An insect is any one of a large class of arthopod animals found in most environments on earth. The many different species of insects have in common a division of the body into three parts1 - head, thorax and abdomen - six legs connecting to the thorax, compound eyes and one pair of antennae. Being arthopods they also possess an exoskeleton.
Internal arrangements are generally quite rudimentary - the body is filled with a (usually yellowish) fluid matrix called haemolymph (since it fulfills the functions of both blood and lymph in higher animals), a primitive CNS, a simple gut and a heart like organ which circulates the haemolymph. Breathing is through vents in the body known as spiracles which let on to simple ducts in contact with the haemolymph.