Riffle Beetle Facts & Information 2022 (Elmidae)


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Riffle Beetle & Riffle Beetle Larvae Facts & Information Guide 2022 (Elmidae)

An adult riffle beetle
Elmids usually live for a year and prefer to live an almost fully aquatic existence. Natural History Museum: Coleoptera Section / CC BY 2.0

Waterfalls, river rapids, and fast-flowing streams are favorite haunts of these aquatic beetles. Despite being known for moving quite slowly, they favor high-energy environments, benefitting from the abundance of oxygen. One might say that these insects live their lives on the edge as they must cling tightly to substrates and vegetation or get washed away! This tough life is permitted by a complex combination of unique adaptations and behaviors, altered throughout their life cycle.

Riffle beetles are classified under the Coleoptera order of invertebrates. The largest of all orders, it has over 400,000 species and constitutes a fourth of all known animal life. Elmidae is quite a diverse family as well, including more than 1,400 species grouped under 150 genera. Although these beetles have a cosmopolitan distribution, only around 10% are native to North America and Europe (with just a dozen species in Britain). It is estimated that hundreds are yet to be discovered and described.

Relatively long-lived for their minuscule size, elmids can live for more than a year. Interestingly, they hardly ever make use of their wings and prefer to live an almost fully aquatic existence. Right after metamorphosis, the young adults may venture away from water for just a single night. They tend to return soon after, never taking flight once more. They are able to mate, lay their eggs, and feed while fully submerged.

RIFFLE BEETLE FACT SHEET
COMMON NAMES
Riffle beetle, elmid, spider water beetles
SCIENTIFIC FAMILY NAME
Elmidae (Coleoptera); subfamilies – Elminae and Larainae
TYPE
Aquatic invertebrate (larvae & adult); terrestrial pulpae
DIET
Detritus, organic debris, microscopic algae and microbes
NATIVE HABITATS
Riffle zones of freshwater bodies
AVERAGE LIFESPAN (IN WILD)
1 year
AVERAGE SIZE
Larvae up to 16 mm; adults up to 8 mm
IUCN RED LIST STATUS
Several species are listed as Endangered, Vulnerable, and Threatened; most are under ‘Least Concern’
COMMON GENERA
Stenelmis, Xenelmis, Hexacylloepus, Grouvellinus, Neoelmis

What Do Riffle Beetles Look Like?

A group of adult riffle beetles
Riffle beetles are small and usually have a distinctly segmented head that is separated from the rest of the body. Natural History Museum: Coleoptera Section / CC BY 2.0

You’ll have to peer closely into clear water to spot a riffle beetle. Adults are notably dark-colored, elongated, and small. Most reach a length of just 5 millimeters, comparable to the length of house ants. They are distinguished by their clubbed or filiform antennae. Up close, these may appear segmented. When peering at them from above, you should be able to see 3 pairs of legs, each bent at 3 joints. These terminate in tiny yet powerful tarsal claws that allow them to maintain their position in a strong current. The head capsule may be distinctly segmented from the rest of the hard body and has several lateral ocelli (simple eyes).

Adult riffle beetles have an extremely useful adaptation, allowing them to breathe in water despite their lack of gills. The ventral (or lower) surface of some species is densely covered in millions of hydrophobic hairs or bristles, collectively termed a ‘plastron’. This structure is able to trap a thin but sufficient layer of air. Thus, the beetles can remain submerged for weeks on end by breathing the trapped gases!

Larvae may be distinctly larger than their adult forms. They are segmented and equipped with a pair of antennae and three pairs of legs. Unlike adults, larvae have filamentous gills occurring at the tip of the abdomen. The gills can be expanded, contracted, or fully retracted depending on the need for oxygen or protection. These are fully lost in adulthood.


Riffle Beetle Habitats – Where Do They Live?

Riffle beetles are usually found in rivers and streams with high oxygen content. Andrew Hoffman / CC BY-NC-ND 2.0

Elmids typically live in the “riffles” of running streams, rivers, and brooks due to the high dissolved oxygen content in these microhabitats. Here, they stay latched onto debris, a rough substrate, wood, or vegetation at a shallow depth. Some species have become adapted to the calmer waters along the margins of lakes and ponds as well. Small variations in habitat preferences exist between genera. For example, Hexacylloepus favor streams with dissolved calcium. Those occurring in tropical areas (Lara, Potamophilops) may be present in swarms close to waterfalls.

When disturbed, some species of riffle beetles may rapidly fly away from the water body. They are more likely to be associated with watershed regions of well-preserved forests as they are sensitive to pollutants. Low precipitation can also result in unstable populations and may limit distribution.

Generally, riffle beetles are more diverse in tropical compared to temperate regions. They prefer humid climates and mild to warm temperatures. Solitary in nature, they may occasionally gather just to mate. Eggs are laid in the same aquatic environments, underneath rocks and wood. Unfortunately, these zones are increasingly threatened by climate change and anthropogenic activity, in the form of polluted groundwater and land use conversion.


What Do Riffle Beetles Eat?

Biofilm on rocks underwater
Riffle beetles like to feed on biofilm that is found on hard surfaces, like rock faces. Lamiot, CC BY-SA 4.0, via Wikimedia Commons

Adult and larval life stages of riffle beetles are sometimes found feeding alongside one another. They both consume organic items that commonly occur along the water’s edge; thus, they are considered detritivores and herbivores. Their favorite food types include decaying plant matter, floating debris, and softened wood.

Riffle beetles also feed on biofilm that accumulates on hard surfaces, such as rock faces, and the spaces between substrate materials. These can be rich in microscopic algae and bacteria. Only a few species are known for feeding on live plant matter.


The Life Cycle of Riffle Beetles

Elmidae larva
Riffle beetle larvae go through 5 – 8 different molting stages, each time becoming slightly larger. Wlodzimierz, CC BY-SA 4.0, via Wikimedia Commons

Elmidae undergo metamorphosis before reaching their mature adult stage. Their life cycle resembles that of many other insects as they must first hatch from eggs, store necessary nutrients as larvae, undergo major anatomical changes as pupae, and then emerge as fertile adults. Their incubation time in eggs, which are usually found underneath submerged stones, lasts for approximately 5 – 15 days. Once they hatch, they undergo a fairly lengthy larval period that lasts anywhere from 6 – 36 months depending on the species.

Riffle beetle larvae undergo developmental changes through 5 – 8 molting phases (instars). Each time they molt, they tend to become slightly larger and their morphology becomes more well-defined. In warmer climates, the larval stage is short enough to ensure that the entire life cycle is completed in a single year. In colder climates, species tend to have lengthier larval stages and may overwinter as larvae in their final instar.

Unlike the larvae and adults, pupae aren’t equipped with adaptations that allow them to respire in water. Larvae that are just about to pupate may sometimes be seen leaving the water and burrowing into soil beneath stones or rotten wood in refuges that are located close to the shore. The larvae may also wait until the waterline recedes, leaving them exposed, before digging a burrow for pupation.

The pupa stage may last for just a few days, after which the adult beetle emerges, takes to the air, and then enters the water. The hind wings of some species are lost upon re-entry into the water. The adults then feed, mate, and lay their eggs while submerged, closing their life cycle.


How to Attract Riffle Beetles to Ponds?

As riffle beetles are attracted to clean, oxygen-rich, and turbulent water, they will seldom venture into still ponds. If you would like to effectively attract them, your pond would have to be equipped with special features that imitate the movement and energy of naturally running water. A makeshift waterfall or fountain pump may work quite well as long as it is forceful enough to generate a strong current. Mist or spray from these features may even encourage some riffle beetles to surface.

To support all stages of their life cycle, the pond should have shallow margins or a slope. Both the larvae and adults are less likely to feed in either deep or fully exposed areas. The shallow sections should come into contact with the running water and should ideally have a natural substrate, like gravel or an assortment of stones.

Sunken leaves, algal growth, and some trailing vegetation would enhance the organic content of the margins, thereby increasing food availability for the beetles. Just off of the pond’s edge, there should also be some sheltered areas that are accessible to larvae when they are ready to burrow and pupate. Slabs of partly damaged wood or decaying logs would help hide the burrows from potential predators. 

If your pond is located far away from other bodies of water, it may, unfortunately, be near impossible to attract these beetles. They will seldom disperse into dry regions, let alone take flight over lengthy distances.


Are Riffle Beetles Beneficial?

Due to their sensitivity to pollutants, reduced dissolved oxygen levels, and changes in water temperature and depth, riffle beetles can serve as indicators of climate change and good water quality. Many species are currently threatened due to the loss of wetland forest regions.

Riffle beetles are highly beneficial insects to have in any type of water feature due to their role as macroscopic decomposers and algae grazers. Moreover, all life stages can serve as food to many larger predators, including amphibians and economically important game fish. It’s vital that even small fragments of wetlands are protected to ensure that your native aquatic beetles are able to survive. The cultivation of high-energy wildlife ponds would also help protect them.

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