A mayfly is a delicate flying insect known for having one of the shortest adult lifespans in the animal kingdom: sometimes just a single day, though some species live a few days or even a week. These fragile creatures spend most of their lives (often a year or more) as nymphs living underwater in streams and rivers. Then, when conditions are right, thousands of mayflies emerge from the water at once, transform into adults, mate, lay eggs, and die, all within hours or days.

Mayflies have transparent wings, long thin bodies, and distinctive tail filaments that trail behind them as they flutter through the air. They're clumsy fliers, and many species don't even have working mouths as adults because they don't need to eat during their brief existence. Their main purpose at this stage is to reproduce.

Because mayflies live such short lives, people sometimes use the word to describe something fleeting or temporary. You might say a friendship that lasted only a few weeks was mayfly-brief, or that a popular trend had a mayfly lifespan. The mayfly reminds us that some things, no matter how brief, still matter: these insects have survived for over 300 million years, making them one of the oldest living groups of insects on Earth.