Chapter Three - Can insects feel pain? A review of the neural and behavioural evidence
Introduction
Sentience is the capacity to have feelings—mental states that are consciously experienced as good or bad. Examples include love and hate, joy and anger, excitement and exhaustion, happiness and depression, hunger and thirst. A particularly salient feeling is pain, such as the “sharp pain” of an injection or the “dull throb” of a headache. These feelings have an important evolutionary function: motivating and teaching us to avoid harm, such as sharp objects or bumps on the head (Kolodny et al., 2021). Yet, due to its intrinsic aversiveness, extreme or unnecessary pain leads to major ethical concerns. Many argue that animal welfare only matters if the animal is sentient and can experience pain (Duncan, 1996; Fraser et al., 1997). Feeling pain is therefore central to whether a living being deserves moral consideration.
How can we tell whether an animal feels pain? First, we must distinguish pain from nociception. Nociception is the detection of noxious stimuli (Tracey, 2005, Tracey, 2017), or stimuli that may cause tissue damage (Cervero and Merskey, 1996). Nociception does not require pain: hand withdrawal from a hot stove is a nociceptive reflex controlled by neurons relaying signals from the nociceptors in the hand, to the spinal cord, and back again (Defrin et al., 2007). All this happens before nerve impulses reach the brain (where pain is experienced). Thus, when animals display nociception, this does not necessarily demonstrate that they can feel pain (Adamo, 2016; Magee and Elwood, 2013; Sneddon et al., 2014). However, when nociceptive signals are transmitted to the brain, this may lead to the aversive, subjective experience of pain (Auvray et al., 2010; Birch et al., 2020).
While it is essential to distinguish between indicators of nociception and pain, the fundamental challenge of pain is that scientists cannot directly measure this subjective and inherently private experience (Frischenschlager and Pucher, 2002). Even in humans, who can self-report their pain and describe its severity (Heft et al., 1980; Wideman et al., 2019), we can never be certain we are accurately measuring pain (Frischenschlager and Pucher, 2002). This issue is exacerbated in non-human animals who cannot verbally self-report. Therefore, researchers rely mainly on two lines of indirect evidence: (1) whether the animal has a nervous system that might support pain, and (2) whether they exhibit behaviours potentially caused by pain (Briffa, 2022; Crump and Birch, 2022). Given functionally similar neuroanatomy and analogous behavioural responses to harm, few dispute that mammals and birds feel pain (Gentle, 1992). There is also growing expert consensus on pain in other animals, including the invertebrate cephalopod molluscs and decapod crustaceans (Crump et al., 2022; Elwood, 2012).
Some early entomologists and naturalists asserted a belief in insect sentience, such as Charles Darwin (1872) and Charles Henry Turner (Galpayage Dona and Chittka, 2020; Turner, 1912). However, by the mid-20th century, the notion that insects are purely instinctual/reflexive had gained popularity (for a historical overview in myrmecology, see Sleigh, 2007). Anecdotal accounts of insects appearing to behave normally after extreme injury were taken as evidence against pain (Eisemann et al., 1984; Wigglesworth, 1980). Despite lacking empirical support, these accounts have received hundreds of citations in entomology, comparative cognition, and welfare/ethics (e.g., Adamo, 2016; Ng, 1995; Sneddon et al., 2014). Another popular argument against insect pain is that insect brains are too small, or lack the appropriate neural connections, to support sentience (Adamo, 2016; Allen-Hermanson, 2008, Allen-Hermanson, 2016; Hill, 2016; Key et al., 2016, Key et al., 2021). For example, Adamo (2019) argued that the lack of direct connections between integrative brain regions that process noxious stimuli, which areessential for pain in vertebrates (Garcia-Larrea and Bastuji, 2018), likely precludes the experience of pain. However, such direct connections have now been found in adult Drosophila melanogaster fruit flies (Diptera: Drosophilidae; Li et al., 2020a). This highlights how lack of evidence may serve as a poor guide for drawing accurate conclusions about insect nervous systems and their psychological correlates.
New psychological evidence is consistent with some form of sentience in insects, such as “emotion-like” cognitive biases (Bateson et al., 2011; Solvi et al., 2016). Further, insects display nocifensive behaviour (defensive or protective behaviours in response to noxious stimuli) that different stimuli and contexts can modulate (Gibbons et al., 2022a, Gibbons et al., 2022b). Although small, insect nervous systems are exquisitely complex (Chittka and Niven, 2009; Giurfa, 2013) and may perform many of the same functions as mammalian nervous systems, even without homologous brain structures (e.g., Varga and Ritzmann, 2016). Insects do not have a visual cortex, for example, but there is no doubt that they can see. It is thus possible that insects may also experience pain, but underpinned by different neural circuits than mammals (e.g., multiple realizability and related theses: Chittka et al., 2012; Mallatt and Feinberg, 2021).
From an ethical standpoint, whether insects feel pain is an urgent question. Trillions of insects are farmed, managed in the wild, and used for research or other purposes every year. There are currently no guidelines for considering their welfare in these settings, and they are almost universally excluded from animal welfare legislation. This is based, at least in part, on the assumption that insects do not feel pain.
In this article, we assess the evidence for pain in insects. First, we outline the assessment framework (Section 2). We then review the neural and cognitive/behavioural evidence for insect pain across six orders, at different developmental stages (Section 3). We use this framework to judge the current likelihood of pain in insects, and consider the review's limitations (Section 4). Finally, we briefly discuss the contexts in which humans use insects and the potential welfare concerns of such usage (Section 5).
Section snippets
How we evaluate evidence for pain
In a report commissioned by the UK government, Birch et al. (2021) developed a new framework for evaluating evidence of animal sentience, with a focus on pain (later published as Crump et al., 2022). Birch et al. (2021) write that “pain is one example within a broader category of negatively-valenced affective states, a category which also includes states of anxiety, fear, hunger, thirst, coldness, discomfort and boredom” (Birch et al., 2021, p. 12). Building on previous work (e.g., Bateson, 1991
Criterion 1: Nociception
The animal possesses receptors sensitive to noxious (i.e., potentially or actually harmful, damaging) stimuli (nociceptors)
This criterion specifies the most basic prerequisite for experiencing pain. If fulfilled, the animal has the neurobiological capacity for nociception. Vertebrates detect noxious stimuli through specialised peripheral sensory neurons: nociceptive neurons (Dubin and Patapoutian, 2010), characterised by free nerve endings under the epidermis. Fruit fly larvae have an
Summary of evidence for insect pain
In Section 3, we assessed the evidence for each criterion in adults and juveniles of six insect orders. Table 11 summarises our confidence levels for adults, and Table 12 summarises our ratings for first (and last) instar juveniles.
Birch et al. (2021) suggested an approximate grading scheme for communicating the strength of evidence for sentience (specifically for pain). The five grades were:
- 1.
Very strong evidence: High or very high confidence that 7–8 criteria are satisfied. Welfare protection
Ethical considerations for the use or management of insects
Insects are managed in a variety of contexts that may raise welfare concerns, including the food and feed industry, silk/shellac/dye production, waste management, pest/invasive species management, wildlife conservation, beekeeping, zoos and insectariums, research/education settings, the entertainment industry, in medicine, and as pets. By far, the largest number of insects with welfare impacted by human management will be in wild/agricultural settings, followed by the growing insects as food
Conclusion
Using the Birch et al. (2021) framework, we reviewed the evidence for sentience (and specifically pain) in six insect orders across their development. We found “strong evidence” for pain experiences in adults of two orders, Diptera (flies and mosquitoes) and Blattodea (cockroaches and termites). There was also “substantial evidence” in adult Hymenoptera (bees, wasps, ants, and sawflies), Orthoptera (crickets and grasshoppers), and Lepidoptera (butterflies and moths), and “some evidence” in
Acknowledgements
We thank Elisabetta Versace, Katrin Vogt, Nikita Komarov and Frederic Libersat for their feedback on the manuscript.
Conflict of interest
Meghan Barrett reports a relationship with Rethink Priorities that includes: consulting.
Funding statement
This research is part of a project that has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme, Grant Number 851145. M.G. received funding from a Queen Mary University of London PhD Studentship. M.B. is currently funded as an NSF postdoctoral research fellow (2109399).
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