Butterflies belong to the great Order of insects called Lepidoptera (Greek _lepis_, a scale, and _pteron_, a wing), that is, insects whose wings are covered with minute structures termed scales. Moths (Heterocera) also belong to the same order, and the first point to deal with is how may butterflies be distinguished from moths? In a broad kind of way they may be recognized by their horns (_antennæ_), which are slender as regards the shaft, but are gradually or abruptly clubbed at the extremity. For this reason they were designated Rhopalocera, or "club horned," the Heterocera being supposed to have horns of various kinds other than clubbed. As a matter of fact this method of separating moths and butterflies does not hold good in dealing with the Lepidoptera of the world, and it is from a study of these, as a whole, that systematists have arrived at the conclusion that there is no actual line of division between moths and butterflies. In modern classification, then, butterflies are reduced from the rank of a sub-order, which they formerly held, and are now dovetailed into the various newer systems of arrangement between certain families of moths. As regards British butterflies, however, it will be found that these may be known, as such, by their clubbed horns. Only the Burnets among British moths have horns in any way similar, and these are thickened gradually towards the extremity rather than clubbed. Day-flying moths, especially the bright-coloured ones, might be mistaken for butterflies by the uninitiated, but in all these the horns will be found not at all butterfly-like. Although varieties of the species will be referred to in the descriptive portion of the book, a few general remarks on variation in butterflies may here be made. All kinds are liable to vary in tint or in the markings, sometimes in both. Such variation, in the more or less constant species especially, is perhaps only trivial and therefore hardly attracts attention. In a good many kinds variation is often of a very pronounced character, and is then almost certain to obtain notice. Except in a few instances, where the aberration is of an unusual kind, it is possible to obtain all the intermediate stages, or gradations, between the ordinary form of a species and its most extreme variety. A series of such connecting links in the variation of a species is of greater interest, and higher educational value, than one in which the extremes alone have a place.