Insects are the most diverse and the largest in terms of numbers, of creatures on the entire planet. They constitute a very important factor in the balance of the planet's bio and ecosystem and many other species of animals, like birds, depend on them. It is a chain dependency though and if it were that one day all of these small creatures would disappear, not only one species of creatures would be gone, but thousands of others will be on the brink of extinction. Let's take this small example: Ant bears feed only on ants mostly and if they would disappear, the bears would have a lot of trouble on changing their eating habits, which I guess they will not cope with very well.
The study of these small creatures is referred to as entomology. They are invertebrate creatures and they are part of the Insecta class and also part of the phylum Arthropoda. When it comes to sizes, they can reach as less than one millimeter to even as high as 18 centimeters or even more. The adult body of an insect is characterized by a body which is segmented into a head and also three pairs of legs, two pairs of wings, abdomen and thorax. They breed through eggs which are laid by a mother insect. As an example, the most common of them (which I am pretty sure you are already familiar with) are bees, mosquitoes, flies, butterflies, beetles and crickets.
On the planet live up to almost two thousand species of praying mantis, one hundred and seventy thousand species of moths and butterflies, five thousand species of dragonflies, twenty thousand species of grasshoppers, one hundred and twenty thousand species of flies, eighty two thousand species of true bugs, three hundred and fifty thousand species of beetles and more than one hundred and ten thousand ant and bee species. These total up to almost nine hundred and twenty five thousand species which is a quite impressive number. But if you think that number is great, wait till you hear this one. Experts say that there are still fifteen to thirty MILLION species on the planet that haven't been yet discovered.
The modern version of insects ranges in size from less than a millimeter to over 18 centimeters in size. Some of the most common and well known insect species include flies, crickets, mosquitoes, beetles, butterflies, and bees.