Mayflies: Ephemeroptera

Mayflies range in length from 0.04 to 3.2 inches (1 to 81.2 millimeters). They come in a variety of colors, including white, yellow, pinkish, gray, or black. The adults always have wings. In fact, this is the oldest group of winged insects alive today. One mayfly fossil (FAH-suhl), an impression of a mayfly left in mud in ancient times and hardened into stone, has a wingspan of 18 inches (45.7 centimeters). The four transparent wings of the mayfly are held together straight over the body. A network of veins supports each wing. The first pair of wings is much larger than the second; in some species the second pair of wings is very small or even missing altogether.

Males and females are usually easy to tell apart. Males have eyes that are each divided into two separate parts. The upper parts of the eyes are directed upward and mounted on short stalks. The eyes are very sensitive, so males can easily find and capture female mayflies in mating swarms or when there is little sunlight. Males have long front legs that are held out in front of the body. They also have slender bodies, and the abdomen is filled mostly with air. Females are shorter and have heavier bodies, and their abdomens are filled with eggs. The abdomens of both males and females are tipped with two or three long, threadlike structures that are used like antennae (an-TEH-nee), to help mayflies "feel" what is going on behind them. These structures are always longer in the males.

Although adult mayflies are amazingly similar to one another, their larvae (LAR-vee) come in a wide variety of shapes. They do not resemble adults. Species living in fast-moving water cling to rocks and are usually flat and shieldlike in shape. Their low, smooth bodies prevent them from being swept away by the swift current. Mayfly larvae living in the slower waters of ponds and lakes are usually free-swimming. Their bodies are slender and shaped more like a cylinder. No matter how and where they live, all mayfly larvae have chewing mouthparts.

The larvae breathe underwater with gills. The gills are attached along the sides of the abdomen. Depending on where they live and the oxygen content of the water, the gills are brushy and tuftlike or flattened into plates. Species with feathery gills often live in low-oxygen water, and the feathery gills have more surface area exposed to the water, for drawing in the oxygen. Species with flattened gills usually live in fast-moving streams. Their streamlined shape prevents them from being swept away by the current. In some species that live in fast-moving water, the gills form a sticky disk that acts like a suction cup to attach the larvae to submerged rocks. Other species use their gills as paddles for swimming.