ApterygotesThese insects are known as the Apterygote orders because all the adults are wingless like the immature stages, these have no true metamorphosis and are the most primitive insects alive today.
Springtails: 1100 species
Like all the suceeding groups of arthropods, springtails have three pairs of mouth-parts and one pair of antennae on the head, a three-segmented thorax with three pairs of legs, and an abdomen which lacks legs. Springtails have six abdominal segments. A forked organ on the end of the abdomen enables the springtail to spring upwards when the fork is suddenly released. They are important animals of the leaf litter, and up to 250 million may live in 1 acre of meadow.
Two-pronged bristletails: 400 species
Bristletails have a ten-segmented abdomen, at the end of which are two filaments. They live under stones or logs, or in buried decaying leaves.
Proturans: 45 species
These tiny creatures have a 12-segmented abdomen with tiny appendages on the first three segments. They use their fore-legs as feelers, holding them in front of their heads.
Three-pronged bristletails: 350 species
The Thysanura hve smooth, tapering bodies with long antennae and three long, slender filaments on the abdomen. As they grow, they cast off the outer skeleton which is replaced by a larger one underneath. This occurs throughout life in Thysanura, unlike most arthropods, whose outer skeleton is not shed once maturity is reached.
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