The Elmidae, commonly known as riffle beetles, are found in freshwater streams all around the world. There are about 1400 species known world-wide, but probably many more have not yet been discovered. About 100 species in 27 genera have been found in North America. These are small (1-8 mm long) aquatic beetles that are most often found crawling on stones and other solid debris in fast-moving streams. A few species are found in slow streams or still water. They have relatively long legs and both adults and larvae are well-sclerotized. Both larvae and adults are fully-aquatic, extracting oxygen from the water around them. (Brown, 1991; McCafferty, 1983; White and Brigham, 1996)
Adult riffle beetles are small (1-8 mm long), dark, elongate, hard-bodied beetles, with relatively long legs and tarsal claws. The antennae are at most slightly clubbed, usually slender (this distinguishes them from species in an otherwise-similar family, the Dryopidae). The ventral surface of the body adult riffle beetles is covered with an extremely dense (millions/mm^2) layer of tiny hydrophobic hairs. This traps a layer of air, called a plastron, on the surface of the body, and the beetle uses this for gas exchange.
Riffle beetle larvae are elongate, up to 16 mm long (most less than 8), with the head and all 3 pairs of legs visible from above. The antennae and mouthparts are shorter than the head. The body segments are usually well-sclerotized, and the body is often hemispherical or concave in cross-section (rarely rounded). One diagnostic feature of the larvae are the filamentous gills that emerge from the tip of the abdomen. These can be retracted for protection, or rhythmically expanded and contracted to increase oxygen flow. A plate called an operculum covers the retracted gills, and has a pair of well-developed claws attached to it. (Brown, 1991; White and Brigham, 1996)
Most riffle beetle species live up to their name, and are found crawling on stones and woody debris in the riffle zones of freshwater streams. Some occur in the depositional zones of streams, on softer sediments, and some are amphibious and feed along the banks of streams. A few have adapted to living in still waters, and are found on vegetation in those habitats. Larvae are strictly aquatic, but otherwise share the same habitats as adults. (Brown, 1991; McCafferty, 1983; White and Brigham, 1996)
Species in the Elmidae tend to be long-lived for beetles their size. Most species take a year or more to mature and reproduce, and some may live for several years. Larvae require 6-8 molts to complete their growth. Pupae are air-breathers, and complete their transformation in soil cavities or other refuges along stream banks or shores. (Brown, 1991; McCafferty, 1983; White and Brigham, 1996)
Adult riffle beetles mate in the water. Females lay single eggs or small groups of eggs in crevices on solid objects on the bottom of the stream where they live. (Brown, 1991)
Riffle beetles tend to move slowly, clinging to the substrate as water moves by. When ready to pupate, larvae either crawl out of the water, or wait until the water level recedes and leaves them in air. In some species, newly-emerged adults may fly significant distances their first night before returning to water. Once they return to the water they no longer fly. (Brown, 1991)
Most riffle beetles are believed to feed on small particles of dead plant material, other organic debris, and periphyton (microscopic algae and other microorganisms growing on hard surfaces in freshwater). A few feed on living plant material. (Brown, 1991; White and Brigham, 1996)
George Hammond (author), Animal Diversity Web.
Brown, H. 1991. Elmidae (Dryopoidea). Pp. 404-407 in F Stehr, ed. Immature Insects, Vol. 2. Dubuque, Iowa, USA: Kendall/Hunt Publishing Company.
McCafferty, W. 1983. Aquatic Entomology: The Fishermen's and Ecologists' Illustrated Guide to Insect and Their Relatives. Boston, Massachusetts, USA: Jones and Bartlett Publishers, Inc..
White, D., W. Brigham. 1996. Aquatic Coleoptera. Pp. 399-473 in R Merritt, K Cummins, eds. An Introduction to the Aquatic Insects of North America. Dubuque, Iowa, USA: Kendall/Hunt Publishing Company.