An Introduction

Did you know that there are many more kinds of insects on earth than any other kind of living creature? It's hard to imagine, but 95% of all the animal species on the earth are insects! Millions of insects can exist in a single acre of land! Over one million species have been discovered by scientists, and they think that there might be ten times that many that haven't been named yet!

They are divided up into 32 orders, or groups of insects. The largest order is the beetles (Coleoptera) with 125 different families and around 500,000 different species. In fact, one out of every four animals on earth is a beetle. Scientists estimate that 10% of the animal biomass of the world is ants, and another 10% is termites. This means that 'social insects' probably make up an incredible 20% of the total animal biomass of this planet!

Insects eat more plants than all the other creatures on earth! They are also so important in the breakdown of plant and animal matter, that without them, we would have a world covered with dead plants and animals! In addition to all of this, insects are a major food source for many other animals.

Insects are incredibly adaptable creatures and have evolved to live successfully in most environments on earth, including deserts and even the Antarctic. The only place where insects are not commonly found is in the oceans. Insects have an amazing number of differences in size, shape, and behavior, but they all have 4 characteristics in common.

All insects must have:

  • three body parts - a head, thorax, and abdomen
  • six jointed legs
  • two antennae to sense the world around them
  • an exoskeleton (outside skeleton)

If all four of these things are not true, then the animal can't be called an insect! Spiders are not insects because they have eight legs and don't have three body parts. Centipedes and millipedes have way too many legs to be called insects! Most insects have one or two pairs of wings, but wings aren't necessary to be classified as an insect.

Scientists believe that insects are so successful because:

  • they have a protective shell or exoskeleton
  • they are small
  • most of them can fly.

Their small size and ability to fly helps them to escape from enemies and travel to new environments. Because they are small they need only small amounts of food and can live in very small cracks and spaces. Insects can also produce large numbers of offspring very quickly.

Insects are directly useful to humans by producing honey, silk, wax, and other products. They also are important as pollinators of crops, natural enemies of pests, scavengers, and food for other creatures. At the same time, insects are major pests of humans and domesticated animals because they destroy crops and carry diseases. Actually, less than one percent of insect species are pests, and only a few hundred of these are consistently a problem.

Insect Anatomy

Insects have a lightweight, but strong exterior (outside) skeleton called an exoskeleton. Their muscles and organs are on the inside. This multi-layered exoskeleton protects the insect from the environment and natural enemies. The exoskeleton also has many sense organs for sensing light, pressure, sound, temperature, wind, and smells. Sense organs may be located almost anywhere on the insect body, not just on the head.

Insects have three main body parts: head, thorax, and abdomen.

The head is used mainly for eating, sensing things, and gathering information . Insect mouthparts have evolved for chewing (beetles, caterpillars), piercing-sucking (aphids, bugs), sponging (flies), sucking (moths), rasping-sucking (thrips), cutting-sponging (biting flies), and chewing-lapping (wasps). That's a lot of ways to eat!

All insects have two antennae that they use to sense the world around them. Whether their antennae are short, long, thick or thin, insects use them to feel, smell, and even taste!

The thorax protects the body and gives support for the three pairs of jointed legs and, on many insects, for one or two pairs of wings. The legs may be adapted for running, grasping, digging, or swimming.

The abdomen contains the organs used for digestion and reproduction.

The inside of an insect's body has an open circulatory system. That means that its body fluids just sort of flow around inside the exoskeleton. It also has many breathing tubes, and a digestive system. It has a heart, a few blood vessels, and insect blood simply flows around inside the body cavity. Air enters the insect through a few openings in the exoskeleton called spiracles. From there oxygen gets to all areas of the insect's body through the breathing tubes, which go everywhere in the body. The insect "stomach", or digestive system, is long and tube-like, and is usually divided into three sections.

The nervous system sends messages from the sense organs (sight, smell, taste, hearing, and touch) to and from the brain. The brain is located in the head and processes information, but some information is also processed at nerve centers at different places in the body.

Insect Reproduction

Most species of insects have males and females that mate and reproduce sexually. Sometimes there aren't many males or they are only around at certain times of the year. When there aren't any males, females of some species may still reproduce! This is common, particularly among aphids.

In many species of wasps, unfertilized eggs become males, while fertilized eggs become females. In a few species, females produce only females.

Insects may reproduce by laying eggs, or in some species, the eggs hatch inside the female and are born a short time later. Sometimes in aphids, the eggs hatch inside the female and the young aphids remain inside the female for quite a while before birth.

Insect Growth and Development (Metamorphosis)

Insects usually go through four separate life stages: egg, larva or nymph, pupa, and adult. Eggs are laid one at a time or in masses, in or on plants, or even inside another insect! Eventually a larva or nymph emerges from the egg. There are usually several larval or nymphal stages, called instars. During each stage the nymph grows larger and molts, or sheds its outer skin before the next stage. They grow the most during the last one or two instars, or stages. All the growing happens during the larval or nymphal stages. The eggs, pupae, and adults don't grow in size.

The two types of metamorphosis typical of insects are incomplete metamorphosis (egg --> nymph --> adult) and complete metamorphosis(egg --> larva --> pupa --> adult).

With incomplete metamorphosis, the nymphal stages look like the adult except that they don't have wings, and the nymphs may be colored differently than the adults. Nymphs and adults usually live in the same kind of habitats. Incomplete metamorphosis is typical of true bugs and grasshoppers; complete metamorphosis is typical of beetles, flies, moths, and wasps. The young insects that go through complete metamorphosis do not look at all like the adults, they often live in different habitats, and feed on different things. Some moth and wasp larvae weave a silken shell (cocoon) to protect the pupa. In flies, the last larval skin becomes a puparium, a kind of hard shell, that protects the pupa.

Insects are cold-blooded, so the rate at which they grow and develop depends on the temperature of their environment. Cooler temperatures cause slow growth; higher temperatures speed up the growing process. If a season is hot, more generations, or life cycles, might happen than during a cool season.

Insect Classification and Identification

It is necessary to classify insects so that we can organize what we know about them and understand their relationships with other insects. For example, all members of a particular species will feed on similar foods, have similar developmental characteristics, and exist in similar environments. Most often, insect species are classified based on similarities in appearance (morphology). The flies, for example, can be distinguished and classified separately from all other winged insects because they have only one pair of wings.










Gypsy Moth


Monarch Butterfly