An insect is an arthropod with three pairs of legs and a body comprising three segments. Insects are the only members of the arthropod phylum with wings. The phylum Arthropoda also includes arachnids, crustaceans, centipedes and millipedes.
The insect body includes a head, thorax and abdomen as well as six legs. The head contains the animal's sensory organs, such as eyes and antennae, while the thorax is the connection point for all three pairs of legs and the wings, if they are present. Insects have no internal skeleton; they rely, instead, upon a hard exoskeleton for support and protection.
Insects breathe via passive respiration. This means oxygen is not actively brought into the body using lungs. Instead, air enters the body through spiracles, or openings in the exoskeleton. From there, oxygen moves throughout the body by way of a system of tracheal tubes, eventually diffusing into the necessary organs.
The open circulatory system of an insect does not contain veins or arteries but consists of a single dorsal tube with many openings to the rest of the body. The dorsal tube circulates a fluid called hemolymph throughout the body cavity. While vertebrate blood is red because of its iron content, hemolymph appears clear or bluish because of the presence of copper instead of iron.