THE XERCES SOCIETY FOR INVERTEBRATE CONSERVATION

Aquatic Invertebrates in Pacific Northwest Freshwater Wetlands
An Identification Guide and Educational Resource

Ephemeridae are both famous and infamous for their prodigous mass emergences. These burrowers are common in sand/gravel or sand/silt in the slow water of large rivers, streams, ponds, and lakes. Fly fishers are always excited to immerse themselves in a "hex" (Hexagenia) hatch, when these large yellow mayflies emerge from lakes or large rivers in overwhelming numbers. The most spectacular mass emergences occur in the Great Lakes and the upper Mississippi where Hexagenia emerge by the millions (sometimes thousands per square yard). They fly inland to mate and the females return to the water to lay up to 8000 eggs each.

The larvae have two curved tusks that project forwards from the front of the head, and long, forked, feathery gills that curve back across the top of the abdomen. They live in U-shaped burrows in soft sediment, feeding primarily on detritus but sometimes on small animals. They wave their long fringed gills to keep fresh, oxygenated water moving in their burrow. They emerge from the water as infertile, yellowish, winged adults (sub-imagos) and cling to a hard surface for a day or so before molting once more into a brown, sexually mature adult (imago). Carcasses of these adults, particularly the males, remain on shore and may cause slick highways, dangerous sidewalk conditions, power outages, and an unpleasant summer aroma.

Although "hex hatches" in the Great Lakes have been very limited for the last 50 years, they have recently returned in large numbers. Their return is seen as a indication of improved water quality in the Great Lakes and a reintroduction into the food chain of an historically important food source for native fish.