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Specialized habitats

called trout stream beetles. They are large water

beetles, 10–15 mm long, represented in the

modern fauna only by the genus Amphizoa, with

five species: three in North America and two in

east Asia. They inhabit clean, well-oxygenated,

rapid-running mountain streams, where adults and

larvae are predators of immature stages of insects

such as caddisflies and stoneflies. The long-lived

larvae of water penny beetles (Psephenidae) cling

to the undersides of rocks in flowing streams,

grazing on algae, and are found in small numbers

throughout the world. They are so called because

their circular larvae resemble small coins. The

small, soft-bodied adults are not aquatic, and live

short lives on streamside vegetation.

Another beetle group that received its common

name from its association with flowing water is the

Elmidae, called riffle beetles. Adults and larvae

live under stones in fast-flowing streams, grazing

on algae and other encrusting organisms. A few

elmid genera, such as the usually rare Stenelmis,

develop as larvae in submerged dead wood at the

bottom of streams and rivers. Several genera of

Elmidae have developed a “plastron,” which

enables them to extract oxygen direct from

water, removing the need to go to the

surface to recharge their air bubble.

Elmidae are much more diverse in

fast-moving water than they are in

ponds or pools, which are preferred

by other water beetle families such

as Hydrophilidae, Dytiscidae, and

Gyrinidae. However, all of those

families also have a few species and

genera that specialize in moving water.

FLOWING FRESH WATER

Like most non-marine habitats, flowing fresh

water such as streams and rivers is home to many

beetles, but fewer than are found in ponds, ditches,

and slower-moving water. There are several

ecological obstacles that keep some beetle genera

and species out of flowing streams. Firstly, most

larvae and adult water beetles need to return to

the surface fairly often to replenish their supply

of air, and in flowing water this exposes them to

the risk of being carried away by the current.

Secondly, the quantity of organic matter, and

so the potential food supply, is usually less in

faster-flowing water. Thirdly, fast-flowing water

has higher levels of oxygen and is able to support

larger populations of fish, many of which will eat

the adult and larval beetles. Despite these hurdles,

a number of families and genera of beetles have

adapted to survive in these environments.

The family Amphizoidae are so closely

associated with fast-flowing streams that they are

left | A typical European riffle beetle

Limnius volckmari (Elmidae), grazing algae

on a submerged rock.