The smell of 'death' that repels insects turns out to be a truly ancient signal for avoiding disease or predators, says David Rollo, professor of biology at McMaster University in the journal Evolutionary Biology.

What do the death stench of corpses of from insects to crustaceans all have in common?   A blend of specific fatty acids. 

Because insects and crustaceans diverged more than 400-million years ago it is likely that most subsequent species recognize their dead in a similar way, that the origin of such signals was likely even older, and that such behavior initially occurred in aquatic environments (few crustaceans are terrestrial). 

Rollo and his team made the discovery while they were studying the social behavior of cockroaches. When a cockroach finds a good place to live it marks the site with pheromone odours that attract others. In trying to identify the precise chemicals involved, Rollo extracted body juices from dead cockroaches. 

"It was amazing to find that the cockroaches avoided places treated with these extracts like the plague," says Rollo. "Naturally, we wanted to identify what chemical was making them all go away." 

The team eventually identified the specific chemicals that signaled death. Furthermore, they found that the same fatty acids not only signaled death in ants, caterpillars, and cockroaches, they were equally effective in terrestrial woodlice and pill bugs that are actually not insects but crustaceans related to crayfish and lobsters. 

"Recognizing and avoiding the dead could reduce the chances of catching the disease, or allow you to get away with just enough exposure to activate your immunity," says Rollo. Likewise, he adds, release of fatty acids from dismembered body parts could provide a strong warning that a nasty predator was nearby.

"As explained in our study, fatty acids—oleic or linoleic acids—are reliably and quickly released from the cells following death. Evolution appears to have favoured such clues because they were reliably associated with demise, and avoiding contagion and predation are rather critical to survival."

The generality and strength of the phenomenon, coupled with the fact that the fatty acids are essential nutrients rather than pesticides, holds real promise for applications such as plant and stored product protection or exclusion of household pests.