their bodies up rivers, over and onto rocks, damaging them horribly in the process: all for purposes of spawning.