Diversity
This small family of beetles gets its common name ("water pennies") from the unusual
shape of the larvae, which have widely expanded dorsal plates that give the animals
a flat round shape, like a coin. The classification of the family is uncertain, some
authorities recognize some subgroups as separate families. There are only about 263
species in 31 genera are known (Lee et alia, 2007) but this family has not received
intensive study, and there are probably more species still undescribed.
Geographic Range
Members of the
Psephenidae
are found on all continents except Antarctica.
- Biogeographic Regions
- nearctic
- palearctic
- oriental
- ethiopian
- neotropical
- australian
- Other Geographic Terms
- cosmopolitan
Habitat
Psephenid larvae cling to hard substrates (rocks, woody debris) in well-oxygenated
water. They are most often found in fast-moving streams, especially riffle beds, but
some species are found on rocky lake shores with wave action. Most pupae and all adults
are air-breathing, but stay near the water. Adults are often found on the underside
of logs and other objects overhanging streams.
- Habitat Regions
- temperate
- terrestrial
- freshwater
- Other Habitat Features
- riparian
Physical Description
Larvae are 3-10 mm long, ranging in color from pale amber to nearly black (usually matching the substrate) and have a distinctive shape. They dorso-ventrally flattened, with expansions of the dorsal plates that extend out so far that they are oblong or nearly round when viewed from above -- the head and legs completely covered. This flat round shape, plus their brownish color, earns them the name "water pennies". Larvae have clawed legs for clinging to rocks in moving water, and chewing mouthparts for scraping food off of rock and other hard substrate.
Adults are dark-colored, 4-6 mm long, with somewhat flattened bodies, short thread-like antennae, and mandibles not visible from above.
Most species in this family have amphipneustic larvae: they can extract oxygen both
from air and directly from the water. They have a few functional spiracles for air
breathing. Most also have filamentous gills that extract oxygen from the water, either
paired structures on abdomenal sterna, or a single caudal structure near the anus.
The genus
Psephenoides
is more fully aquatic, neither larvae nor pupae have functional spiracles, and both
stages have gills. In the rest of the family, pupae and adults are air-breathers,
with functional spiracles.
- Other Physical Features
- ectothermic
- heterothermic
- bilateral symmetry
Development
Female water penny beetles lay their eggs above or at the edge of the stream or shore
habitat where their larvae will live. Aquatic larvae hatch from the eggs and drop
or crawl into the water. They cling to gravel and other hard debris in the water,
grazing on the algae and other micro-organisms that grow there. In this stage they
grow and molt several times during warm months. If the warm season is short, they
may need more than one year to complete the larval stage. When read to transform,
larvae small chamber where they enter the pupal stage and metamorphose into an adult.
Some species create an air-filled chamber under water, but most pupate on land, in
moist soil. Pupation is completed inside the larval skin. Adults don't live long.
- Development - Life Cycle
- metamorphosis
Reproduction
Reproduction information only known for one or a few species.
Psephenus herricki
females lay hundreds or thousands of small, bright yellow eggs on submerged and emergent
objects in stream riffles with fast current. Probably semelparous, adults only live
a few weeks. Believed to all be sexual dioecious species (each individual either male
or female), with internal fertilization.
- Key Reproductive Features
- semelparous
- seasonal breeding
- gonochoric/gonochoristic/dioecious (sexes separate)
- sexual
- fertilization
- oviparous
Behavior
Water penny larvae tend to move away from light, cling to whatever they touch, and
move into any water current they encounter. This helps them keep their place in streams,
and not be washed into the water. Larvae of some species seem to be attracted to each
other, but some are solitary and it is rare to find more than one.
Food Habits
Water penny larvae feed on periphyton, the thin layer of algae and micro-organisms
that forms on stones and other objects in moving freshwater. Adults may not feed.
- Primary Diet
- herbivore
Predation
Water pennies hide from predators during the day, and are most active at night.
Freshwater trout
are reported to be able to pluck the larvae off of stones.
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).
- terrestrial
-
Living on the ground.
- 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.
- coastal
-
the nearshore aquatic habitats near a coast, or shoreline.
- 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.
- semelparous
-
offspring are all produced in a single group (litter, clutch, etc.), after which the parent usually dies. Semelparous organisms often only live through a single season/year (or other periodic change in conditions) but may live for many seasons. In both cases reproduction occurs as a single investment of energy in offspring, with no future chance for investment in reproduction.
- seasonal breeding
-
breeding is confined to a particular season
- 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
- social
-
associates with others of its species; forms social groups.
- herbivore
-
An animal that eats mainly plants or parts of plants.
References
Brown, H. 1991. Psephenidae (Dryopoidea) (including Eubriidae, Psephenoididae). Pp. 395-397 in Immature Insects , Vol. 2. Dubuque, Iowa, USA: Kendall/Hunt Publishing Company.
Lee, C., M. Satô, W. Shepard, M. Jäch. 2007. Phylogeny of Psephenidae (Coleoptera: Byrrhoidea) based on larval, pupal and adult characters. Systematic Entomology , 32(3): 502-538. Accessed April 30, 2009 at http://www3.interscience.wiley.com/journal/117964692/abstract .
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..
Shepard, W. 2002. Pesphenidae Lacordaire 1854. Pp. 133-134 in American Beetles , Vol. 2. Boca Raton, Florida, USA: CRC Press.
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.
Wichard, W., W. Arens, G. Eisenbeis. 2002. Biological Atlas of Aquatic Insects . Stenstrup, Denmark: Apollo Books.