Fact about dragonfly
A dragonfly's brain invests 80% of its ability in controlling sight, and processing of images the insect sees.
Dragonflies have been known to buzz around the Earth for about 300 millions years. These insects dwell in almost all continents, except Antarctica. Most dragonfly species are found in tropical regions of the world. Of the 5900 species of dragonflies found of the planet, about 450 inhabit the United States.
Dragonflies and Damselflies
The Megaloprepus caerulatus, native to Costa Rica is believed to be the largest dragonfly species with wingspan of 7½ inches, in the world. It is, however, a damselfly. (The term 'dragonfly' is most often used for species belonging to the Odonata order. In the true sense though, only those species belonging to suborder Epiprocta are 'dragonflies', while those belonging to Zygoptera suborder are 'damselflies'.) So the largest dragonfly species by wingspan are the Tetracanthagyna plagiata females.
Diet and Eating Habits
Dragonflies are classified as strict carnivores. They usually feed on mosquitoes, gnats, midges, ants, termites, bees, butterflies, and flies. They also have larvae, tadpoles, and even tiny fish in their menu. Dragonfly nymphs (the first stage after hatching) love to feed on mosquitoes. An adult dragonfly can savor about 50 mosquitoes in a day while flying around, which by the way, is an important factor in the biological control of mosquitoes.
Wings and Flight
Dragonflies are equipped with two sets of wings, so they do not have to beat their wings simultaneously. They flap their wings at a rate of 30 beats per second. These fliers can reach a top speed of 100 km/h. Their flight is similar to that of a helicopter; they can fly forward, backward, up, and down. And guess what? They are also capable of hovering! Such flying skills are achieved by powerful flight muscles and wings that can move independently.
Dragonflies are able to distinguish between colored, ultraviolet, and even polarized light. This ability enables these insects to detect reflection in water. Each of the compound eyes of these insects contains about 30,000 individual lenses. Furthermore, they are able to have a 360º field of vision due to their eyes' position.
Territorial Behavior and Mating
Like most males in the animal kingdom, male dragonflies are very particular about their territory. They stake their claim to a particular area alongside a river, pond, and other water bodies. Often, two dragonflies can be seen chasing each other. When it comes to mating, these insects undergo a complex process. What is normally seen in a mating is a couple attached to one another while flying around at the same time.
Birds are the natural predators of dragonflies, and species like flycatchers and swallows are known to feast on them. In the larval stage, they mainly fall prey to aquatic and semi-aquatic species like water beetles, turtle, frogs, ducks, etc. Adult dragonflies, on the other hand, are preyed upon by fish, frogs, spiders, lizards, bats, and, in rare cases, larger species of their own kind.
Dragonflies and Water
Dragonflies are commonly found near water bodies like ponds, lakes, slow moving streams, and other wet lands. These insects prefer healthy and breathable water; which is why they are considered as indications of good water source. Water bodies that support submerged and emergent vegetation provide shelter for these insects. They spend a major part of their lives underwater as nymphs in their larval stage. Sometimes this period goes up to 3 years, depending on the type of the species. Adult dragonflies' life span lasts for a very short time - about a month.
Some More Facts
Native American culture attributed these fliers as the souls of the dead.

The wings of a dragonfly have ridges like formation. This helps the insect to detect the slightest brush of wind to take flight.

The efficient speed and maneuverability that the insect achieves in its flight is because of its front wings which are slightly longer than the wings at the rear.
When a dragon fly reaches its adult stage, it crawls out of water and its exoskeleton is cracked open exposing its abdomen. When the wings come out, they are still wet and fragile. So the wings dry up and harden over the next several days after which the insect gets ready to go airborne.

Paleontology records suggest that the members of genus Meganeura, the extinct genus of flying insects closely related to dragonflies, were exceptionally large in size. With a wingspan of 2.1 ft, Meganeura monyi was one of the largest insects from the Carboniferous Period.
Dragonflies and Humans
The existence of dragonflies are surrounded by a myriad of myths. It was believed that dragonflies have deadly stingers and people were allergic to them. It was also believed that, these insects used to sew the eyes of people who slept outside. On the contrary, no cases have been recorded of any stings or bites from dragonflies on humans. In fact, they are beneficial to us. They help in reducing the population of mosquitoes and other harmful insects. Dragonflies also have some funny stories attached to them. One of them was a belief that they are snake doctors; they have the ability to bring dead snakes back to life.
Along with other insects and animal species, dragonflies also face a threat of extinction. The British Isles have witnessed the complete extinction of almost three species of these insects. The remaining population has become rare and has been restricted to a few areas of the world. These species face a constant threat from habitat destruction, pollution, and other environmental degradation.
Dragonflies form a small but an important part of the ecosystem. In fact, all animals and insects contribute to the ecological balance of nature, in someway or the other. The facts that have been mentioned above, have been put up with an intention which can be of some help in preserving and respecting these wonderful fliers of nature.