The most important criterion for classification of insects is the structure of the wing (hence the abundance of '-pter-' in most taxonomic levels). Even at the species level, the wings often indicate best to which group a specimen belongs The following tree shows the relation of insects to some other animals.

Phylum Annelida (worms)

Phylum Arthropoda

Superclass Trilobitomorpha (including Trilobites and the Trilobitoidea)

Superclass Crustacea (including crabs and lobsters)

Superclass Chelicerata (including spiders, scorpions, and horseshoe crabs)

Superclass Uniramia *

Class Myriapoda

Subclass Collifera

Infraclass Diplopoda (millipedes)

Subclass Atelopoda

Infraclass Chilopoda (centipedes)

Class Insecta

Order Coleoptera (beetles)

Order Hymenoptera (ants, bees, wasps)

Order Diptera (flies, mosquitos, gnats)

Order Lepidoptera (butterflies, moths)

The diversity of the insects is staggering. Approximately three quarters of all animals are insects and the proportion on land is much greater, as virtually all non-insects live in the oceans. Insects have adapted to every habitat on land except for the polar regions (where it is impossible for them to survive, as only warm-blooded animals can withstand the cold).

Despite the relatively short life span (compared to terrestrial vertebrates) of the insects, their high fecundity produces by far the largest biomass among animals. They so extensively pervade every terrestrial ecosystem that most plants and many animals have evolved to take advantage of their presence in many ways. The insects are the principal source of food for many birds, other small animals (particularly other insects and arthropods of the other terrestrial orders), and even some plants. A large proportion of flowering plants depend on insects for pollination.

* Dr. Kukalova-Peck, a researcher here at Carleton University has demonstrated that this widely recognized superclass is bogus as insects are not uniramous (having only one ramus, or exite, on the leg) but are, in fact, polyramous.