Subphylum Hexapoda
The distinguishing characteristics of the Hexapoda are as follows: maxillary plate present; body divided into distinct head, thorax, and abdomen; thorax with three pairs of legs; legs composed of six segments (coxa, trochanter, femur, tibia, tarsus, pretarsus);abdomen consisting of 11 segments; "knee" formed by the fermoral-tibial joint; second maxillae fused to form the labium; epimorphic segmental growth; ommatidia (planar cells that make up compound eye) with two primary pigment cells; trochantin present; arolium present. The subphylum is divided into 31 orders, with 3 in the Entognatha (mouthparts withdrawn into head) and the rest in class Insecta.
Class Insecta
Early insects from the Devonian were undoubtedly wingless, and hence primitive orders are the Apterygotes, wingless insects. Most more recent orders have wings or are secondarily wingless, the Pterygotes.
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