page_title: Pain in invertebrates
section_title: Mechanical stimulation
hierarchical_section_title: Pain in invertebrates / Protective motor reactions / Mechanical stimulation
caption_attribution_description: Deutsch: Raupe des Tabakschwärmers, Urbana, Illinois, USA English: Tobacco Hornworm, found in Urbana, Illinois, USA Français&#160;: Larve de sphinx du tabac à Urbana, Illinois, États-Unis
context_page_description: Pain in invertebrates is a contentious issue. Although there are numerous definitions of pain, almost all involve two key components. First, nociception is required. This is the ability to detect noxious stimuli which evokes a reflex response that moves the entire animal, or the affected part of its body, away from the source of the stimulus. The concept of nociception does not imply any adverse, subjective feeling; it is a reflex action. The second component is the experience of "pain" itself, or suffering—i.e., the internal, emotional interpretation of the nociceptive experience. Pain is therefore a private, emotional experience. Pain cannot be directly measured in other animals, including other humans; responses to putatively painful stimuli can be measured, but not the experience itself. To address this problem when assessing the capacity of other species to experience pain, argument-by-analogy is used. This is based on the principle that if a non-human animal's responses to stimuli are similar to those of humans, it is likely to have had an analogous experience.
context_section_description: Rather than a simple withdrawal reflex, the flatworm, Notoplana aticola, displays a locomotory escape behaviour following pin pricks to the posterior end. Touching the larvae of fruit flies, Drosophila melanogaster, with a probe causes them to pause and move away from the stimulus, however, stronger mechanical stimulation evokes a more complex corkscrew-like rolling behaviour, i.e. the response is plastic. When a weak tactile stimulus is applied to the siphon of the sea-hare Aplysia californica, the animal rapidly withdraws the siphon between the parapodia. It is sometimes claimed this response is an involuntary reflex (e.g. see Aplysia gill and siphon withdrawal reflex), however, the complex learning associated with this response (see 'Learned Avoidance' below) suggests this view might be overly simplistic.
  In 2001, Walters and colleagues published a report that described the escape responses of the tobacco hornworm caterpillar (Manduca sexta) to mechanical stimulation. These responses, particularly their plasticity, were remarkably similar to vertebrate escape responses.
caption: Tobacco hornworm larva, Manduca sexta
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