Insect external structure
Insect internal structure
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Insect identification > Apterygota
These are all relatively small insects, some being nearly microscopic in size, while the largest are not more than 2 in, in length. They are all land animals, though a few live near the ocean and are occasionally found in tide pools. They are widely distributed over the earth, some living in arctic conditions while others occur in the tropics, but nearly all at least require a somewhat humid atmosphere.
In this group the mouth-parts seem to be typically of the chewing type. In many cases they are as much exposed as in most insects, but, in some, folds of the cheeks extend over them so that they are almost concealed. Under such conditions they are often so slender as to be no longer of value for chewing and are probably used for piercing and sucking. Some Apterygota have traces of abdominal legs; spine-like appendages, attached to the hinder margins of some of the abdominal segments beneath and called styli, may also be present.
Bringing together these facts, the Apterygota may be characterized as Wingless insects having the mouth-parts either exposed and of the chewing type or almost entirely concealed by folds of the cheeks, where they are often slender and probably used for piercing and sucking. They have no metamorphosis.
Very few of the Apterygota are of any importance from an economic standpoint, but they are of much interest, being the simplest insects known and throwing some light upon the subject of the ancestry of the insect group.
Two subdivisions, the orders Thysanura and Collembola, are generally recognized in the Apterygota, of which over two thousand species are known.