They may have well developed toothed mandibles or hooks for locking onto prey, large eyes for nocturnal vision, venom glands, fast running ability to chase down prey and some even secrete a sticky glue to stick insects in a trap.
Ladybird (Rodolia cardinalis or Coccinella repanda)
The red and black Ladybird beetle is one of the most universally recognised beneficial insects, although not all of the 300 species of Australian Labybirds are useful, twenty-six spotted ladybird (Henosepilachna vigintiocto-punctata) for instance is a noted pest of potatoes and other crops in the north.
The red and black Rodolia cardinalis is a ferocious and greedy predator of soft-bodied insects in both the adult and larvae stages. It saved the Californian citrus industry from the introduced (Australian) pest Icerya purchasi (cottony cushion scale) and became a icon of biological control in the process. Yellow or orange eggs can be found on plants in groups of five to thirty five, usually near a food source such as an established aphid colony.
The Rodolia population was decimated in the 1940s in California by use of DDT and provides a good example of how pesticides may work against the best interests of the farmer. Cryptolaemus or the black ladybird is another lady beetle which is a very voracious destroyer of mealybug and other sift-bodied insects.
Ground Beetles (Carabids)
Ground Beetles are common in Australia, except in Tasmania. They a also known as carabids (they belong to the Carabidae family). This beetle lives on the ground and is often found around the compost heap or sheltering under logs and rocks. It is a scavenger and a hunter in both the adult and larvae stages, although some will supplement their diet with organic matter.
I have found ground beetles eating slug and snail eggs, adult snails, larvae of other insects and adult moths but it is likely that they will eat a very much larger range of prey, including larger animals such as tadpoles, frogs and small fish, especially where the large prey have been weakened or injured, and are easy to pull down.
The female lays eggs in a brood burrow and stays guard over them for weeks until they hatch, a rare trait in the beetle family. If provoked the adults can squirt a nasty-smelling defensive chemical from a gland on the tip of the abdomen.
Carabids are beneficial in the garden because they attack a wide range of other insects, including many that are generally regarded as pests.
Rove beetles (Staphylinidae family) look almost unlike a beetle, being very long in the body with a short elytra and an exposed, flexible abdomen. They are also ground dwelling and sometimes look a little like an earwig and they seem to slither along dragging this long body behind them.
They often have a bright red or orange head or under-abdomen and they do have full size wings folded under the short elytra. which can be quickly unfolded for rapid escape. However if approached the rove beetle will often arch its abdomen and disperse a strong smelling chemical deterrent. This chemical can cause blistering on the skin of sensitive people. Most rove beetles are predatory on other insects.
They are called rove beets because they are great wanderers - and undertake long roaming journeys to search for prey or for mates. They will eat carrion, invertebrates, dung, decomposing vegetable matter or fungi. Some species live in any nests or termite mounds.
Soldier beetle, Chauliognathus lugubris. Soldier beetles are brightly coloured predators, up to 18 mm in length.
They lack the hard elytra of most beetles, and are found swarming on warm summer evenings or feeding on pollen and nectar.
They also consume other small insects which visit the blooms.
Larvae live on the soil surface and are also predatory.
Scarab Beetles (common name: Dung Beetle)
Scarab beetles (family Scarabaeidae) are burrowers. They do not predate on other insects, but the larvae feed mainly on decaying vegetable matter, roots of plants and dung - leading to their common name of 'dung beetle'.
They are beneficial because the adults remove and bury large quantities of dung, which they use to line the brood tunnels for the developing larvae.
The dung removal plays an important part in nutrient recycling and removes large quantities of dung which would otherwise stay on the surface and reduce the area available for production of pasture plants.