What is an insect?

Introduction
The Insect Body
Insect Evolution

Cockroach. Photo: D Britton

Cat Flea. Electron micrograph: S Lindsay

Dragonfly. Photo: D Britton

Moth. Photo: D Britton

Introduction

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.

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:

Crustacea (crabs, crayfish, prawns)
Arachnida (spiders, mites, scorpions)
Myriapoda (millipedes & centipedes)
Insecta (insects)

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 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.

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

The Insect Body

Beetle

Earwig and Bug

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).

What is an insect?

Introduction

The Insect Body

Insect Evolution

Insects navigation

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