Benthic Macro Invertebrate (BMI) Lifestyles

BMI nymphs have five general feeding strategies. First are the shredders, who shred leaves and other loose organic matter. Second are the scrapers, who scrape algae, etc., off underwater rocks. There are two types of collectors that depend on fine particulate organic matter. Collector-filterers obtain material from the water column. Collector-gatherers, as the name implies, gather detritus from the substrate. Lastly, there are the predators who eat other animals. Most BMIs live on the bottom of the body of water.

Stone flies live several years and need cold, well oxygenated, moving water. They generally favor rocky areas, but different species prefer different size rocks, and some are found in leaf packs or even sandy areas. They are very sensitive to pollution, so a population of large stone flies is a good indicator of a healthy creek.

There are thousands of species of caddis flies worldwide. The nymphs are an important part of the freshwater food web; the adults are terrestrial. Caddis flies are closely related to butterflies and moths. In fact, the adults look like moths and the nymphs spin silk like caterpillars. They use it to make nets or tubes to live in, or as glue to hold together small pebbles, twigs, leaves, or other debris that they use as portable cases. A few species are free-moving predators, but even they lay down a line of silk when they move. Most species eat detritus. Caddis fly nymphs are widely used as indicator species; different species are sensitive to different types of pollution.

Mayfly nymphs eat bacteria although most of the material passes through their bodies unused. Most species are collectors and scrapers. Mayflies are microhabitat specialists, each species needing a specific type of substrate at a particular depth with a certain amount of wave action. Species vary in their sensitivity to pollution; the sensitive ones are good indicator species. Caddis fly nymphs eat mayfly eggs. Many creatures eat mayfly nymphs – birds, fish, and other insects. Like caddis flies, mayflies differ in their sensitivity to pollution; some make better indicator species than others.

While nymphs are the most important BMI life stage for assessing water quality, most of these insects are much more familiar as flying adults. Two of the most popular and familiar groups are damsel flies and dragon flies. They look similar, but here's an easy way to tell them apart:  Damsel flies are smaller and perch with their wings up. Dragon flies perch with their wings out.