Cicadas are classified in the order Hemiptera, then the Homoptera, although the Homoptera is considered an order in its own rights, in some publications it is designated as a suborder of the Hemiptera. They are then members of the superfamily Cicadoidea, the the Family Cicadidae, or in the case of two unusual Australia species Family Tettigarctidae. The order Hemiptera include all insects with piercing and sucking mouth-parts, such as aphids and scale insects.

Cicadas are mainly found in warm-temperate to tropical habitats. There are more than 200 species of cicadas in Australia, most of which belong to the one large family, the Cicadidae. There are about 1,500 species world-wide.

Singing
The song of the cicada is only produced by the males as a mating call. Each species has its own distinctive call and will only attract females of its own kind, even though similar species may co-exist. Some large species such as the Green Grocer, Yellow Monday, and Double Drummer produce a noise in excess of 120 decibels at close range, this is approaching the pain threshold of the human ear. In contrast, some smaller species have songs so high in pitch, that the noise is beyond the range of human hearing.

How they produce their songs is still being researched today, but the organs that produce the sound are the tymbals, a pair of ribbed membranes at the base of the abdomen. Contracting the internal tymbal muscles causes the tymbals to buckle inwards and produce a pulse of sound. By relaxing these muscles, the tymbals pop back to their original position. In some species of cicadas, a pulse of sound is produced as each rib buckles.
 

Both male and female cicadas have organs for hearing. A pair of large, mirror-like membranes, the tympana, receive the sound. The tympana are connected to an auditory organ by a short tendon. When the male sings, it crease it's tympana so that it won't be deafened by its own noise.

Many species of cicada sing during the warmth of the day. This noise actually repels birds, probably because the noise is painful to the birds' ears and interferes with their normal communication. The males of many cicada species group, such as the Green Grocer, Yellow Monday and Double Drummer, group together when singing. This increases the volume of noise, makes it harder to locate where the sound is coming from and reduces the chances of bird predation. Other species of cicada only sing at dusk. These species tend to be weak fliers, such as the Bladder Cicada. They gain some measure of protection from birds by confining their activity to dusk.

In addition to the calling or mating song, many species possess a distress song. This is usually a broken and erratic noise emitted when an individual is captured. Some species also have a courtship song, a quiet call that is sung only after a female has been attracted nearby using the calling song.

Feeding
Cicadas feed on a huge range of plants, including eucalypts and grasses. They feed by piercing the surface of plants with their mouth stylets. The sap is then sucked up through a tube formed by the concave surfaces of two of the stylets. They are not harmful to trees, although in some cases the growth of the tree may slow, due to the amount of sap consumed. Cicadas do not bit, even when handled, although their claws may feel sharp as they cling to the skin.

Birds, bats, spiders, ants, mantids and tree crickets all prey on cicadas. They also provide food for the larva of the Cicada-killer Wasps and are also parasitised by the larvae of Feather-horned Beetles (family Rhipiceridae).

Lifecyle
It is thought that the nymphs of the larger, common Australian species of cicada may live underground for around 6-7 years. This may explain why adult cicadas are more abundant during some seasons than others, with peaks occurring every few years.

In contrast to its nymph stage, the life of the adult cicadas is very short, lasting a matter of weeks. Once the adult cicadas have mated, the female cicada lays its eggs by piercing the plant stems and inserting the eggs into the slits. The eggs hatch into small wingless cicadas known as nymphs. They fall to the ground and burrow below the surface, where they live on the sap from plant roots. Over the period of several years, the nymphs grow, shedding their skin at intervals.

Once the nymph reaches full size, it will dig its way to the surface with specially adapted front legs. It usually surfaces as night falls in the late spring or early summer. Then, climbing the nearest tree trunk or other fixture, it will shed its skin for the last time, emerging as a fully-winged adult cicada.
 

Source: Various including Australian Museum Online: Fact Sheets Cicadas,
The Wonderful World of Insects: The Singing Cicadas