Insects are the most numerous, diverse and successful group of animals in the history of the planet. They are found in almost every environment, and range from the minute (less than a millimeter long for the feather-winged beetle) to the large (more than 15 cm for the South American longhorn beetle). But the longhorn is nothing compared to the monster insects that lived in the Paleozoic era, which ended about 250 million years ago.
One species of fossil dragonfly, Meganeura monyi, had a wingspan of almost 1 meter. Toward the end of the Paleozoic there was a jump in the levels of oxygen in the atmosphere, and this, some scientists believe, allowed insects to reach giant proportions.
This is how they did it. Rather than a fluid "blood" that carries oxygen to the tissues, as in vertebrates, insects take in oxygen by a process known as diffusion. Their bodies are punctured with small holes called spiracles. Air diffuses through the holes, down narrow tubes called trachea and directly on to the muscles, cells and tissues that need oxygen.