Insects are one of the most successful  groups of living organisms on earth.
Insects comprise about 95% of all known animal species. . Actually it is insects instead of human (nor mammal) who dominate this planet. . 

insects have one of the widest distributions on the globe. . Insects are found almost everywhere on the planet earth. . Insect species live in deserts. . Other species are found in hot springs, where temperatures reach 180°°F, others are found on mountains at an elevation of 18,000 feet.
Insects are very abundant in tropical rain forests, and still other species are found in the arctic, where temperatures temperatures drop to --40°°F. . 

One explanation for their great diversity and abundance is based on their high reproductive capacity. Most insects produce huge numbers of offspring and many species produce several generations each year. This large reproductive capacity is related to insects' adaptability to a wide range of environmental factors.
Small Size
Water Loss
Special Appendages
Diverse Development