Creek Critters: May Flies

by Bonnie Ross

Mayflies are found in a diverse array of aquatic habitats from standing water to cold and rapid head-water streams. They are ecologically very important; essentially being the first-order consumers in all water bodies they inhabit. As a preferred for most aquatic and terrestrial insectivores, they survive by sheer numbers alone. Mayflies were originally named because the majority of adults emerge from the water in May, although California mayflies emerge between February and November. As with all arthropods, their exoskeletons require that they molt (shed their shell-like skin) prior to growing. This is a dangerous time since they are most vulnerable while their new skins are hardening. Merritt and Cummins reports that depending on the species, mayfly nymphs can molt between 12 and 45 times prior to becoming adults and each molt moves the nymph into a new instar. While the great majority of mayfly nymph species live in the water for one year, some stay for only a few months while others remain aquatic for two years.

What happens during their last instar is what makes the mayfly unique among flying insects. They first emerge from the water in a sub-adult, sexually immature stage called the “subimago”. This stage can last from 1 1/2 to 48 hours. They are covered with hair-like microtrichia which renders them waterproof. Within hours they molt one last time. Adult mayflies take to the air and form huge swarms. Most swarms are made up of males with the occasional female flying in to mate. As their Order Ephemeroptera implies, adult mayflies have short lives. They have no mouth parts and live just long enough to reproduce. In his book Aquatic Insects of California, Robert Usinger mentions that swarms in other areas of the world just contain a few hundred specimens, but swarms of many millions of these insects occur in the Sacramento Valley. Here in the Valley, they become an important food source for young salmon and other freshwater fish. Mayflies need clean water to survive to adulthood and are very sensitive to chemical pollution. As we creek stewards restore our urban creeks, we shall see many more salmon and steelhead and the swarms of mating adult mayflies that provide so much energy and benefit to the ecosystems they inhabit.

This article originally appeared in our Winter 2004 Newsletter


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