7.4. 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 in flighted adult stages, associated with development of wings and the required musculature; it is weakly developed in wingless taxa. Thoracic legs have more than five segments. 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, that is, with the number of body segments constant during development.
The 30 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 (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 group Apterygota for the primitively wingless taxa Archaeognatha + Zygentoma appears paraphyletic on most (but not all) modern analyses (Figs. 7.2 & 7.3).
Italicized names indicate likely paraphyletic taxa. Broken lines indicate uncertain relationships. (Data from several sources)
Broken lines indicate uncertain relationships. Thysanura sensu lato refers to Thysanura in the broad sense. (Data from several sources)