Water Beetles (Coleoptera)
Water beetles are part of the largest order of insects, containing approximately 24000 species in North America alone. Of these, only about 1000 are water beetles.
Water beetles undergo complete metamorphosis, meaning that their lifecycle is similar to that of a butterfly: they have a larval stage, a pupal stage during which metamorphosis occurs, and an adult stage that looks entirely different from the larval stage. In water beetles, both the larval and adult stages are usually aquatic, while the pupal stage is always spent out of the water.
Water beetle larvae differ widely in appearance, but can be distinguished from other insect larvae by the hardened skin on their heads, lack of wing pads, 3 pairs of segmented legs, lack of filaments or gills on the sides of the abdomen, and lack of prolegs or a long, tapering filament on the end of the abdomen. They range in length from 2-70mm long. Adult beetles range from 1-40mm long. They can be identified by their hard bodies, and hard front wings that cover their hind wings and most of the abdomen.
Because they are such a large order of insects, water beetles can be found in almost all feeding groups and most habitat types. There are beetles that scrape algae, shred plants, prey on other invertebrates, and eat detritus, amongst other feeding habits. They can be found in standing and flowing water bodies of all sizes and velocities, although they are more common in calm waters. Although the order is very diverse, species within this order can have very specific feeding strategies and habitat requirements. Some examples of commonly found beetle families are listed below:
Crawling water beetles are common in dense beds of vegetation in standing water and slow moving parts of rivers and streams. They are primarily herbivores that feed by shredding and sucking the juices from large plants, but some are predators.
Predacious diving beetles account for about ½ of all water beetle species. They are found mostly in weedy areas of shallow, still, small waterbodies such as ponds and slow streams. As the name implies, they are predators. Larvae feed by piercing their prey and ingesting its juices, while adults have chewing mouthparts and consume whole prey.
Riffle beetles are found in areas with lots of water movement, such as fast portions of streams and the littoral (near-shore) zones of ponds and lakes. Most species of riffle beetle are scrapers that feed on algae that grows on rocks and debris.
Water penny larvae occur on stones in areas with moderate to fast current, and occasionally in wave-washed zones of large lakes. They feed by scraping algae from rock surfaces. Water penny adults are terrestrial.
Most water beetle larvae have a closed breathing system, meaning that they obtain oxygen by diffusion across their body surfaces, gills, or a combination of the two. Almost all adult water beetles have an open breathing system, like terrestrial beetles. They breathe through openings in their skin called spiracles that are designed to obtain oxygen from the air. Adult water beetles often carry a store of air beneath their hardened wings so that they can breathe while underwater.
As a group, water beetles cannot all be categorized as either sensitive to or tolerant of pollution and other environmental stressors. Because some beetle species have very narrow habitat requirements, an index of beetle species can provide valuable information about the types of habitat and disturbances that may be present at a specific water body.
Picture key
Top Left: Water Penny Larva(Family Psephenidae)
Bottom Left: Adult Predacious Diving Beetle (Family Dytiscidae)
Right: Water Scavenger Beetle Larva (Family Hydrophilidae)
Source: Photos taken from The North American Benthological Society
Bibliography
Voshell, Jr., J. Reese; illustrated by Amy Bartlett Wright. 2002. A Guide to Common Freshwater Invertebrates of North America. Blacksburg (VA): The McDonald & Woodward Publishing Company. 442 p.
