The great order Lepidoptera, including butterflies and moths, seems to the non-scientific mind to be composed of members which are pretty much alike, the differences being of slight importance; but this is not in reality the case, for the lepidoptera might, with some accuracy, be compared to the mammalia, with its two divisions of the placental and non-placental animals. Comparing the butterflies (Rhopalocera) to the placental mammals, we may look upon the different families as similar to the orders of the mammalia. Were we as accustomed to notice the differences of butterflies as we are to remark the various forms of familiar animals, we should no longer consider them as slight, but accord to them their true value. When in the mammalia we find animals whose toes differ in number, like the three-toed rhinoceros and the four-toed tapir, we admit the distinction to be great, even apart from other outward forms. So, too, the seal and lion, though both belonging to the carnivora, are readily recognized as distinct, but the seals may easily be confounded by the casual observer with the manatees, which belong to quite a different order. Thus it is with the Lepidoptera, for from six-legged insects, whose pupæ lie buried beneath the soil, like most moths, we pass to the highest butterflies, whose fore-legs are atrophied, and whose pupæ hang suspended in the open air; and this by easy intermediate stages. Surely, if six-legged mammals were the rule, we should look upon four-legged ones as very distinct; and this is the case with the butterflies. It is necessary to make this clear at starting, in order that we may appreciate to its full value the changes that have taken place in the insects under study. Butterflies (_Rhopalocera_) are grouped into four sub-families, as under:-- 1. _Nymphalidæ_, having the fore-legs rudimentary, and the pupæ suspended from the base of the abdomen. 2. _Erycinidæ_, in which the males only have rudimentary fore-legs. 3. _Lycænidæ_, in which the fore-legs of the males are smaller than those of the females, and terminate in a simple hook. 4. _Papilionidæ_, which have six perfect pairs of legs, and in which the pupæ assume an upright posture, with a cincture round the middle.