Along the western bank of the Ebro River in Tudela—a small town in northern Spain—a hungry mantis (Mantis religiosa) waits for one of the millions of recently hatched mayflies to flit within range of its raptorial legs. Given the sheer number of mayflies the odds might seem in the mantis’s favor, but the predator has just a few hours to take advantage of this frenzy, after which all of the mayflies will be gone.

This mass hatching and subsequent die-off of mayflies is part of the peculiar life cycle of these insects. After spending a year or so underwater in the relative safety of their larval stage, the mayflies emerge for just a handful of hours to mate and lay their eggs before dying. More fascinating still is the way in which they perish. Adult mayflies do not have functional mouthparts and are therefore unable to eat. Why a species would evolve to reach adulthood, reproduce, and die from starvation all in the same day remains a mystery, but what scientists do know is that mayflies are not the only animals that seem biologically programmed for a premature death. Indeed, the mantis falls into the same category.

If you know one thing about these predatory insects, it is most likely their proclivity for post-coital cannibalism. Before, after, or during sexual reproduction, the female mantis—which is usually much larger than the male—may decapitate and then devour her mate. While this behavior—known as sexual cannibalism—is relatively rare, it still happens about 30 percent of the time in some species. And just like the mayfly with its missing mouthparts, the real mystery is in the details, since even after decapitation the male mantis is still able to complete copulation. In fact, a 2016 study suggests that when sexual cannibalism does occur, reproduction tends to be more successful, with females laying larger clutches of eggs.

In recent years, researchers have started to investigate why this phenomenon, known as phenoptosis—the programmed death of an organism based on behavioral or physical characteristics—occurs. While the mantis and mayfly offer extreme examples of phenoptosis, some scientists think that understanding the process on a biochemical level could help to answer fundamental questions about why living beings—including humans—age and eventually die in the first place. For now, we will leave these existential questions to the scientists investigating them and answer a more immediate one: On this particular night, the mantis’s efforts to catch even a single mayfly before they died were unsuccessful.