Insects Class Insecta

Identification: Three pair of legs. 3 body regions (head, thorax, and abdomen). Often 1 or 2 pairs of wings. 1 pair of antennae (antennae rarely absent). Mouth parts typically consist of a lab rum, a pair of mandibles, a pair of maxillae, a hypopharynx, and a labium. Genital ducts open near posterior end of body. Winged insects differ from all other invertebrates in the possession of wings; wingless insects differ from most other arthropods in having 3 pairs of legs and a pair of antennae. Caterpillars appearing to have more than 3 pairs of legs have the first 3 pairs behind the head normally leglike in structure and the remaining pairs (prolegs) stout and fleshy and quite different in structure. Similar groups: (1) Arachnida: larval mites with only 3 pairs of legs lack antennae, the body is not differentiated into 3 regions, and the abdominal region is not segmented. (2) Diplopoda: newly hatched millipedes, with 3 pairs of legs, have the head structure characteristic of millipedes (see illus., p. 51) and a single liplike structure (gnathochilarium) behind the mandibles instead of maxillae and a labium. (3) Annelida: some resemble legless insect larvae but have more than 13 body segments and lack a tracheal system.

Classification: The class Insecta is divided into 2 subclasses, the Apterygota and Pterygota. The Apterygota include the orders Protura, Thysanura, and Collembola, and the Pterygota include the remaining orders. The Apterygota are wingless, and most Pterygota have wings. The wingless Pterygota are thought to have evolved from winged ancestors because they have certain features of thoracic structure (for example, the thoracic pleura divided by a pleural suture into episternum and epimeron, and the meso- and metanotum divided by sutures) correlated with the development of wings; Apterygota have a simpler thoracic structure (these sclerites not divided by sutures). The Apterygota usually have stylelike appendages on the pregenital segments of the abdomen; such appendages are lacking in the Pterygota. The orders of insects are separated principally by the characters of the wings, mouth parts, legs, and the metamorphosis. No. of species: World, 703,500; N. America, 88,600.

Some of the outstanding features of the 26 orders of insects are outlined in the following table.

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