Classification of The number of known species of insects is from 200,000 to 300,000, but it is estimated that there are up ward of a million species now living. In fact, the class of insects vastly outnumbers all other groups of animals. This is probably due to their being winged, and to their great fecundity. At present the class of insects is divided into two subclasses, that is, the (1) Synaptera, rep resented by the wingless orders Thysanura and Collembola • and (2) Pterygota, comprising 15 winged orders and which may be thus tabu lated Fossil species of fossil insects have been described, of these from 200 to 300 are Paleozoic, 500 Mesozoic and the re mainder are Tertiary. The oldest fossil insect remains is the wing of a supposed bug rimer) from the Ordovician of Sweden. The wing of a cockroach (Pahroblattina) has been detected in the middle Silurian of Calvados, France. From the Devonian shales of Saint John, N. B., nine species of primitive net veined insects have been collected. The coal measures are characterized by cockroaches, primitive dragon-flies, May-flies and grasshop per-like forms, phasmids, etc., also occurring.
All of the Palaeozoic insects known are very primitive. Modern forms, those having a com plete metamorphosis, begin to appear in the Triassic and Jurassic, where remains of beetles, a and a moth occur. Ants, bees and butterflies date from the Oligocene and Mio cene Tertiary. See FRESH-WATER INsEcTs; MARINE IxsEcrs, and the names of groups and species, as Fuss, HYMENOPTERA, Months, etc.
The latest general works on insects are Carpenter's 'Insects, their Structure and Life' (London 1899), and Sharp's 'Insects' (Vols. V and VI of Cambridge Natural His tory 1895-99) • both contain sufficient references to other works. For American insects consult Packard's 'Text-book of Entomology> (18'98), and 'Guide to the Study of Insects' (1889) ; Comstock's 'Manual for the Study of Insects' (1895) ; 'Insects and Crustacea' (Vol. II, Standard Natural History 1884) ; Howard's 'The Insect Book) (1901). The last named contains a copious bibliography, especially to the voluminous publications of the Entomolog ical Division of the United States Department of Agriculture.