Why do adult mayflies have functionless mouthparts and digestive tracts?

Adult mayflies possess functionless mouthparts and digestive tracts because their adult stage is extremely brief, often lasting only a few hours to a few days.1

In the life cycle of these insects, the primary purpose of the adult stage is reproduction and dispersal rather than growth or nutrition. Because they do not feed during this final winged phase, they have no biological need for functional mouthparts or a digestive system. All the energy and nutrients required for the adult's activities—such as mating flights and egg deposition—are acquired during the aquatic nymph stage. This evolutionary strategy of limiting feeding to the larval stage and dedicating the adult stage solely to reproduction is also observed in various other insect groups where adult mouthparts have become vestigial or lost.123