Mayflies are ancient insects often found near fresh water sources like rivers, streams, and lakes. Their appearance changes significantly throughout their life cycle, transitioning from an aquatic nymph to a winged adult. Understanding these distinct forms aids in their identification.
The Adult Mayfly
Adult mayflies are delicate, slender, and soft-bodied, typically ranging from 0.5 to 2.5 centimeters in length. Their most prominent features are two pairs of wings: large, triangular forewings and smaller hindwings. In some species, hindwings may be reduced or absent. These membranous wings are often transparent or slightly smoky and are characteristically held vertically above the body at rest, resembling a butterfly.
Extending from the abdomen are two or three long, thread-like tails, known as cerci. These tails can be as long as or longer than the body, contributing to their distinctive silhouette. Adult mayflies also have delicate legs, often held forward and primarily used for clinging rather than walking. Their antennae are short and inconspicuous.
Adult mayfly coloration is typically dull, earthy tones, including browns, grays, and yellows, though hues vary by species. Before reaching the final adult stage, some mayflies pass through a subimago stage, often called a “dun.” This form resembles the adult but is duller in color and has a less refined wing structure, undergoing one more molt before becoming a mature adult.
The Mayfly Nymph
Mayfly nymphs are the aquatic larval stage, appearing markedly different from adults. They inhabit freshwater environments like stream bottoms, rivers, and lakes, adapted for a submerged existence. Their body shape can be flattened for clinging to surfaces or cylindrical for burrowing into sediment.
A distinctive feature of mayfly nymphs is their external gills, visible as plate-like or feathery structures along their abdomen. These gills are essential for extracting oxygen from water and serve as a primary identifying characteristic. Nymphs also possess strong legs, well-suited for crawling across substrates or digging into soft bottom materials.
Similar to adults, mayfly nymphs have two or three thread-like tails, though often shorter and thicker than adult tails. Their mouthparts are adapted for various feeding strategies, such as scraping algae from rocks or filter-feeding particles. Nymphs are smaller than adult mayflies, increasing in size through multiple molts before their final transformation.