All living things can be divided into two main groups: the Animal Kingdom and the Plant Kingdom. There are many different kinds of creatures in the Animal Kingdom, and there is a scientific system for grouping animals into smaller and smaller groups that are more and more similar.
The Animal Kingdom is first subdivided into groups called phyla (singular: phylum). The phyla are divided into "classes," the classes into "orders," and so on until we get to the smallest division called species.
The ranking of the different subdivisions is shown in the examples of four different animals in the following chart, Examples of the Scientific Classification System. This method of classifying animals is used by scientists. The genus and species names are italicized, and the two names together are the scientific name of the animal. For example, the scientific name of the house fly is Musca domestica.
There are 15 phyla of animals, and each phylum contains animals that have a combination of characteristics that animals in other phyla do not have. The phylum Arthropoda are the animals with jointed legs, segmented bodies, and a tough or hard outer covering that also serves as their skeleton (exoskeleton). Insects have these characteristics, so they belong to Phylum Arthropoda, but so do millipedes, spiders, ticks, crabs, lobsters, and crayfish, which are not insects.
The class Insecta, or insects, are the Arthropoda that have three pairs of legs, a segmented body divided into three regions (head, thorax, and abdomen), one pair of antennae and, usually, wings. Other Arthropoda classes have more than three pairs of legs and only one or two body regions, and they never have wings. Other common classes of Arthropoda are Crustacea (such as sowbugs, crayfish, crabs), Diplopoda (millipedes), Chilopoda (centipedes), and Arachnida (such as spiders, ticks, mites, scorpions).