Because they have a separation in the middle of their bodies, whereby they are cut (insectus, cut or notched, Lat.) into two parts, joined by a small ligature, as in the common house-fly.
Because of the origin of that term from the two Greek words, entoma, insects; and logos, a discourse.
Because the localities of insects are, to a certain extent, constantly changing. Insects, also, which are plentiful one year, frequently become scarce, or disappear altogether the next - a fact, strikingly illustrated by the uncommon abundance in 1826 and 1827, of the seven-spot lady-bird in the vicinity of London, though during the two preceding summers, this insect was comparatively scarce, while the small two-spot lady-bird was plentiful. - J. Rennie.
Because they were found about animal bodies in a state of decomposition; and the circumstance was merely noticed without any previous or accurate observation of the means by which they were first produced. The fact is now established, that all insects come from eggs, as plants do from seeds.
Because the males rarely survive the inclemency of the first winter, and the females die after having deposited their eggs.
Because the wonderful metamorphoses to which they are subjected, conceal their identity from the eye of the uninitiated, and greatly interfere with a continuous tracing of the same individual, from the egg to the perfect form. For example - many aquatic flies, such as the Ephemerae and others, whose declared and more obvious existence, does not exceed a few hours, have, previous to their assuming the winged state, spent months or even years in the banks of rivers, and beneath the surface of the stream.
Because the primary divisions of Linnaeus are taken from circumstances connected with the condition of the wings. The simplicity of this method, and the obviousness of the characters which have been employed, have secured for this system a decided preference among the entomologists of Britain. - Fleming.
Because they have wings in sheaths: (koleos, a sheath - pteron, a wing;) as the common black-beetle : - 4,087 species.
Because they have half of one wing overlaid by the other: (hemisu, half - pteron a wing;) as the common cockroach : - 1,427 species.