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Why do mayflies not eat as adults?

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Two wild horses graze on tall grass in the open prairie of Theodore Roosevelt National Park, with rolling badlands visible in the background. — Feeding behaviour Wild horses graze in the Badlands of North Dakota.

Adult mayflies do not eat because they possess nonfunctional mouthparts and digestive tracts.1

Because their adult life span is extremely brief—often lasting only a few hours or days—they are biologically specialized for reproduction rather than feeding. All the energy and nutrients required for their adult activities, such as mating and egg deposition, are acquired during their long-lived aquatic nymph stage. Once they emerge as winged adults, they lack the physical structures necessary to ingest or process food.12