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AQUATIC BEETLES

SCIENTIFIC NAME: Coleoptera

NUMBER OF SPECIES/DISTRIBUTION
Beetles comprise the largest, and perhaps the most diverse group of life forms on Earth.
  • There are an estimated 350,000 to 375,000 species of beetles, making coleoptera the largest order of insects in the animal kingdom.
  • The coleoptera order contains 40% of all the insects, and almost 30% of all animal species.
  • In North America, there are approximately 5,000 species of beetles that live in water for at least one stage during their life cycle.
DIET/FEEDING
Beetle larvae can be herbivores, scavengers, or predators.
  • Herbivorous larvae eat only plants. They eat aquatic vegetation such as algae, roots, and leaves.
  • Herbivorous may also suck juices from the stems of plants.
Water Beetle
  • Scavengers feed on decomposing organic material that has been deposited.
  • This scavenged material can come from aquatic vegetation, feces, or other organisms that have died.
  • Beetle larvae that are predators feed on other invertebrates.
  • Predator beetles have strong specialized feeding appendages (called 'mandibles') on the sides of their mouths. These are used to to catch and hold prey. Smaller invertebrates may also be swallowed whole.
  • Some species of beetles also have specialized piercing/sucking mouth parts that they use to inject their prey with special chemicals called 'enzymes'.
  • These enzymes can break down the body tissue of the prey.
  • By liquifying the body tissue, the can suck out the insides of the prey animal.
  • These mouth parts can also be found on some herbivorous beetle species, which use them for sucking out plant juices.
  • Large adults of the predatory suborder Adephaga can catch and kill tadpoles and small fish.
  • Beetles of the same species may have very different feeding habits during different life stages. For example, a beetle may be predatory in its larval stage but may be herbivorous as an adult.

HABITAT
Aquatic beetles are found in a wide range of habitats. They are most frequently found in both fast and slow moving fresh water.
  • Beetles inhabit every terrestrial and aquatic environment in the world except Antarctica.
  • The greatest diversity of beetle species occurs in tropical regions.
  • Most beetles tend to avoid open water because it makes them a target for predators; as a result, they have a tendency to live in sheltered areas that offer greater protection.
  • The majority of species dwell along the substrate (making them 'benthic dwellers'), although different species inhabit different specific areas within aquatic environments.
  • Many species can be found living in rock crevices, while others may create (and live in) little burrows within the substrate.

LIFECYCLE
Aquatic beetles undergo complete metamorphosis, passing through 4 complete life stages. These are egg, larvae, pupa, and adult.

Eggs
  • Female beetles can lay anywhere from one or two eggs up to hundreds of eggs at a time, depending on the species.
  • Eggs are usually deposited on or near the larval food source, such as along the river's substrate, in the soil, or on a host plant.
  • Locations for egg deposition also depend on the type of species.
  • Eggs usually take approximately one to two weeks to hatch.
Larvae
  • Upon hatching, the beetle larvae pass though three to eight stages of development, called 'instars', before being ready to pupate and transform into adults.
Pupae
  • The pupal stage of the beetle occurs on land.
  • When ready to pupate, the larvae will leave the water to find a spot on land, but still near the water, where it can pupate safely.
  • The pupal stage tends to last several few weeks, and the exact duration depends on the type of species and on environmental conditions.
Adults
  • Terrestrial adults of aquatic beetles have short lives, living only to mate; some species feed as adults, while others do not.
  • A complete lifecycle of a beetle may take anywhere from a few weeks to several years.

ROLE IN FOOD CHAIN
Although important, beetles are not a significant source of food for other animals in North America. They can, however, cause damage to aquatic vegetation when there are too many herbivorous beetles in one area.

INTERESTING FACTS
  • The word 'Coleoptera' comes from the Greek words 'koleos', meaning sheath, and 'ptera', meaning wings. It refers to the hard sheath-like shells that adult beetles have to cover their softer hind wings.
  • The beetle is one of the world's strongest creatures: A species known as the rhinoceros beetle can carry up to 100 times its own weight for short periods, and up to 30 times its weight for any length of time. Proportionally, that is like a 150 pound person walking with a car on his or her head!
  • Some adult terrestrial beetles store water underneath their hard exoskeleton to help them survive in hot desserts.
  • Adult aquatic beetles, such as the riffle beetle, carry their own oxygen supply in an air bubble underneath their hard shell. That is why riffle beetles must live in habitats with high oxygen levels, such as fast-flowing water with lots of riffles.
  • By mimicking the behaviour and odour of ants, over 1000 terrestrial beetle species can live in the nests of ants.
  • Two families in the Coleoptera order are 'bioluminescent'. This means that they can glow. These families are more commonly known as fireflies and glow-worms.
  • One species of beetle, known as the bombardier, can create a popping noise to scare predators away by mixing gases that are stored separately within their bodies.
  • To communicate with each other, beetles can use chemicals (such as pheremones) or sounds. They may also communicate by sight, as fireflies do.

Sources
Animal Corner (2010). All about Coleoptera.  Available here.
Brusca, R. C, and Brusca, G. J., 2003. Invertebrates, 2nd ed. Sinauer Associates, Inc., Publishers, Massachusetts. Page 599.
California State University (2001). Coleopterea. Available here.
Canada's Aquatic Environments (2002). Coleoptera. Available here.
DMI International Corporation (2003). LaMotte Aquatic Macroinvertebrate Insect Identification Flashcards.
North Carolina State University (2005). Coleoptera. Available here.
Soil and Water Conservation Society of Metro Halifax (2004). Order Aquatic Coleoptera. Available here.
The Commonwealth Scientific and Industrial Research Organization (2010). Coleoptera: Beetle and Weevils. Availablehere
Tree of Life Web Project (2000). Coleoptera. Available here.
US Environmental Protection Agency (2009). Riffle Bettles. Available here

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