It may be easily understood that such differences in the estimation of the primitive characters of the embryology of a group may arise. Embryology has been compared by one of the greatest of modern embryologists to "an ancient manuscript with many of the sheets lost, others displaced, and with spurious passages interpolated by a later hand." What wonder is it that different people examining such a record should come to different conclusions as to the more doubtful and difficult portions of it. It is this very difficulty which makes the principal interest in the study, and although our knowledge of the language in {80}which this manuscript is written is as yet imperfect, still we hope that constant study may teach us more and more, and enable us to read the great book of nature with more and more ease and certainty. If any of my readers should wish for a more full account of the natural history of this group I must refer them to the following works, which I have used in compiling the above account. In the first of these there is an excellent bibliography of the subject:— LATZEL, Die Myriapoden der Oesterreichisch-Ungarischen Monarchie, Wien, 1880. ZITTEL, Handbuch der Palaeontologie, 1 Abth, II. Bd., Leipzig, 1881-1885. KORSCHELT AND HEIDER, Lehrbuch der vergleichenden Entwicklungsgeschichte der wirbellosen Thiere, Jena 1891. INSECTA BY DAVID SHARP, M.A., M.B., F.R.S. {83}CHAPTER III CHARACTERISTIC FEATURES OF INSECT LIFE–SOCIAL INSECTS–DEFINITION OF THE CLASS _INSECTA_–COMPOSITION OF INSECT SKELETON–NUMBER OF SEGMENTS–NATURE OF SCLERITES–HEAD–APPENDAGES OF THE MOUTH–EYES–THORAX–ENTOTHORAX–LEGS–WINGS– ABDOMEN OR HIND BODY–SPIRACLES–SYSTEMATIC ORIENTATION. Insects form by far the larger part of the land animals of the world; they outnumber in species all the other terrestrial animals together, while compared with the Vertebrates their numbers are simply enormous. Yet they attract but little attention from the ordinary observer, this being probably primarily due to the small size of the individual Insect, which leads the unreflecting to treat the creature as of little importance. "It can be crushed in a moment" is perhaps the unformulated idea that underlies the almost complete neglect of knowledge concerning Insects that prevails even in the educated classes of society. The largest Insects scarcely exceed in bulk a mouse or a wren, while the smallest are almost or quite imperceptible to the naked eye, and yet the larger part of the animal matter existing on the lands of the globe is in all probability locked up in the forms of Insects. Taken as a whole they are the most successful of all the forms of terrestrial animals.