Insects are invertebrates (animals without a backbone) that have a hard chitinous exoskeleton, three body parts (head, thorax and abdomen), and three pairs of jointed legs. They are the only invertebrates in which the adult stages of most species have wings. Their dominance of most terrestrial and freshwater environments in terms of number of species and in numbers of individuals is indicative of their ecological importance. However, many people consider insects as pests: flies, mosquitoes and ants disrupt outdoor activities, garden pests devour both native and exotic plants, and wasps and bull ants can inflict painful stings, yet these represent only a tiny fraction of insect species.
The high diversity of insects in Melbourne is due to the range of available habitats that include grasslands, woodlands, forests, swamps, billabongs, and rivers and creeks, especially the Yarra River. There are no known marine insects from Port Phillip Bay. This diversity is increased by structural complexity within each habitat. Some live on plants (on or inside leaves, wood, bark, flowers, fruits, seeds); each plant species often has its own specific suite of insect species. Others live in the leaf and woody debris on the ground or in the soil, under rocks and in rotting logs. Aquatic species live on the substrate (benthic), float freely (planktonic) or live on aquatic plants.
Insects belong to a wide range of trophic levels that include feeding on plants (herbivores), predation (carnivores), parasites (either internal or external), pollinators, scavengers, fruit and seed feeders, and the decomposers of organic matter such as leaves, wood, dung and carrion. They carry on these functions quietly and often unnoticed, but without them the ecological processes that maintain life on earth would stop.
Before European settlement the insects may have been in some state of dynamic equilibrium with their environments. No doubt insects that are capable of inflicting a painful sting or bite would have annoyed local Koories, who in turn would have harvested the nutritious beetle grubs that are found in older logs or in the trunks of trees. European settlement altered the composition of the insect fauna by destroying the habitats for some species, providing better breeding conditions for other species, and by introducing pest insects from overseas. The latter two activities have had profound effects on the way Melburnians live. The production of organic waste which was left to rot in rubbish heaps or taken to open tips resulted in massive increases in fly numbers. The dung of introduced stock animals such as cattle and horses also resulted in large numbers of blowflies because the small native dung beetles were only used to dealing with the smaller-sized dung pellets of native mammals. The fly situation has abated dramatically because of better waste disposal practices (such as regularly covering open tips with soil), and the introduction of exotic dung beetles has resulted in the more rapid breakdown of cattle and horse dung.
Other native species that benefited from European settlement include timber-destroying termites, which moved from native trees to structural timber in buildings. This resulted in the heavy use of insecticides in many parts of Melbourne when new buildings were established; however, physical barriers to prevent colonisation by termites are now available. The small black thrips that normally are found on the flowers of eucalypts occur on garden plants and are also attracted to washing left hanging out to dry. One insect that is very popular because of the sounds it makes in summer is the greengrocer cicada. This normally feeds on eucalypt roots but has readily adapted to many different introduced trees, especially species that are used as street trees.
There are many introduced insects. Some introductions were accidental (for example, European wasp, American and German cockroaches, and the Cabbage White butterfly), while others were deliberate (for example, European honey bee and various biological control agents for pest plants or insects). There are few dangerous insects in Melbourne. There are some species that can inject venom to which the victim may be very sensitive or allergic (such as the introduced European honey bee and the European wasp, some native wasps and bulldog ants).
While these 'pest' insect species can occur in very large numbers, the number of species involved is actually relatively small. Yet this small number of species has resulted in many cultural icons: the great Australian wave (to shoo flies away from the face), the fly swat, the pesticide sprayer (in all forms), fly papers, light traps that lead insects to electrocute themselves (which have been shown to attract more insects rather than reduce numbers), and various water traps of differing degrees of ingenuity and complexity.
Unfortunately information on the numbers of species, their original distributions and changes in distributions for the insects of Melbourne, is in most cases lacking. Melbourne has undergone massive environmental change since European settlement, and the number of known threatened species of insects is very small. This may reflect lack of information rather than an accurate assessment of the conservation status of the insect fauna. Among the known threatened insects is the Eltham Copper butterfly, which has declined through increasing urban development.