Elmidae
Phylum Arthropoda
Class Insecta
Order Coleoptera
Family Elimidae
Common Name Riffle Beetle
 
Distinguishing Characteristics
  • The terminal abdominal segment is bifid or slightly emarginated (notched) posteriorly and with lateral ridges
  • Head capsule with groups of 5 lateral ocelli
  • Ninth abdominal segment with a lid-like operculum
  • Adbominal sternites 1 8 sometimes never bearing gills
  • Body cylindrical, subcylindrical, or fusiform
  • Head and legs are visible from a dorsal view
Additional Pictures
         
         
         
Ecology
Geographic Distribution
  • The genus Xenelmis is common and widespread, ranging from central Mexico through much of South America (2).
  • Hexacylloepus has been found in southern North America in Oklahoma, Texas, and New Mexico. As well as in the Caribbean; Grenada and Trinidad. The genus has been recorded in South America in Venezuela, Brazil, Paraguay, and Bolivia (3).
  • 10 of the 22 Nearctic genera are indigenous to the United States. Ordobrevia, Ampumixis, Cleptelmis, Narpus, Lara, and Heterlimnius are confined to the western states; Dubiraphia and Optioservus are transcontinental; Promoresia and Gonielmis occupy only in the East and Southeast states (4).
  • Phanocerus, Cylloepus, Elsianus, Neoelmis, Heterelmis, Microcylloepus and Hexacylloepus are confined to the Southwest, being a northern extension of fauna in Mexico and tropical fauna (4).
  • Stenelmis is the most widely distributed genus, with species on every continent, except for South America. Macronychus has a single species in the eastern half of the United States, as well as a species in Europe and India. Zaitzevia species were distributed in Asia and found in the west of North America. The Limnius species are found in the Appalachian Mountains. Ancyronyx has one species in the Eastern U.S., one species in northern South America, and several species in the Oriental Region and Africa (4).

    Habitats
  • Elmidae live in rubble, debris, and sometimes vegetation in stream riffles(5).
  • Macronychus glabratus has the greatest densities in wood debris streams. The adults and larvae are completely aquatic and live in similar habitats: well-conditioned wood, or in wood that is just beginning to decay and has loose bark (6).
  • Xenelmis and Atractelmis can be found embedded in mixed substrata in clear, fast flowing water anywhere from small brooks to medium sized rivers (2).
  • Hexacylloepus, Microcylloepus and Neoelmis live in streams with dissolved calcium content. Larvae were collected from underneath travertine covered rocks and pebbles. Certain species can live in fast flowing water just a few centimeters in depth, while others can be found in slow flowing water just below a riffle (3).

    Life History
  • Elmidae have short incubation times of 5 to 15 days and the larval period can last anywhere from 6 to 36 months. Elmidae can have between 5 and 8 instars (6).
  • M. glabratus has univoltine life cycles with 6 larval instars (6).
  • Primary recruitment months in Arkansas were March and April; in Texas they are February and March. The larvae hollow out a small oval space in the wood to pupate; pupation occurs under bark or in rotted wood (6).

    March and April were the months that recruitment occurred in Arkansas based on the study performed by Phillips (1997). The majority of samples taken between the months of October and January were comprised of larvae in the sixth instar and close to maturity. The warmer climate found in southern North America encourages the completion of a life cycle in one year compared to the two years it takes for M. glabratus to develop in colder environments such as Quebec, Canada. Based on the data collected from the 1997 Arkansas study, larval growth from the first instar to the sixth takes around six months and suggests that the larvae over-winter while in the sixth instar (6).

    Genetics and Evolution
    Adults of Hexacylloepus are closer to the genus Cylloepus than any other genus in North or Central America based on external morphology. Hexacylloepus larvae are much different than Cylloepus, and more closely resemble Elsianus, Microcylloepus, Neocylloepus, or Neoelmis. This is because they have a posterior prothoracic sternum, the mespleura and the metapleura are divided into two parts, the abdominal pleura are only on the first seven segments, and the body shape is hemicylindrical versus the cylindrical shape of Cylloepus (3).

    Physiological Ecology
  • Elmidae have powerful claws that allow them to maintain their position in rapid streams. This characteristic is unique because Elmidae are completely aquatic (7).
  • Elmidae have a hydrofuge hair system on the ventral surface that provides a space for air to be. A series of hairs are curved and covered by another layer of hairs that act as a cover. Elmidae are capable of living eight weeks submerged in aerated water (7).
  • In Donacia larvae, the 8th segments have two spines that are used for piercing the air cells of aquatic plants to support life in muddy banks (7).

    Feeding Ecology and Food Web
  • The adults and larvae of Macronychus occur and feed on wood, whereas the larvae and adults of Xenelmis occur in sand or gravel (2).
  • Macronychus glabratus can be important to secondary production in stream ecosystems. They provide food and habitat to other stream organisms (6).

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    Bibliography

    Merritt, Richard W., and Kenneth W. Cummins. "Aquatic Coleoptera." An Introduction to the Aquatic Insects of North America. Dubuque, IA: Kendall/Hunt Pub., 1978. 203-32. Print.

    (2) Brown, Harley P. "Neotropical Dryopoids. III. New Records of Xenelmis, with a Description of the Larva (Coleoptera: Elmidae)." The Coleopterists Bulletin 25.3 (1971): 95-101. JSTOR. Web. 16 Apr. 2012. .

    (3) Brown, Harley P. "The True Larva of Hexacylloepus, with a Description of the Larva of H. Ferrugineus and a Summary of the Records for the Genus. (Coleoptera: Elmidae)." The Coleopterists Bulletin 27.3 (1973): 143-50. JSTOR. Web. 16 Apr. 2012. .

    (4) Sanderson, Milton W. "A Revision of the Nearctic Genera of Elmidae." Journal of the Kansas Entomological Society 26.4 (1953): 148-63. JSTOR. Web. 18 Apr. 2012. .

    (5) Brusven, Merlyn A. "Drift Periodicity of Some Riffle Beetles (Coleoptera: Elmidae)." Journal of the Kansas Entomological Society 43.4 (1970): 364-71. JSTOR. Web. 16 Apr. 2012. .

    (6) Phillips, Edward C. "Life Cycle, Growth, Survival, and Production of Macronychus Glabratus (Coleoptera: Elmidae) in Northwest Arkansas and Southwest Texas Streams." Invertebrate Biology 116.2 (1997): 134-41. JSTOR. Web. 16 Apr. 2012. .

    (7) Leng, Chas W. "Aquatic Coleoptera." Journal of the New York Entomological Society 21.1 (1913): 32-42. JSTOR. Web. 19 Apr. 2012. .