are a class within the arthropods that have a chitin as exoskeleton, a three-part body (head, thorax, and abdomen), three pairs of jointed legs, compound eyes, and two antennae. They are among the most diverse group of animals on the planet and include more than a million described species and represent more than half of all known living organisms.
The number of extant species is estimated at between six and ten million, and potentially represent over 90% of the differing metazoan life forms on Earth. Insects may be found in nearly all environments, although only a small number of species occur in the oceans, a habitat dominated by another arthropod group, the crustaceans.
Insects are arthropods with the segments of the body fused to form 3 distinct body regions: head thorax and abdomen. The head bears the eyes, one pair of antennae and the mouthparts. The thorax bears 3 pairs of legs and in most cases one or two pairs of wings.
Insects develop from birth to maturity in stages called life cycles. Unlike humans, insects have a hard outer shell called an exoskeleton. As they grow, they have to shed the tough outer skin. Depending on the type of insect, this will occur a number of times throughout the life cycle until they reach adulthood.
Most insects begin life as an egg. When an insect has developed fully, the insect inside of the egg will break out of the shell by chewing the covering or expanding and contracting its muscles to break the shell. This usually happens seven to 10 days after the egg is laid. These insects then progress through the life cycle stages, often ending up looking far different than they did at birth.
The nymph stage is the second part of the simple development life cycle. It is called simple because these insects are born looking just as they do as adults, only smaller. Molting is still a part of this process. Insects who undergo simple development include grasshoppers and cockroaches. Winged insects that are part of this group, like dragonflies, do not undergo the pupa stage in which most winged insects develop their wings.
The larvae stage is the second part of the complex developmental life cycle. Insects that undergo a dramatic transformation from birth until they reach adulthood develop partially during the larvae process. They always appears like worms or small caterpillars. The shedding of the exoskeleton during the larvae stage occurs from three to six times, with the insects growing larger each time.
Winged insects that undergo the larvae stage, progress to the pupae stage for final development into adulthood. Only small, gradual changes occur during the larvae stage, but the pupae stage involves major changes. Because of this, it is often known as metamorphosis. The pupae stage involves the insects wrapping themselves into cocoons where they develop slowly over time. When the insects emerge from the cocoons, they look completely differently from what they looked like during the larvae stages, and the pupae are complete.
After the nymph stage (for insects with simple life cycles), larvae and pupae stage (for insects with complex life cycles), the final stage of the life cycle is adulthood. During adulthood, the insects are fully developed. Ironically, the developmental stages of the life cycle sometimes last longer than adulthood itself. For instance, cicadas spend 17 years in the larvae and pupae phases, and just a few weeks as adults.