Why do adult mayflies have such short life spans?

Adult mayflies have extremely short life spans—often lasting only a few hours—primarily because they lack the biological equipment necessary to sustain life beyond their reproductive phase.

Unlike the aquatic nymph stage, which can last up to two years, the adult mayfly possesses nonfunctional mouthparts and a vestigial digestive tract. Because they cannot feed, they are unable to obtain the energy required for long-term survival. Consequently, the adult stage is dedicated entirely to reproduction and dispersal. Once mating and oviposition (egg deposition) are complete, the insect dies. This brief existence serves as a highly specialized strategy that focuses all remaining energy on ensuring the next generation.12