Insecta
Collier's New Encyclopedia (1921)
insects; a class of Annu- Insects, losa, 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 oldest known insects are from the Devonian rocks of this country. They seem to have belonged to the Neuroptera.
What Has Changed in the Last Century
When Collier's wrote this in 1921 it put the known insects at "more than 200,000" species and guessed the true total might be half a million; today about a million species have been described and credible estimates of the total run to five million or more. The basic anatomy given here — three pairs of legs on a distinct thorax, one pair of antennae, tracheal breathing, a chitinous exoskeleton — is still what a textbook says, but the classification has been rebuilt: DNA evidence places insects inside the crustacean radiation (the Pancrustacea), termites are now sunk into the cockroach order Blattodea, and the old catch-all Neuroptera has been broken into several separate orders. Collembola, listed here as insects, are usually treated as a separate group of hexapods rather than true Insecta. The largest new question is one the entry could not have imagined: since the 2010s repeated surveys of insect biomass and abundance have reported steep declines in monitored regions, making conservation, not cataloguing, the field's most argued-over problem.
Written for this edition, 2026.