similarity of habit; and _Cœlioxys_ to the constructive _Megachile_. None of these parasites resemble their sitos, but _Nomada_ is exceedingly different, being in its gay array more like a wasp than a bee. The only close approach in the appearance of a parasite to the insect upon which it is parasitical is in the resemblance between _Apathus_ and _Bombus_, which are so alike that they were long continued to be united in the same genus, until the peculiar characteristic of the parasitical bees was detected, when they were readily separated. Although, cuckoo-bees as they are familiarly called, they could not be associated with the _Nudipedes_, because their posterior legs, though not pollen-conveying organs, are hairy; but the _Cenobites_, to which section they belong, have a peculiar and distinguishing structure of that limb. They are further separated from the _Nudipedes_ by several frequenting the same nest, thus habitually associating with their sitos. Some of the _Chrysididæ_ are likewise, as I shall have occasion to notice in the description of the habits of the genera, similarly parasitical upon some of the species of the family of bees. The genus _Mutilla_ is also probably entirely parasitical upon bees, for _Mutilla Europæa_ is a parasite upon _Bombus lapidarius_, from whose nests it has been dug in winter, by my friend the late Mr. Pickering, whose activity and accurate observation once promised to be very beneficial to the science, but he, like many others of my entomological friends, is now no more! ------------------------------------------------------------------------ CHAPTER VI GENERAL PRINCIPLES OF SCIENTIFIC ARRANGEMENT. THE following rapid observations are addressed to those whom it is the desire that this series of volumes may induce to take up the study of Nature in a methodical manner. With this view, the merest summary of the principles upon which scientific arrangement is based, is here exhibited. The study requires method as a lodestar to guide through its intricacies, but it is one which, pursued simply as a recreation, yields both much amusement and gratifying instruction. It shows us that when we unclasp the book of nature, and wherever we may turn its leaves, every word, the syllables of which we strive to spell, is pregnant with the fruitfulness of wonderful wisdom, whose profound expression the human intellect is too limited thoroughly to comprehend.