Elmidae  

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 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

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

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