Insects are invertebrates, which means that they are animals without a backbone. As a class they have what is called an exo-skelleton. This is a skelleton on the outside of the body. It serves to support the organs and tissues and also as a protection for the creature, a kind of armor. One of the problems with having an exo-skelleton is that they generally do not grow with the body, and as insects grow they must slough off their exo-skelleton periodically until they reach adulthood.
Another characteristic of insects is that they all have six legs. This means that eight legged creatures like tarantulas and scorpions are not insects. The legs may be used for digging, holding, and balancing, but their primary use is locomotion. Insects walk by planting three of their feet (two on one side and one on the other) then using the other three feet to step forward (one on one side and two on the other), alternating each foot between planting and stepping.
Insects come with a head, thorax and abdomen. The head usually has two compound eyes and up to three ocelli (which are simple eyes). From the head will protrude a pair of antennae which are used for smelling or sensing pheromones in the air. The head will also sport a mouth or gullet used for the intake of food. Appendages for chewing, sucking, etcetera, are present in various forms depending on the food source of the insect. Mosquitoes, for example, have very elongated mouth parts that allow for the insertion of the mouth beneath the skin of their victims.
Many, but not all, insects have wings. They vary in speed and maneuverability depending upon their wing structure, musculature and nervous system. Some insects spring into the air with their legs before their wings go into action. Flying beetles often have a shell covering that protects their wings, which is extended when the creature takes flight. Butterflies are decked out with spectacular wings that are sometimes used for gliding. Some butterflies are bitter or poisonous to predators. These display their markings quite openly to warn off those who would consume them.
Insects may fall into one of two types of life-cycles. Insects such as the silverfish are hatched from eggs looking like little adults. As they grow they molt one or more times and gain the capacity for reproduction. Finally, they lay eggs and begin the process again. Higher orders of insects such as the hummingbird moth have a more complicated cycle that begins with the creature hatching from an egg into a larval stage. In butterflies and moths this is the caterpillar stage. They tend to eat a lot and finally create a cacoon or some other protective cover and enter the pupa stage where their basic components are broken down and reordered. The protective covering is shed and the reproductive capable adult (often capable of flight) emerges to mate and ultimately lay eggs for the next generation.
Insects have many specialized activities regarding food sources and reproduction. Some like to live in houses where they can get scraps left behind by humans. Others eat dung or decaying carcasses. Some prey on other insects. Bees get their food from the nectar of flowers and at the same time further the reproduction of the flowering plant. Their highly social behavior helps them to be efficient in this activity. Amazingly, bees can communicate the location of food sources to each other. Insects can be both desireable and undesireable. In the garden beneficial insects include the lacewing, lady beetles, and praying mantids.
Insects are one of the most successful classes within the animal kingdom. Well over a million different kinds of insects have been identified and catalogued. They have been around for many millions of years and will probably be around for many million more. Take a look at your own back yard and you will find ant-hills, bee-hives, mosquitoes, gnats, flies, fleas, cicadas, aphids, butterflies, and more in abundance. It has been estimated that for every human on earth there are at least two million insects.
People who study insects are called entomologists.
Interesting Link: Insects that are Garden Pests