The various branches of knowledge connected with Insects {86}are called collectively Entomology. Although entomology is only a department of the great science of zoology, yet it is in practice a very distinct one; owing to its vast extent few of those who work at other branches of zoology also occupy themselves with entomology, while entomologists usually confine themselves to work in the vast field thus abandoned to them. Before passing to the consideration of the natural history and structure of the members of the various Orders of Insects we will give a verbal diagrammatic sketch, if we may use such an expression, with a view to explaining the various terms that are ordinarily used. We shall make it as brief as possible, taking in succession (1) the external structure, (2) internal structure, (3) development of the individual, (4) classification. In the course of this introductory sketch we shall find it necessary to mention the names of some of the Orders of Insects that will only be explained or defined in subsequent pages. We may therefore here state that the term "Orthoptera" includes grasshoppers, locusts, earwigs, cockroaches; "Neuroptera" comprises dragon-flies, May-flies, lacewings, stone-flies and caddis-flies; to the "Hymenoptera" belong bees, wasps, ants, sawflies, and a host of little creatures scarcely noticed by the ordinary observer: "Coleoptera" are beetles; "Lepidoptera," butterflies and moths; "Diptera," house-flies, blue-bottles, daddy-longlegs, and such; "Hemiptera" or "Rhynchota" are bugs, greenfly, etc. CLASS INSECTA: OR INSECTA HEXAPODA.