Beetles!

Skyrail Nature Diary: April 2013


This month we'll be looking at the beetles of the Wet Tropics. Beetles are the largest group of animals on the planet with an estimated 300,000 species identified so far. The majority are amazingly less than half a millimetre long. In Barron Gorge quite a few live in the compost of Basket Ferns where they help to break down and aerate the leaf litter.

So what exactly is a beetle? The Latin name Coleoptera gives a valuable clue. It means shield wing. Most insects have two pair of wings on their thorax. In beetles the first pair has been modified into a pair of shields to protect the remaining pair of wings. The shields are held to the side during flight. The disadvantage to this arrangement is that it makes beetles among the clumsiest of fliers around. Beetles have pretty basic jaws with little or no modifications of the basic type found in all insects. Beetles often have very visible mandibles, the sideways jaws used for biting and chewing.

The life cycles of beetles involves what is known as complete metamorphosis. This means that young and adult 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 onto 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 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 biggest 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 on agricultural crops. Intriguingly one particular weevil pest from California, the Boll Weevil, has a statue erected in its honour. The reason for this particular lunacy has to do with the Boll Weevil's effect on cotton crops which were once widespread in California. The weevil devastated one year's harvest and the local farmers decided to change crops and grow oranges instead. The oranges thrived and now the farmers make more money than when they were growing cotton, hence the bizarre statue. Weevils can benefit us as well. Nobody at Skyrail would have failed to notice the recent infestation of the Salvinia weed in Lake Smithfield. Well, it so happens that there is a particular weevil that specializes in feeding on Salvinia to the exclusion of all else. When conditions are right these weevils can completely eradicate Salvinia in a given area. We are now making use of this weevil in the lake - fingers crossed. Weevils are also surprisingly important pollinators of primitive plants, especially cycads and the most primitive flowers in the world.

Rhinoceros Beetles are perhaps the most noticeable beetles in the Cairns region. This is partly because of their unusually large size for an insect. The males have horn like protuberances on their thoraxes 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 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, slightly more colourful beetles that spend most of their time under water. They often have a yellow band running along the edges of the 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.