“They show that a specific set of serotonin-receptor-possessing neurons are used” by living flies to perceive dead ones, said Marc Tatar, a biologist at Brown University who was not involved with the study. “That is the beauty of this paper.”

It’s not abundantly clear why seeing dead flies would cause those still alive to rush to join them. Dr. Tatar suspects that dead flies are a sign of danger for those still living, so he expects that seeing them leads flies to put more energy into reproduction at the expense of longevity.

A 2022 paper reported that female flies exposed to dead bodies laid more eggs, but it found no effect on life span; Dr. Pletcher said the authors used “significantly less severe” corpse exposures, which could lead to different effects. In their experiments, Dr. Pletcher and Dr. Gendron didn’t see increased reproductive output from death-exposed flies.

The other hypothesis is that the shorter life span results from stress caused by perceiving death. Chronic stress in animals leads to health problems and shortens life spans, and flies have a stress response, too. “If we suddenly found ourselves in a sea of dead humans, that would be very stressful,” Dr. Pletcher said.

The researchers hope to take a broader view of their results and look at other ways that social interactions, or the lack of them, influence aging in flies. And to understand if aging faster after seeing death is somehow beneficial for the flies, Dr. Tatar says we need to take time to study the fruit fly in its natural habitat — instead of just in the lab.

The species has “been in the lab for 120 years,” he said, adding, “We think of them as genetic organisms, instead of as free, natural insects.”