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.

In those kinds of butterflies that attain the perfect state twice in the year, the individuals composing the first flight are somewhat different in marking from those of the second flight. Such species as the large and small whites exhibit this kind of variation, which is termed seasonal dimorphism. The males of some species, as for example the Common Blue and the Orange-tip, differ from the females in colour; this is known as sexual dimorphism. The Silver-Washed Fritillary, which has two forms of the female, one brown like the male, the other green or greenish in colour, is a good example of dimorphism confined to one sex. Gynandrous specimens, sometimes called "Hermaphrodites," are those which exhibit both male and female coloration, or other wing characters; when one side is entirely male and the other side entirely female, the gynandromorphism would be described as complete.

The ornamentation on the under side of a butterfly differs from that of the upper side, and is found to assimilate or harmonize in a remarkable manner with the usual resting-place. It is therefore of service to the insect when settled with wings erect over the back, in the manner of all butterflies, except some few kinds of Skippers.

The number of known species of butterflies throughout the world has been put at about thirteen thousand, and it has been suggested by Dr. Sharp that there may be nearly twice as many still awaiting discovery. Dr. Staudinger in his "Catalog" gives a list of over seven hundred kinds of butterflies as occurring in the whole of the PalÆarctic Region. This zoological region embraces Europe, including the British Islands, Africa north of the Atlas range of mountains, and temperate Asia, including Japan. The entire number of species that can by any means be regarded as British does not exceed sixty-eight. Even this limited total comprises sundry migratory butterflies, such as the Clouded Yellows, the Painted Lady, the Red Admiral, the Camberwell Beauty, and the Milkweed Butterfly; and also the still less frequent, or perhaps more accidental visitors, the Long-tailed Blue and the Bath White. Again, the Large Copper is now extinct in England, and the Mazarine Blue does not seem to have been observed in any of its old haunts in the country for over forty years. The Black-veined White is also scarce and exceedingly local.

The majority of the remaining fifty-seven butterflies may be considered natives, and of these about half are so widely distributed that the young collector should, if fairly energetic, secure nearly all of them during his first campaign. The other species will have to be looked for in their special localities, but a few kinds are so strictly attached to particular spots, that a good deal of patience will have to be exercised before a chance may occur of obtaining them.

A few remarks may here be made in reference to the names and arrangement adopted in the present volume.

As will be adverted to in the descriptive section, the English names of our butterflies have not always been quite the same as those now in general use. There has, however, been far less stability in scientific nomenclature, and very many changes in both generic and specific names have been made during the past twenty years, more especially perhaps within the last decade.

Genera are now founded by some specialists on characters which formerly served to distinguish one species from another, whilst other authorities merge several genera in one upon certain details of structure that are common to them all.

Patient research into the entomological antiquities has revealed much important material, some of which may furnish a new interpretation of the Linnean classification of Lepidoptera.

The discovery of the earliest Latin specific name bestowed upon an insect, is a labour which entails a large expenditure of time and requires fine judgment. Great credit is therefore due to those who undertake such investigations, the result of which may tend to the establishment of a fixed nomenclature in the, probably not remote, future, although it sadly hampers and perplexes students in the meanwhile.

All things considered then, it has been deemed advisable not to make many changes in specific names, and to retain the old genera as far as possible. The arrangement of families, genera, etc., will be found to accord with that most generally accepted both in England and on the continent.