Elmidaeriffle beetles

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Diversity

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)

Geographic Range

The Elmidae are found on all the continents except Antarctica. (Brown, 1991)

Habitat

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)

  • Aquatic Biomes
  • benthic
  • lakes and ponds
  • rivers and streams

Physical Description

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)

  • Sexual Dimorphism
  • sexes alike

Development

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)

Reproduction

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)

Behavior

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)

Food Habits

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)

Economic Importance for Humans: Positive

The Elmidae are often considered useful indicator species for the environmental quality of streams. (Brown, 1991)

  • Positive Impacts
  • research and education

Contributors

George Hammond (author), Animal Diversity Web.

Glossary

Australian

Living in Australia, New Zealand, Tasmania, New Guinea and associated islands.

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Ethiopian

living in sub-Saharan Africa (south of 30 degrees north) and Madagascar.

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Nearctic

living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

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Neotropical

living in the southern part of the New World. In other words, Central and South America.

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Palearctic

living in the northern part of the Old World. In otherwords, Europe and Asia and northern Africa.

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benthic

Referring to an animal that lives on or near the bottom of a body of water. Also an aquatic biome consisting of the ocean bottom below the pelagic and coastal zones. Bottom habitats in the very deepest oceans (below 9000 m) are sometimes referred to as the abyssal zone. see also oceanic vent.

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

cosmopolitan

having a worldwide distribution. Found on all continents (except maybe Antarctica) and in all biogeographic provinces; or in all the major oceans (Atlantic, Indian, and Pacific.

detritivore

an animal that mainly eats decomposed plants and/or animals

ectothermic

animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature

fertilization

union of egg and spermatozoan

freshwater

mainly lives in water that is not salty.

herbivore

An animal that eats mainly plants or parts of plants.

heterothermic

having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.

hibernation

the state that some animals enter during winter in which normal physiological processes are significantly reduced, thus lowering the animal's energy requirements. The act or condition of passing winter in a torpid or resting state, typically involving the abandonment of homoiothermy in mammals.

internal fertilization

fertilization takes place within the female's body

metamorphosis

A large change in the shape or structure of an animal that happens as the animal grows. In insects, "incomplete metamorphosis" is when young animals are similar to adults and change gradually into the adult form, and "complete metamorphosis" is when there is a profound change between larval and adult forms. Butterflies have complete metamorphosis, grasshoppers have incomplete metamorphosis.

motile

having the capacity to move from one place to another.

native range

the area in which the animal is naturally found, the region in which it is endemic.

nocturnal

active during the night

oriental

found in the oriental region of the world. In other words, India and southeast Asia.

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oviparous

reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.

riparian

Referring to something living or located adjacent to a waterbody (usually, but not always, a river or stream).

sedentary

remains in the same area

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

solitary

lives alone

temperate

that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).

tropical

the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.

References

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.