Insecta
insects; a class of Annulosa, division Anthropoda. Formerly it was made to include, among other animals, the centipedes and spiders. Now these are made distinct classes, and the Insecta confined to those arthropodous animals which have three pairs of legs; these are affixed to the thorax which is distinctly separated from the head and the abdomen. There are compound and simple eyes. In the highest orders there are four wings; in another order, Diptera, but two; and in several more the wings are rudimentary or totally absent.
There is one pair of antennæ. The respiration is by tracheæ. Of the 13 segments, of which a typical insect consists, one constitutes the head, three the thorax, and nine the abdomen. The cutaneous skeleton is composed of chitine. There is generally a more or less complete metamorphosis. Insects exist in all countries. The species existing may be half a million, those known more than 200,000. Most of them are confined to particular regions; some insects, however, like the Painted Lady Butterfly (Cynthia cardui), are widely diffused over the world. Insects exert a powerful influence in fertilizing plants. The classification now commonly adopted divides insects thus: Sub-class I., Ametabola: (1) Anoplura, (2) Mallophaga, (3) Collembola, (4) Thysanura. Subclass II., Hemimetabola: (1) Hemiptera or Rhynchota, (2) Orthoptera, (3) Neuroptera. Sub-class III., Holometabola: (1) Aphaniptera, (2) Diptera, (3) Lepidoptera, (4) Hymenoptera, (5) Strepsiptera, (6) Coleoptera. The old- INSECTICIDE est known insects are from the Devonian rocks of this country. They seem to have belonged to the Neuroptera.