Class Insecta Insects are by far the largest group of organisms on earth, and probably include 1 to 10 million species. Three separate body regions and six thoracic legs are the major diagnostic features of insects. Insects possess all the major characteristics of arthropods, but their ability to fly is probably the key to their great success. Although other groups of organisms can fly, insects were the first fliers. There are many advantages to flying, such as avoiding predators.
Most insects have two pairs of wings.
The success of insects on land is further enhanced by an efficient respiratory system of tubes called tracheae that conduct air throughout the body. Insects also have highly modified mouthparts. For example, mouthparts of grasshoppers grind coarse plant tissue, whereas mosquito mouthparts puncture and suck fluid from animals. Subphylum Chelicerata Chelicerates such as spiders and scorpions are arthropods with the appendages of their most anterior segment modified into feeding structures called chelicerae. The second pair of appendages are pedipalps and are modified for capturing prey, sensing the environment, or copulating.