INSECTA, insects; a class of Annu losa, division Arthropoda. Formerly it was made to include, among other ani mals, 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, Dip tera, but two; and in several more the wings are rudimentary or totally absent. There is one pair of antenn. The res piration is by trache. Of the 13 seg ments, of which a typical insect consists, one constitutes the head, three the thorax, and nine the abdomen. The cu taneous skeleton is composed of chitine. There is generally a more or less com plete 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, hol.v ever, like the Painted Lady Butterfly (Cynthia curdici), are widely diffused over the world. Insects exert a power ful influence in fertilizing plants. The classification now commonly adopted di vides insects thus: Sub-class I., Arne tabola: (1) Anoplura, (2) Mallophaga, (3) Collembola, (4) Thysanura. Sub class II., Hemimetabola: (1) Hemip tera or Rhynchota, (2) Orthoptera, (3) Neuroptera. Sub-class III., Holometa bola : (1) Aphaniptera, (2) Diptera, (3) Lepidoptera, (4) Hymenoptera, (5) Strepsiptera, (6) Coleoptera. The old est known insects are from the Devonian rocks of this country. They seem to have belonged to the Neuroptera.