Insects are among the most abundant and diverse groups of organisms on Earth. They can be found everywhere, from pristine bushland to the concreted heart of a city’s CBD. Parks and gardens support hundreds of different species. Even the barest entertainment area will be populated with insects and leafy gardens have a veritable 'microcosm' of six-legged beasts.
Insects are an integral part of the ecology of natural, agricultural and urban ecosystems. The 'ecosystem services' provided by insects should not be underestimated. They play vital roles in the decomposition of organic matter, nutrient recycling and soil formation, and in the pollination and dispersal of plants. They also provide a food source for many other groups of animals. Only a very small percentage of insect species are troublesome or destructive pests.
An insect's body consists of three parts:
- The head is used for feeding and sensing the environment and has a pair of compound eyes and up to three simple eyes, one pair of antennae and a set of mouthparts which may be piercing, chewing or sucking types depending on the insect.
- The thorax is made up of three segments, each having a pair of legs. In most adult insects the last two segments of the thorax may each have a pair of wings. The thorax is filled with muscles which power the insect's legs and wings.
- The abdomen is the largest and softest of the three body parts. It houses the vital organs for digestion and reproduction. It can swell up to store food and, in females, the growing eggs before they are laid.
Adult insects have three pairs of legs and most have two pairs of wings, although some have only a single pair, or have lost their wings altogether. Only the most primitive insects, such as silverfish and their relatives, have never possessed wings as part of their evolutionary history.
Insects begin life as an egg, although a rare few give birth to live young. Insects can be divided into two groups on the basis of how the immature stages develop.
In one group, metamorphosis (the transformation to an adult insect) is gradual. The immature stages, called nymphs, closely resemble the adults except that they lack wings and reproductive structures. They generally live in the same sorts of places and eat the same food as the adults.
In the second group, those with abrupt metamorphosis, the immature stages are grub-like larvae. To make the transformation from larva to adult they must undergo an intermediate stage, the pupa, which does not feed. The larval stages of this second group of insects usually differ from the adults in their habitat and food requirements.
Insects are classified into about 25 Orders, many of which correspond to familiar groups such as:
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