Seven orders of insects were originally recognized by Linnaeus, namely, Neuroptera, Diptera, Hemiptera, Lepidoptera, Coleoptera, Hymenoptera, and Aptera. This classification was based on the organs of flight only, and while in the main resulting in natural divisions which still furnish the basis of more modern classifications, was faulty in several particulars. For instance, the Aptera, which included all wingless insects, was soon found to be a very unnatural assemblage and its components were distributed among the other orders. The establishment of the order Orthoptera by Olivier to include a large and well-defined group of insects associated with the Hemiptera by Linnaeus, restored the original seven orders, and this classification has, in the main, been followed by entomologists up to the present time. [Illustration: FIG. 1.--Pyramid showing the nature of the mouth, and relative rank of the Orders, and the affinities of the Suborders of Insects.] All insects are, in a broad way, referable to one or the other of these seven primary orders by the structure of the wings and the character of the mouth-parts in the imago, and by the nature of their transformations. Some of these orders are connected by aberrant and osculant families or groups, which have by other authors been variously ranked as independent orders, but which, following Westwood substantially, I have considered, for convenience, as suborders. (_See_ Fifth Report, Insects of Missouri, etc., 1872.) In the article just cited, I made use of the accompanying diagram in the form of a pyramid (Fig. 1), which gives a graphic representation of the distinguishing characters and the relative rank as usually accepted, of the orders and suborders. Full discussion of the different classifications is unnecessary in this connection. Authors have differed in the past and will differ in the future as to what constitutes a natural system, and it would require many pages to give even a brief survey of the various schemes that have been proposed. As I have elsewhere said, "We must remember that classifications are but a means to an end--appliances to facilitate our thought and study--and that, to use Spencer's words, 'we cannot, by any logical dichotomies, actually express relations which in nature graduate into each other insensibly.'" The most philosophical, perhaps, of the more modern systems of classification is that of Friedrich Brauer, who has carefully studied the subject, and has given us an arrangement consisting of sixteen orders. This has many merits and has been adopted, with slight modifications, by Packard in his "Entomology for Beginners," and by Hyatt and Arms in their recent and valuable text-book "Insecta." Comstock, in his "Introduction to Entomology" strongly recommends Brauer's classification, but for reasons of simplicity and convenience adheres to a modification of the old classification of Westwood.