Entognatha
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| Entognatha | ||||||||
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 Collembola (Isotoma sp.)
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The Entognatha is a class of ametabolous arthropods, which, together with insects, makes up the hexapods. Their mouthparts are entognathus, meaning they are retracted within the head. Entognatha are apterous, meaning they lack wings. The class contains three orders: Collembola (springtails), Diplura and Protura. It is agreed that the hexapodous condition of these animals has evolved independently from that of insects, and independently within each order. The orders are not closely related, and so Entognatha is considered a polyphyletic group.
These three groups, if united with the order Thysanura, form the class Apterygota.
[edit] Morphology
These minute arthropods are apterous or wingless, unlike some other orders of insects that have lost their wings secondarily (but are derived from winged ancestors). Their name comes from their mouthparts that are enclosed within a pouch in the head capsule, so that only he tips of the mandibles and maxillae are exposed beyond the cavity. This pouch is created in the embryo by a flap or lateral head sclerite near the mouth on each side of the head which fuses with the labium. Other differences with insects are that each antennal segment is musculated; in insects, only the two basal segments are. Of the three orders, only collembolans possess eyes. Nevertheless, many collembolans are blind, and even when compound eyes are present, there are no more than eight ommatidia.
The sperm transfer is always indirect, and there is ovipositor in the females.
Collembola have a ventral tube termed a collophore on the first abdominal segment. The collophore is involved in moisture absorption. On the third abdominal segment is the tenaculum that holds the furcula. The furcula is the spring by which the Collembola are given the name springtails.
Proturans also known as "coneheads," do not have eyes or antennae. They possess abdominal styli thought to be vestigial legs.
The Diplura have a pair of caudal cerci.
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