Little workers: Beetles of the Wet Tropics

Skyrail Nature Diary: July 2011

Beetles are the largest group of animals on the planet, with an estimated 300,000 species identified. The majority are less than half a millimetre long and are found in composting environments such as the Basket Fern (Drynaria rigidula: Polypodiaceae), where they help to break down and aerate the leaf litter.

Most insects have two pairs of wings on their thorax, however a distinct difference with a beetle’s wing structure is that its first pair of wings has been modified into a pair of shields to protect the remaining pair. The shields are held to the side during flight. The disadvantage to this arrangement is that it makes beetles clumsy in flight.

The life cycles of beetles involves what is known as complete metamorphosis. This means that both young and adult beetles are totally different from each other, interspersed by a pupa stage where the young, known as larvae, change into the adult, known as an imago. The reason for this arrangement is twofold. One, it removes competition between the adults and their larvae since the two have different needs. Two, it divides functions between the two generations. Larvae do most of the eating and all of the growing in the insect’s life. The imago can fly and spread the larvae out away from their parents’ birthplace and on to an appropriate food source.

Beetles are known to eat almost anything - there are beetles that eat oil! Some beetles have unique ways of defending themselves. Bombardier Beetles (Pheropsophus verticalis) spray their enemies with hot corrosive liquid at boiling point! They do this by separating two chemicals necessary for the production of the noxious fluid in the abdomens. When threatened they eject the two chemicals under high pressure toward the outside where they meet just outside the beetle’s abdomen. The reaction is virtually instant and highly unpleasant for the attacker.

Weevils are by far the largest group of beetles in the world, with an estimated 60,000 species. These often small beetles are characterized by a “beak” on the front of the thorax with two tiny antennae on either side. Being so amazingly diverse it’s unavoidable that they should affect humans one way or another. Several are serious pests for agricultural crops. They can also benefit humans by eradicating certain weed pests if used in a controlled environment.

Rhinoceros Beetles (Oryctes rhinoceros) are perhaps the most noticeable beetles in the Cairns region. This is partly because of their unusually large size. The males have horn-like protuberances on their thoraxes which they use when fighting other males over mating privileges. The males make a hissing sound when threatened. The females lack these horns and are generally smaller than the males. The grubs are also quite large and a common sight after heavy rain when they are forced out of their burrows.

Water Beetles of various kinds, are quite common in the Wet Tropics. The Whirligigs (Gyrinus sp.) are small, round, black beetles that prefer to stay on the surface of ponds and billabongs. They are quite gregarious and whirl around on the surface when disturbed. When many Whirligigs do this at the same time it confuses any potential predator. Swimmers, also known as Water Calves, are much larger, more colourful beetles that spend most of their time under water. They often have a yellow band running along the edges of their thorax and abdomen. Their last pair of legs is shaped as oars and is used for propulsion. Swimmers always bring a bubble of water with them. These impressive looking beetles are voracious predators of insects, prawns and small fish. They can deliver a painful bite when handled.