Entomology/What are insects?
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Insects are animals with three body segments (the word insect means "segmented"): the head, the abdomen, and the thorax. Insects have, typically, three pairs of legs located on the abdomen; however, there are exceptions, such as brush-footed butterflies.
Insects belong to the taxonomic class Insecta, which is just one class of Phylum Arthropoda. The arthopods also includes Crustacea (crabs, lobsters, etc.), Arachnida (spiders, mites, etc.), and Myriapoda (Centipedes and Millipedes). The class Insecta is closely related to the class Entognatha (Collembola, Diplura, and Protura), which is usually included in the study of entomology. Both Insecta and Entognatha are within the subphylum Hexapoda.
Insects have a wide range of life cycles, but there are some basic similarities. They almost all begin as eggs, which vary widely in shape and size. Then insects undergo a period of growth. Some then undergo a metamorphosis ("huge change"), into an adult form.
Insects such as silverfish (order Zygentoma) do not undergo metamorphosis at all — they grow directly from hatching to their full size, and molt several times as a mature adult.
Some Insects, such as true bugs (order Hemiptera) and grasshoppers (order , undergo an incomplete metamorphosis. When they hatch from eggs they are called nymphs. These nymphs will eventually grow into their final adult form. They do not molt after that.
Insects such as butterflies, moths, and flies undergo a complete metamorphosis. When they hatch from eggs, they are called larvae. Once they have grown through several periods called instars, they become pupae (in butterflies it's called a chrysalis). Eventually, an adult, or imago phase, will emerge from the pupa.
Insects are very successful organisms. They exist in every type of terrestrial habitat, including some flies native to Antarctica. Insects also vary widely in shape, size, colour, etc. There are currently well over 900 thousand named species of insect, and estimates place the total number of species between 2 and 30 million.