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.
Development - Life Cycle: metamorphosis
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.
The Elmidae are often considered useful indicator species for the environmental quality of streams.
Positive Impacts: research and education
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.
Primary Diet: herbivore ; detritivore
The Elmidae are found on all the continents except Antarctica.
Biogeographic Regions: nearctic (Native ); palearctic (Native ); oriental (Native ); ethiopian (Native ); neotropical (Native ); australian (Native )
Other Geographic Terms: cosmopolitan
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.
Habitat Regions: temperate ; tropical ; freshwater
Aquatic Biomes: benthic ; lakes and ponds; rivers and streams
Other Habitat Features: riparian
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.
Other Physical Features: ectothermic ; heterothermic ; bilateral symmetry
Sexual Dimorphism: sexes alike
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.
Key Reproductive Features: gonochoric/gonochoristic/dioecious (sexes separate); sexual ; fertilization (Internal ); oviparous
Elmidae, commonly known as riffle beetles, is a family of beetles in the superfamily Byrrhoidea described by John Curtis in 1830. Both adults and larvae are usually aquatic, living under rocks in fast-flowing shallow areas of streams, such as riffles, feeding on algae and biofilms.[1] There are more than 150 genera and 1,500 described species in Elmidae.[2][3][4][5] The oldest record of the group is Cretohypsilara from the Cenomanian aged Burmese amber.[6]
Hydora picea illustration by Des Helmore Elmidae, commonly known as riffle beetles, is a family of beetles in the superfamily Byrrhoidea described by John Curtis in 1830. Both adults and larvae are usually aquatic, living under rocks in fast-flowing shallow areas of streams, such as riffles, feeding on algae and biofilms. There are more than 150 genera and 1,500 described species in Elmidae. The oldest record of the group is Cretohypsilara from the Cenomanian aged Burmese amber.
