Wikijunior:Bugs/Insects Intro

Introduction
The first signs of life on Earth appeared in the oceans around 3,500 million years ago. Arthropods were the first creatures to emerge from the oceans to live on land about 420 million years ago.

Smaller groups of arthropods are called classes. Insects form the Class Insecta, the most successful group of arthropods. Classes are divided into groups called orders. For example, butterflies belong to the Order Lepidoptera and beetles belong to the Order Coleoptera.

Orders are made up of families. Ladybug beetles are in the Family Coccinellidae. Families are made up of groups called genera which are divided into even smaller groups called species. When people refer to a specific kind of insect, they are usually thinking of a species, such as the Monarch Butterfly.

What do they look like?
An insect’s body is has three main parts: the head, thorax, and abdomen. Eyes, mouthparts, and antennae are located on the head. Legs and often wings are on the thorax. Females have an ovipositor at the tip of the abdomen for laying eggs. Some insects have a pair of sensory organs called cerci at the tip of the abdomen. A hard outer covering called an exoskeleton protects the soft inner parts.

Most adult winged insects have two pairs of wings. Flies have only one pair of wings for flying and a pair of club-shaped halteres for balance. The hard, armor-like fore wings of beetles cover and protect the hind wings used for flying. Experts who study insects (entomologists) use the various arrangements of veins in the wings for identification.

How do they defend themselves?
Insects were the first animals to take to the air. Their ability to fly helped them evade enemies more easily and find food and mates more efficiently. Most insects have the advantage of being able to fold their wings back along the body. This gives them a wider range of small places to hide from predators. Today, the only insects that cannot fold back their wings are dragon flies and mayflies.

Unlike birds, insects need to warm up their flight muscles. They do this by vibrating their wings or basking in the sun before takeoff.

What stages of metamorphosis do they go through?
Because the exoskeleton is hard and cannot stretch, insects must shed (molt) several times as they grow. Many insects have four growth stages—egg, larva, pupa, and adult. The larva is an active feeding stage. The pupa is an inactive resting stage. During the pupal stage, the larva is transformed into an adult. This kind of development is called complete metamorphosis. Butterflies, flies, beetles, and bees undergo complete metamorphosis. Scientists believe the pupal stage evolved millions of years ago from a need to survive long periods of cold. Today 85% percent of all living insect species develop through complete metamorphosis.

Other insects have only three stages—egg, larva, and adult, with no pupal stage. This is known as simple or incomplete metamorphosis. Grasshoppers undergo simple metamorphosis. Their young are called nymphs and often resemble small adults. Dragonflies and mayflies are aquatic insects. Their offspring are called naiads or water nymphs.

How do insects affect people?
Insects are seen as pests by most people. Some destroy crops and carry diseases that harm animals and human beings. Others are beneficial and most are harmless. Honey, silk, waxes, oils, natural medicines, and dyes are among the many useful products from insects. Without pollinators, such as bees, many plants would be unable to produce fruit and seeds and so would die out. Predacious insects, such as lady bugs, help control harmful aphid populations. In some countries, crickets, grasshoppers, and caterpillars provide nutritious food for humans.