No, despite some of the headlines that are spreading across the Internet, scientists have not found that flies are emotional beings, nor did they demonstrate that the insects experience feelings like fear in a similar way to us. However, what they did find was that flies respond to threatening situations in a manner more complex than can be explained simply by reflex alone. The way that the flies reacted to fake predators in the lab indicated that these animals may possess the basic building blocks of emotion, but the researchers didn’t assert they had anthropomorphic emotions like anxiety.

“No one will argue with you if you claim that flies have four fundamental drives just as humans do: feeding, fighting, fleeing and mating,” first author William Gibson, California Institute of Technology, said in a statement. “Taking the question a step further- whether flies that flee a stimulus are actually afraid of that stimulus- is much more difficult.”

The reason that emotions can be so tricky to study in other animals is because scientists often attempt to look for behaviors that display similarities to our own emotions. That means researchers may miss or ignore emotion states in distantly related species, like insects, that are different to our own behaviors. And to make things even more complicated, researchers still haven’t quite come to an agreement on what precisely emotion means, nor is there a gold standard definition of what it is.

But Gibson and colleagues have put forward their own idea which gets around some of the hazards of studying emotions, suggesting that they are “a type of internal brain state with certain general properties that can exist independently of subjective, conscious feelings, which can only be studied in humans,” said lead researcher David Anderson. “That means we can study such brain states in animal models like flies or mice without worrying about whether they have ‘feelings’ or not.”

For their objective approach, the team first broke down emotion states into their fundamental building blocks, called “emotion primitives.” These aren’t specific to one particular emotion, and can be applied to different species. These included: persistence, meaning the response lingers for a period of time after a particular stimulus or trigger; scalability, meaning the reaction will intensify if the trigger is repeated, like a gunshot sound; and generalizability, meaning the same response would occur in different contexts or situations.

The researchers then set out to examine whether these primitives were apparent in fruit flies after exposing them to a fear-inducing stimulus. This involved analyzing their responses to a fake aerial predator in the lab, which was just an overhead shadow resembling another animal, like a bird. As described in Current Biology, they found that the shadow triggered reactions like jumping or freezing, sometimes causing them to enter a state of elevated arousal. And repeating the procedure increased their response, demonstrating scalability. Furthermore, they would even ignore food until their arousal levels returned back to normal, suggesting generalization and persistence.  

Although this apparent state does display similar qualities to fear in mammals, the scientists are careful not to classify their response as an “emotion.” But the observations do suggest that the animals’ response was more complex than a simple avoidance reflex.

“It shouldn’t surprise us to find something resembling fear in flies, as it’s a mechanism that evolved over millions of years to protect organisms from harm, so its retention across those millions of years attests to its efficiency,” Aarhus University’s Mathias Clasen, who was not involved in the study, told New Scientist. “If it didn’t work, natural selection would have got rid of it.”