Class Insecta True Insects

Insects range from minute to large (0.2 mm to >30 cm long) with very variable appearance. Adult insects typically have ocelli and compound eyes, and the mouthparts are exposed (ectognathous) with the maxillary and labial palps usually well developed. The thorax may be weakly developed in immature stages but is distinct and often highly developed in winged adults, associated with the sclerites and musculature required for flight; it is weakly developed in wingless taxa. Thoracic legs each have six segments (or podites): coxa, trochanter, femur, tibia, tarsus, and pretarsus. The abdomen is primitively 11-segmented with the gonopore nearly always on segment 8 in the female and segment 9 in the male. Cerci are primitively present. Gas exchange is predominantly tracheal with spiracles present on both the thorax and abdomen, but may be variably reduced or absent as in some immature stages. Larval/nymphal development is epimorphic.

The orders of insects traditionally have been divided into two groups. Monocondylia is represented by just one small order, Archaeognatha, in which each mandible has a single posterior articulation with the head. Dicondylia (see Fig. 7.3), which contains all of the other orders and the overwhelming majority of species, has mandibles characterized by a secondary anterior articulation in addition to the primary posterior one. The traditional groupings of "Apterygota" for the primitively wingless hexapods and "Thysanura" for the primitively wingless taxa Archaeognatha + Zygentoma are both paraphyletic according to most modern analyses (see Figs. 7.2 & 7.3).

The previous edition of this book recognized 30 orders of true insects. However, new data have shown that two of the traditional orders (Blattodea and Psoco-ptera) are each paraphyletic. In each case, another group nested within had been accorded order status due to its possession of diagnostic autapomorphic features. The requirement for monophyly of orders means that only 28 orders of insects are recognized here, with the Isoptera (termites) subsumed into the Blattodea, and the Phthiraptera (parasitic lice) + Psocoptera forming the order Psocodea. Evidence for these relationships is discussed below. Although a new ordinal-level classification is used for these groups, separate taxoboxes have been provided for the termites and parasitic lice due to the distinctive biology and morphology of each group. Hypotheses of relationships for all insect orders are summarized in Fig. 7.2, with uncertain associations or alternate hypotheses shown by broken lines.

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