The _Lycaenidae_ or gossamer-wings are the bright little butterflies known as “blues,” “hair-streaks,” and “coppers.” The males do not use the first pair of legs in walking, but the females do. The caterpillars are small and usually slug-shaped, and the chrysalids are held closely attached to some object by a girdle of silk. The _Papilionidae_ is the family of the swallowtails and allied forms. Both sexes use all six feet in walking. The butterflies are usually large or medium-sized; many of the caterpillars produce disagreeable odors; the chrysalids are suspended by the tail and provided with a girdle of silk, but not drawn up to the surface to which they are attached, as in the case of the _Lycaenidae_. The _Hesperids_ or skippers are small butterflies with thick, moth-like bodies and a peculiar manner of flight. Both sexes have six walking feet. The caterpillars are smooth and thin-necked, with large globular heads. The chrysalids are usually enclosed in a flimsy cocoon of leaves fastened together with a few silk threads. SUBFAMILIES, GENERA, AND SPECIES Each one of these four families is divided into several subfamilies; each subfamily is divided into several genera, and each genus is divided into several species, all of these divisions being based on an increasing anatomical similarity. Thus the common Milkweed Butterfly belongs to the class _Insecta_, the order _Lepidoptera_, the suborder _Rhopalocera_, the family _Nymphalidae_, the subfamily _Euploeinae_, the genus _Anosia_, and the species _Plexippus_. THE VALUE OF SCIENTIFIC NOMENCLATURE One may well ask why we do not simply say “milkweed butterfly” and have done with it, but this term refers merely to the fact that the caterpillar feeds upon a certain plant, while _Anosia plexippus_ places the specimen definitely in the scientific scheme of things, and designates important structural distinctions which have nothing to do with milkweeds. The average American is prone to call any insect a “bug” or “moth-miller” or some such unenlightening name, so that many butterflies have no common English name at all, while others have many different names which vary widely in different parts of the country. Besides, the Greek and Latin names are understood by scientists in all countries, and are less liable to corruption than terms taken from the modern languages.