The internal organs of a tiger beetle Insects belong to a group of invertebrates (animals without backbones) called arthropods. Instead of having an internal skeleton, arthropods have a hard outer skeleton. This is made of a light, strong material called chitin that supports and protects their soft inner parts. All insects have six legs and a body divided into three sections or segments: the head, thorax and abdomen. Insects have a pair of long sense receptors on their heads, called antennae. Flying insects have either one or two pairs of wings. Many insects are plant-eaters, but others are carnivorous, feeding on other insects or small animals. An insect's mouth shape reflects its diet: some have tube-like mouths for sucking up fluids; others have jaws for chewing up their food.
The body parts of an insect (a parasitic wasp)Insect skeletons are found on the outside of their bodies—unlike ours, which are on the inside. Called exoskeletons, they form a protective armour around the soft parts inside. The insect breathes through tiny holes, called spiracles, in the sides of its body. The hard covering of the legs is jointed to enable the insect to move. Tiny bristles on the legs and body can feel vibrations caused by sound or movement. The insect's foot is called the tarsus. The claw is used for gripping surfaces.
Like its legs, an insect’s antennae are made up of jointed sections. They are highly sensitive to smells, flavours and touch. The insect uses them to detect its food and to sense objects. Ants, for example, use their antennae to pass on “messages” to each other.
Insects have two kinds of eyes: simple eyes and compound eyes. Simple eyes work by detecting changes in light. Compound eyes are made up of thousands of tiny units, each of which sends an image to the insect’s brain. This means that insects have good all-round vision, and are able to sense movements and judge distances very accurately. This is important when flying at high speed.
After mating, a female insect lays eggs. The young are usually left to hatch and fend for themselves. Some insects, such as bugs, mantises, dragonflies and grasshoppers, look like tiny adults when they hatch. Known as nymphs, they change gradually into adults by shedding their exoskeleton several times, after a new one has grown underneath. This is called moulting. The exoskeleton does not stretch and so would otherwise prevent the insect from growing. The final moult results in the adult insect. Each stages of development in the nymph is called an instar. The change from nymph to adult is called incomplete metamorphosis: there is no pupa stage.
Butterflies, beetles, fleas, ants and bees undergo complete metamorphosis. They hatch out from eggs as larvae: grubs or caterpillars, shedding their skin several times as they grow. They then develop a protective coating, called a cocoon, inside which they pupate: change into their adult form. The adults that emerge from their cocoons, known as imagos, are fully formed with body segments and wings. They are now able to reproduce and lay eggs themselves.
Many insects are masters of disguise, using camouflage to hide from predators or to stay out of sight while they hunt. Some species are covered in mossy growths that make their disguise even more effective. Stick and leaf insects live in trees, feeding on vegetation. They are coloured green or brown, and look exactly like twigs or leaves. Some stick insects have the ability to change colour as their surroundings change around them. Many even sway from side to side if the plant is moved by the breeze.
Dead leaf mantis, camouflaged among dead leaves
This hoverfly’s markings make it look like a bee
Some insects make no attempt to blend into the background. Instead they have bright colours. These warn predators that they are dangerous to eat, and so deter them from attacking them. Reds, yellows and blacks warn predators that they sting or are poisonous. Some harmless insects use these colours to pretend to be dangerous.Elegant grasshopper
Consultant: Chris Jarvis