The insects are Arthropods. Other members of the Arthropoda that I have discussed in past articles are the spiders (see my article, "Spiders: Our Eight Legged Friends") and horseshoe crabs (see my article, "Horseshoe Crabs").
Spiders have two body parts, the cephalothorax ("head thorax") and the abdomen. Insects, however, have three body parts: the head, thorax, and abdomen. Insects have six legs, which are all found on the thorax. If you are looking at a winged insect, the wings are also found on the thorax.
The insects first appear as fossils in Devonian rocks (395 million years ago). The earliest known insect is a spring-tail, a flightless insect, it was found in a fossilized peat bog called the Phynie Chert in Scotland. Flying insects did not appear until the Middle Carboniferous (about 320 million years ago). Development of flight in insects was probably similar to that seen in the vertebrate animals, with the wings probably only making simple vertical flapping movements at first. (See my article, "The Development of Flight," for more information.)
In the Upper Carboniferous Period (about 300 million years ago), insects developed more advanced wings that could be folded back when the insect was at rest. Dragonflies and other primitive groups of insects cannot fold their wings back along their bodies when they are at rest. Instead, they hold them perpendicular from their bodies at all times.
During the Carboniferous, the only predators that could threaten the insects were primitive terrestrial vertebrate animals (reptiles and amphibians), and other Arthropods. This lack of predation allowed some insects to reach enormous sizes. Meganeura, a gigantic dragonfly, had a wing span of 70 cm (28 inches)!
The development of insects was dependent on the development of the plant life. During the Mesozoic Era (230 to 65 million years ago), most of our modern groups of insects were already around. When flowering plants, or Angiosperms, appeared in the Middle Cretaceous Period (about 100 million years ago), insects could really proliferate. Insects that served as pollinators, such as butterflies and bees, became common at this time. (See my article, "Paleobotany II," for more information about the interrelationship of insects and plants.
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