Insects are by far the most diverse group of multicellular organisms on our planet. Of about 1,625,000 described species of prokaryotes, protoctists, fungi, plants and animals, more than 1 million is represented by arthropods, of which insects constitute the largest group with about 854,000 described species. The estimations of the number of still undescribed species, especially in the vanishing tropical rainforests, are ranging from 2 million to 80 million species! The most species-rich groups within insects are the holometabolous orders Coleoptera (beetles), Hymenoptera (ants, wasps and bees), Diptera (mosquitoes and flies) and Lepidoptera (moths and butterflies). Among the hemimetabolous orders, which lack a pupal stage in their ontogenetic development, the Hemiptera (aphids, scale insects, cicadas and bugs) are the largest group, while all other insect orders have much fewer species.
Even though relatively small animals, the extremely large number of individuals makes insects a very significant part of the total terrestrial biomass in many biotopes. For example, in tropical rainforests, the ants and termites have a higher total biomass than all the vertebrates combined.
Insects are not only diverse in terms of species number and number of individuals, but also in their astonishing anatomical and ecological variability. Insects populate nearly every available habitat on the planet, except for the open seas and the frozen polar regions.