Insects: The greatest conquerors of all?
The characteristics of six legs and a body divided into three parts became numerically the most successful group of animals: the insects. Although ancestral insects probably climbed about the vegetation, one important ingredient for their success was the development of wings and the ability to fly. How wings evolved is unknown but it may have reflected attempts on insects to increase surface area and become more efficient at warming up their bodies so that they can become active (thermoregulation). Winged insects appeared some three hundred million years ago with animals resembling dragonflies. In the absence of early competition, early dragonflies radiated with some species developing enormous sizes (eg wingspan of 700 mm). Dragonflies have two pairs of wings with a simple up and down movement, and consequently cannot be folded back. Today's dragonflies have large compound eyes and catch smaller insects in flight, but are able to hunt only during the day. Consequently today's carnivorous dragonflies must have been preceded by herbivorous animals or carnivorous forms that prey on non-flying insects. Modern dragonflies probably evolved from primitive omnivorous or herbivorous insect forms such as cockroaches, grasshoppers, locusts or crickets.
Much larger dragonfly species existed in the distant past than occur on earth today. The largest, found as a fossil, is an extinct Protodonata named Meganeura monyi from the Permian period with a wingspan of 70-75 cm (27.5-29.5 in). This compares to 19 cm (7.5 in) for the largest modern species of odonates, the Hawaiian endemic dragonfly, Anax strenuus. The smallest modern species recorded is the libellulid dragonfly, Nannophya pygmaea from east Asia with a wingspan of only 20 mm, or about 3/4 of an inch.