Class Insecta Winged and Wingless Insects

The million or so species in the subclass Pterygota include all winged invertebrates and some insect species that have secondarily lost wings during evolution. They include two orders of ancient winged insects (Ephemeroptera and Odonata) and some 25 to 30 (depending on the classification system) orders of modern folding-wing insects. Most have 11 abdominal segments. The head features two antennae and compound eyes. Respiration is generally with internal tracheae, but aquatic species may use external, tracheate gills or other means to obtain sufficient oxygen. Fertilization is usually direct, distinct developmental stages are common, and molting generally stops with attainment of reproductive maturity. Their most prominent features are two pairs of wings, but a great many insects (e.g., fleas) lack wings or have dispensed with either the hind (e.g., flies) or fore pair (beetles). Among their beneficial attributes are pollination of most flowering plants, production of honey and silk, predation on harmful insects, decomposition of animal wastes and carcasses, and facilitation of ecological processes at all trophic levels above primary producer. Negative attributes include transmission of diseases, annoying bites, and damage to crops, stored food, ornamental plants, forests, and wooden structures.

All insect species that did not evolve from a winged hexapod and whose adults all lack wings are in the subclass

Apterygota, order Thysanura. This small group of 600 or so primitive species includes bristletails, silverfish, and rock jumpers. These are small to medium-sized insects (5—25 mm) without compound eyes. They have an 11-segmented abdomen with a prominent caudal filament between two terminal cerci. Fertilization is indirect, and molting continues after the reproductive state has been reached (unlike insects). No pronounced metamorphosis is evident from subadult to adult stages. Thysanurans are swift, agile runners (probably to avoid predators) and are omnivorous scavengers of animal and plant matter. Most live in litter of forests and grasslands, but silverfish also infest houses, where they can extensively damage clothing and books.

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