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.
Geographic Range
The
Elmidae
are found on all the continents except Antarctica.
- Biogeographic Regions
- nearctic
- palearctic
- oriental
- ethiopian
- neotropical
- australian
- Other Geographic Terms
- cosmopolitan
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.
- Habitat Regions
- temperate
- tropical
- freshwater
- Aquatic Biomes
- benthic
- lakes and ponds
- rivers and streams
- Other Habitat Features
- riparian
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.
- Other Physical Features
- ectothermic
- heterothermic
- bilateral symmetry
- 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.
- Development - Life Cycle
- metamorphosis
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.
- Key Reproductive Features
- gonochoric/gonochoristic/dioecious (sexes separate)
- sexual
- fertilization
- oviparous
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.
- Key Behaviors
- flies
- nocturnal
- motile
- sedentary
- hibernation
- solitary
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.
- Primary Diet
- herbivore
- detritivore
Economic Importance for Humans: Positive
The
Elmidae
are often considered useful indicator species for the environmental quality of streams.
- Positive Impacts
- research and education
Additional Links
Contributors
George Hammond (author), Animal Diversity Web.
- 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.
- native range
-
the area in which the animal is naturally found, the region in which it is endemic.
- Palearctic
-
living in the northern part of the Old World. In otherwords, Europe and Asia and northern Africa.
- native range
-
the area in which the animal is naturally found, the region in which it is endemic.
- oriental
-
found in the oriental region of the world. In other words, India and southeast Asia.
- native range
-
the area in which the animal is naturally found, the region in which it is endemic.
- Ethiopian
-
living in sub-Saharan Africa (south of 30 degrees north) and Madagascar.
- native range
-
the area in which the animal is naturally found, the region in which it is endemic.
- Neotropical
-
living in the southern part of the New World. In other words, Central and South America.
- native range
-
the area in which the animal is naturally found, the region in which it is endemic.
- Australian
-
Living in Australia, New Zealand, Tasmania, New Guinea and associated islands.
- native range
-
the area in which the animal is naturally found, the region in which it is endemic.
- 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.
- 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.
- freshwater
-
mainly lives in water that is not salty.
- 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.
- riparian
-
Referring to something living or located adjacent to a waterbody (usually, but not always, a river or stream).
- ectothermic
-
animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature
- 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.
- 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.
- 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.
- sexual
-
reproduction that includes combining the genetic contribution of two individuals, a male and a female
- fertilization
-
union of egg and spermatozoan
- internal fertilization
-
fertilization takes place within the female's body
- oviparous
-
reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.
- nocturnal
-
active during the night
- motile
-
having the capacity to move from one place to another.
- sedentary
-
remains in the same area
- 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.
- solitary
-
lives alone
- herbivore
-
An animal that eats mainly plants or parts of plants.
- detritivore
-
an animal that mainly eats decomposed plants and/or animals
References
Brown, H. 1991. Elmidae (Dryopoidea). Pp. 404-407 in 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 An Introduction to the Aquatic Insects of North America . Dubuque, Iowa, USA: Kendall/Hunt Publishing Company.