Several lines of evidence suggest that insects' central nervous system processes pain in a way far more like ourselves than anyone has been willing to admit.

Scientists once performed what we now consider horrific experiments on animals out of curiosity. These days, studies on vertebrates have to go before ethics committees to show the value of the research outweighs any harm to the subjects, even if not everyone agrees on where to draw the line. Cephalopods like octopus and squid are starting to gain the same protection.

Insects, on the other hand, are usually treated as fair game. Fruit fly researchers are not put to the same standards as those who work on mice, let alone monkeys. The justification – that insects don't experience pain the way “higher” animals do – is challenged by a new paper in the journal Proceedings of the Royal Society B. 

The authors note the question hasn't been researched all that much, possibly because people have been scared of the possible answer.

In the language of neuroscientists, nociception is the nervous system's encoding of harmful or unpleasant stimuli like extreme temperatures, pressure, or intense chemical attack. Animals (insects included) respond to these signals to limit damage to their bodies that might impede their survival. What is debated is whether insects experience pain through the central nervous system, or if the response is localized, for example in an injured limb.

Insects have much less sophisticated central nervous systems than mammals, after all, with only a tiny fraction of the brain cells devoted to processing such inputs. In particular, they lack the opioid receptors so crucial to pain control in our own brains. However, Queen Mary University PhD student Matilida Gibbons and co-authors argue that doesn't mean they lack simpler versions of the same capacity.

Nociception is closely related to pain, but they're not the same thing. Our bodies can sometimes modulate pain without changing nociceptive reflexes, particularly in emergencies when too much pain might distract us from what we need to do. The pain comes later, forcing us not to use an injured limb, for example. Curiously, the reverse has also been observed with nociception enhanced without changing pain levels.

Nevertheless, we lack an understanding of how nociception and pain are related in insects, so the authors explore insects' capacity to control nociception, which they consider indicative, if not proof.

“Behavioural work shows that insects can modulate nocifensive behaviour. Such modulation is at least in part controlled by the central nervous system since the information mediating such prioritization is processed by the brain,” the paper points out.

The authors identify specific neuropeptides produced in insects during traumatic events that might act as pain suppressors, similar to the role performed by opiates in humans.

Additional evidence is how insects, like other animals, can become sensitized to particular threats. If fruit flies are repeatedly exposed to high temperatures, they start to respond more quickly when heat is applied. Some of the molecules involved in this sensitization are the same as those seen in humans. Pathways for sending nociceptive messages to the brain have also been identified.

Even one of the most famous insect behaviors – female praying mantises sexual cannibalism – may shed light on the question. Infamously, male mantises respond to having their heads chewed off by mating harder. To do this, the male must suppress his typical response to attack.

“This evidence has been suggested to indicate the absence of pain in insects,” the paper notes. “However, it is more likely that it demonstrates that insects can prioritize other behavioural needs and reduce the nocifensive behaviour in certain contexts.” That in turn points to a centralized response, which in turn makes pain sensations more plausible, not less.

We still don't know how pain is processed in the insect brain, if it is, but that's less important than working out our response if it proves to be true. If we find out insects do feel pain, can we really go on treating them as we do?