Our environment is gifted by the nature to nourish the life on the earth. Everything which we used to continue our lives comes under the environment such as water, air, sunlight land plants, animals, forest and other natural things. Trees, animals, birds and insects were creates most beauty to our nature. Numerous insects were seen in our world. Insects are the largest group among animals and plants in the world. Insects form the largest and the most successful group of the existing animals in terms of the number of species, number of individuals, extensive distribution and extreme adaptive radiation. Our earth is diversified with much more type of insect groups. They are the group within the arthropod phylum. Arthropoda is the largest animal taxon, comprising nearly 80% of the known living species. They range in size from mites less than one hundredth of an inch to the giant spider crab Macrocheira with a claw –spread of nearly 3 m. To be an arthropod phylum, insects have many characteristics. Insects have a chitinous exoskeleton, a 3 part body (head, thorax and abdomen), 3 Pairs of jointed legs, compound eyes and one pair of antennae. Insects are the most diverse group of animals; they include more than a million described species and represent more than half of all known living organisms. Many insects groups were found on our earth. Each of them carried their own special characters. Some of them are smaller and some are larger in size. Some insects may show carnivorous behavior but, some may show herbivorous in nature. Majority of them are free-living, but some are parasitic on plants and animals. Class Insecta is broadly divided into two sub-classes, namely Apterygota and Pterygota. Apterygota includes wingless, primitive insects while, Pterygota includes advanced winged insects. Many orders were included in these two sub-classes.
Odonata is one of the group coming under sub-class Pterygota of class Insecta. Dragonflies and damselflies were included in this group. They are amongst the most attractive of creatures on earth. They are brilliantly colored, predacious and medium sized to large insects with two sub equal pairs of membranous wings. Odonata, dragonflies and damselflies, constitute a small, well known order of insects that are widely distributed over the world [Tillyard RJ .The Biology of Dragonflies. Cambridge University Press, Cambridge]. Dragonflies are insectivorous diurnal insects. They feed on insects which they catch in flight. They are found worldwide except in the extreme cold regions such as Antarctica, Northern Canada and Iceland. The name Odonata derived from the Greek word ‘Odonto’ meaning tooth, it refers to the strong teeth found on the mandibles of most adults. The largest living dragonfly appears to be the central and South American species Megoprepus caerulatus (wing span nearly 20 cm).Odonates appeared on Earth over 300 million years ago, that is even before the dinosaurs. Odonates were well appreciated by their bright colors, wing patterns and equilibristic flight. There were 8 super families, 29 families and some 58 sub – families of dragonflies for appropriate 600 genera and 6000 named species have been described all over the world [Silsby,2001]. Dragonflies are a predaceous, hemimetabolous and amphibiotic insect. They inhabit all fresh water habitats either permanent or temporary. As insects, they follow the typical pattern of body: head-thorax-abdomen, have 2 pairs of wings and 6 pairs of legs. They have reduced antennae therefore their use is limited. They have huge and multifaceted eyes, thus providing excellent eyesight.
Odonates are broadly divided into 3 suborders, the Zygoptera, Anisozygoptera and Anisoptera. Zygoptera includes damselflies and Anisoptera includes dragonflies. Dragonflies and damselflies are medium to large insects with body lengths ranging from 15-120 millimetres. They have very similarity in first look. But there were certain differences between them. Even before hatching from the egg, differences in morphology of the egg distinguish dragonflies from damselflies. Dragonfly eggs are round and about 0.5 mm long, whereas damselfly eggs are cylindrical and longer, about 1mm long. Also, the nymphs (larvae) of the 2 groups differ. Abdomen size is also different in 2 groups. A larval damselfly abdomen is longer and narrower with 3 fin- like gills projecting from the end. Dragonfly nymphs are shorter and bulkier, and the gills are located inside the abdomen. Damselflies have narrowly rectangular heads and eyes and are separated by more than their own width while dragonfly eyes are not separated by more than their own width actually. Both of them are good predators throughout their lifespan, play an important role in wetland ecosystem.