In order to answer this question we must look at where insects fit in the animal kingdom. The animal kingdom is divided into several groups called phyla. An example of a phylum is the Chordata, which holds all the backboned animals. Insects belong to the phylum Arthropoda.

Insect - Hairy grub SEM image

Insect - Hairy grub SEM image

Image: Sue Lindsay
© Australian Museum

Insects are arthropods

Arthropods are characterised by having the following features:

  • a hard external skeleton (called a exoskeleton)
  • a segmented body
  • at least three pairs of jointed legs

The Arthropoda is divided into a number of classes. These include the:

Robber Fly
Robber Fly Image: Bruce Hulbert
© Bruce Hulbert

Insect are successful and important

The insects have proved to be the most successful arthropods. There are far more species in the class Insecta than in any other group of animals. These amazingly diverse animals have conquered all the environments on earth except for the frozen polar environments at the highest altitudes and in the immediate vicinity of active volcanoes.

Insects are the only invertebrates (animals without backbones) with wings. Much of their success results from their ability to fly and colonise new habitats. The study of insects is called entomology and entomologists are scientists who study insects.

Insects play a very important role in the web of life, in every environment. Some of their jobs include pollinating flowering plants, being a source of food for insectivorous animals and assisting in the decomposition of plants and animals.

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Insect classification

Modern insect classification divides the Insecta into 29 orders, many of which have common names. Some of the more common orders are:

  • Mantodea - praying mantids
  • Blattodea - cockroaches
  • Isoptera - termites
  • Siphonaptera - fleas
  • Odonata - dragonflies and damselflies
  • Dermaptera - earwigs
  • Diptera - flies
  • Lepidoptera - butterflies and moths
  • Orthoptera - grasshoppers, katydids, crickets
  • Coleoptera - beetles
  • Hymenoptera - wasps, bees, ants, sawflies

Insect Diagram

Insect Diagram

Image: Design Unit
© Australian Museum

The insect body

It is very difficult to provide a simple answer to the question: What external features characterise an insect? This is because the class Insecta is full of exceptions. It is not easy to produce a typical body plan for what most insects look like, but there are some very general features that most insects possess.

Insect features

  • The insect body is divided into three main parts, the head, thorax and abdomen.
  • Insects have no internal skeleton, instead they are covered in an external shell (exoskeleton) that protects their soft internal organs.
  • No insect has more than three pairs of legs, except for some immature forms such as caterpillars that have prolegs. These are appendages that serve the purpose of legs.
  • The typical insect mouth has a pair of lower jaws (maxillae) and upper jaws (mandibles) which are designed to bite. There are many variations to this structure, as many moths and butterflies have tubular sucking mouthparts, many bugs and other blood-sucking insects have sucking stabbing mouthparts and some adult insects simply don't have functional mouthparts.
  • Insects have one pair of antennae located on the head
  • Most insects have one or two pairs of wings although some insects such as lice, fleas, bristletails and silverfish are completely wingless.

Together these features can help us distinguish insects from other arthropods.

Insect evolution

Insects are an ancient group of animals. The first insects probably appeared before the Devonian period (400 - 360 million years ago) and by the Carboniferous period (360 - 285 million years ago) had taken to the air.

Adaptation to flight proved a highly successful strategy and during the Permian period (285 - 245 million years ago) insects achieved their greatest diversity. No other arthropod group has achieved flight. By the Permian, the basic physical structure of many of the modern orders of insects had evolved.

The more recently evolved Hymenoptera (ants, bees, wasps and sawflies) and Lepidoptera (butterflies and moths) appear as fossils in the Jurassic period (210 - 145 million years ago). The Mantodea (praying mantids) appeared in Eocene period in fossilised amber (60 - 35 million years ago).

Types of insects

The straight-winged insects, Order Orthoptera, and the skin-winged insects, Order Dermaptera.

The two-winged insects, Order Diptera; the toothed insects, Order Odonata; and the net-winged insects, Order Neuroptera.