There are around 24,000 known insect species in Britain, from ‘Featherwing’ beetles under 1mm long, to our largest resident moth, the Privet Hawk-moth, with a wingspan reaching 12cm.  So, what do they all have in common?

The simple answer is that all insect species possess, (in the adult stage), three pairs of jointed legs and three main body parts - head, thorax and abdomen. Examples of insects include beetles, bugs, bees, butterflies, moths, flies, grasshoppers, crickets, wasps, dragonflies, ants, caddisflies, earwigs, and lacewings, but just to confuse things, there are also some tiny creatures with six legs that are not classed as insects.

Insects are invertebrates – they lack a vertebral column, or backbone – but not all invertebrates are insects.  Invertebrates commonly mistaken for insects include spiders, harvestmen, woodlice, centipedes and millipedes.  Wildlife can sometimes be very confusing!

On the outside

Instead of an internal skeleton, insects have an outer casing called an exoskeleton, or cuticle. This contains a substance called chitin, a complex carbohydrate molecule comparable to the cellulose found in plants. The cuticle acts as a waterproof barrier to prevent internal body fluids from drying out, it forms an anchor for muscles, and helps guard against invasive microorganisms such as bacteria and fungi.

The thickness, rigidity and function of the cuticle varies according to the insect species, the stage of development, and the body region.  The tough outer armour of a shieldbug or beetle, the fine hairs of a bumblebee, the soft body of a caterpillar and colourful scales of a moth, are basically all made from the same material.

Wings

The thorax is the middle section to which the six legs and wings (if present) are attached. Insects with wings have either one or two pairs. Flies have one pair for flying plus two small paddle-shaped structures, called halteres, which are small, modified wings to help stabilise them during flight. Bumblebees have two pairs of wings linked together by rows of microscopic hooks.  Beetles have four wings, but the hardened or leathery forewings (elytra) cover the delicate membranous hind wings when the insect is at rest. Not all insects with wings can fly though. Some species of beetle rarely fly, the females of various bug species have much reduced wings that barely function, and several species of bush cricket are flightless.

On the inside

Insects have complex digestive, respiratory, reproductive, circulatory, and nervous systems. The heart pumps a fluid called haemolymph, the equivalent of blood, but rather than a closed system of veins and arteries this fluid flows freely throughout the body, lubricating tissues and cells, transporting nutrients and minerals, transmitting hormones, and carrying away waste products. Haemolymph does not carry oxygen, as insects breathe through small openings called spiracles, situated in the thorax and abdomen.  Spiracles can be opened to let in air and closed to prevent water loss, or in the case of aquatic insects, to stop water from entering.

Head

The head accommodates the brain and many other organs. Antennae enable the insect to detect smells, sounds and movement, to sense hot and cold, and explore the physical environment. Many species have two types of eye, both simple and compound. A compound eye comprises thousands of minuscule lenses, providing the wide field of vision needed for flying, hunting for food or avoiding predators; some also have small shiny bumps on the head, called ocelli, consisting of single lenses that detect light and dark.  

The structure of mouthparts depends on the species, the stage of development, lifecycle, and diet.  Carnivores such as dragonflies and damselflies have powerful cutting jaws to dismember other insects including flies and wasps. Species of ‘social’ wasp, including the Common Wasp, consume only nectar and sugars as adults, but they hunt and break up caterpillars and flies into smaller parts to feed their developing larvae. Adult butterflies, bugs, and many species of moth have a flexible feeding tube (proboscis) through which they suck nutritious fluids, and flies have an assembly of mouthparts that are designed to suck or pierce, rather than bite, as they are all liquid feeders. 

Depending on the species, the structures of the mouth may also be used for digging into soil, tunneling into wood, for grooming, moving or breaking through obstacles, defending against predators, and collecting and positioning nesting materials.

Insects are extremely complex animals that, no matter what shape or size, have evolved to fit perfectly into their ecological niche and keep our environment healthy.