The Louie The Fly character from the Mortein flyspray advertising campaigns. He may have feelings after all. Credit:Mortein And that's a bit troubling. You might not want flies in your house, but do you want their agony on your conscience? "I do have the instinct that flypaper seems more cruel than swatting," Klein says, "even if it's more convenient for us. I think if you're governed by the idea that it's just a fly, or just a bee, so it doesn't matter, then that's probably worth rethinking." The question of whether insects are conscious – both in the sense of being able to learn and adapt from events, and of having a subjective inner life – has long been a subject of fascination. Although seemingly recondite, the answer carries some very significant implications. Because it is impossible to know the subjective reality of another species, the question of whether consciousness is unique to humans is unanswerable. Most biologists assume it is present in many animals – mammals, at least, and probably birds – but what about fish, grasshoppers or sea slugs?

"One of the things we're very interested in is in the origins of consciousness, why consciousness might have evolved in the first place," Klein says. "One of things that's really important about the origin is that it arises from integrating functions – taking together sensory information, the state of the body, memories of where the organism has been – and putting these together into a single point of view that allows navigation in a complicated world. "And so we think things that are simple, such as bacteria, or don't face those pressures, won't evolve the kind of complicated brains that you need to be conscious." Which is why, just quietly, a jellyfish will never become prime minister, but cockroaches will always know when you creep into the kitchen at night. Based on their understanding of insect brain structure, Klein and Barron conclude that far from being a recent evolutionary development, consciousness is very ancient, emerging first in the Cambrian Period, about 500 million years ago.

If, thus, consciousness is widespread through the animal kingdom, ethical questions about how we treat even tiny household pests become a bit more complicated. It's long been recognised that there is a huge moral difference between slaughtering a lamb quickly, and letting it die slowly, even if in both cases the intention is to barbecue its meaty bits. Does the same distinction thus apply between a cockroach stomped on, and one left to expire over time, fearful and pain-racked by pesticide? It's a question that up until now hasn't really arisen, even among the most conscientious animal rights activists. Writing in a newspaper in 2013, philosopher Peter Singer tackled the issue of whether killing a fly or mosquito could be considered murder. "I don't think so," he wrote. "I'm doubtful that insects such as flies and mosquitoes are conscious – that is, that they can feel pain, or enjoy their lives." The Macquarie findings seem to invalidate that reasoning, although to what extent, Klein happily admits, remains unclear.

"Certainly for Peter Singer, the morally relevant animals are the ones that feel pain," he says. "So in Singer's view, if insects feel pain then they in some sense become part of the moral community which we have to take into account." One key consideration, he says, is the question of to what degree "primordial emotions" such as fear and pain are experienced by insects. Subjective experience is not the same as self-awareness, he adds, the kind of keen insight enjoyed by humans. There is no suggestion that Louie the Fly is really a closet existentialist. But what of Louie's cousin, helplessly glued to flypaper, struggling until thirst and exhaustion take their fatal toll? He might be in trauma, but, equally, Klein suggests, he might be simply experiencing "dim confusion". So has Louie been libelled? Perhaps, but to Klein he is also an indication that his and Barron's conclusions might not be so controversial after all. "On one hand we really want to resist the tendency to anthropomorphise flies," he says.

"On the other, the fact that people are willing to think of Louie the Fly as having internal mental states and goals and things like that, however seedy, is evidence that they are willing to accept the consciousness hypothesis even in the case of insects."