According to new research, insects experience pain in a manner similar to vertebrates.

Praying mantises’ famous sex lives has important implications for whether they, and other insects, can feel pain. Image credit: Don Sutherland

Scientists have combined behavioral, neuroscientific and molecular evidence to prove that the central nervous system of insects processes pain in a way much more similar to ourselves than anyone has been willing to admit, IFL Science reports.

In the past, scientists performed what we would now consider horrific experiments on animals without much constraint, if any. But ethical standards have changed and today research on vertebrates has to go before ethics committees to prove that the value of the research outweighs any potential harm to the subjects. Cephalopods like octopus, squid and lobsters are starting to gain similar recognition. But not insects.

The justification for the different treatment of insects – that they don’t experience pain the way “higher-level” animals do – is what the new study, published in the journal Proceedings of the Royal Society B, challenges. As the researchers note, the question hasn’t been researched extensively, possibly because people have been scared of the possible answer.

Animals (including insects) respond to extreme temperatures, pressure, or intense chemical attack in a way to limit damage to their bodies that might impede their survival. The central nervous system’s encoding of these harmful or unpleasant stimuli is called nociception – something closely related to pain, but still different. Our bodies can sometimes modulate pain without changing nociceptive reflexes, particularly in emergency situations when too much pain might distract us from what we have to do. The feeling of pain comes later, forcing us not to use an injured limb, for example. Interestingly, the opposite has also been observed with nociception enhanced without changing pain levels.

Sensory nerve fibres stimulated beyond harmful intensity causing nociceptive pain. Image credit: Manu5

As for insects, it has been long debated whether they experience pain through the central nervous system, or if the response is localized, for example in an injured limb. After all, insects have much less sophisticated central nervous systems than mammals, and only a tiny fraction of their brain cells are devoted to processing such inputs. Unlike our brains, they lack the opioid receptors so crucial for pain control but, according to the study authors, that doesn’t mean they lack simpler versions of the same thing.

To understand how nociception and pain are related in insects, the authors explored 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 study underlines.

The authors identify specific neuropeptides produced in insects during traumatic events that have a similar role performed by opiates in humans: suppressing pain. Furthermore, additional evidence shows that insects, like other animals, can become sensitized to particular threats. When fruit flies are repeatedly exposed to high temperatures, for example, they start to respond more quickly – and some of the molecules involved in this process are the same as those seen in humans. Scientists have even identified the pathways for sending nociceptive messages to the brain.

And there’s something else that may shed light on the question whether insects feel pain: female praying mantises sexual cannibalism, one of the most widely known insect behaviors. Infamously, mantis males respond to having their heads chewed off by mating harder. For that to happen, the male has to suppress its typical response to attack.

Caught in the act of cannibalism! Image credit: djfrantic

“This evidence has been suggested to indicate the absence of pain in insects,” the study authors write. “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 makes pain sensations more plausible, not less.

We still don’t know exactly how pain is processed in the insect brain, but if it is, that has serious implications as to how we treat them.