Insects are the most diverse group of animals on Earth. There are over a million described species of insect, and an estimated 6-10 million species total. Insects are found in nearly every above-ground environment, even in Antarctica, which has a species of insect called springtails. There is even one that lives on the surface of the open ocean, walking on the water tension using tiny leg hairs. These are insects of the genus Halobates, otherwise known as sea skaters or water striders. Around Antarctica, where there is greater oxygen in the sea than the Equator, these can grow to 30 cm (1 ft) in width.
So far, there have been observations of 5,000 dragonfly species, 2,000 praying mantis, 20,000 grasshopper, 170,000 butterfly and moth, 120,000 fly, 82,000 true bug, 360,000 beetle, and 110,000 bee, wasp and ant species. Keep in mind that arachnids, such as mites — of which there are over 1 million described species — and crustaceans, like lobsters, are not insects, which only includes members of the class Insecta.
Species of insect range in size from 0.139 mm (0.00547 in, fairyfly) to 55.5 cm (21.9 in, stick insect), with most falling between 0.5 mm (0.02 in) and 50 mm (2 in). In general, smaller insects are capable of reproducing more rapidly due to their larger counterparts, allowing them to maintain their biomass over generations. If the biomass is not regularly replenished, then a species will go extinct. It is thought that the all the species of insects alive today only represent about 1% of all insects that have ever lived.
Insects are truly a success story for animals. Although most people don't think of insects when the word "animal" is mentioned, they are both the most successful and numerous. The global insect biomass is estimated at 1012 kg, with approximately 1018 distinct individuals. Divided by the total number of species, this gives an average of 100 million individuals per species. Of course, like with other animals, the majority (over 66%) of insect species are found in the world's rainforests.