Much care has been taken to give due credit either to the original sources from which the illustrations are copied, or to the artist; about ninety of the simpler figures were drawn by the author, many of them for this work. For the use of certain figures acknowledgments are due to the Boston Society of Natural History, to the Division of Entomology, U. S. Department of Agriculture, through the kind offices of Mr. L. O. Howard, and to the Illinois State Laboratory of Natural History, through Professor S. A. Forbes and Mr. C. A. Hart. Professor W. M. Wheeler, of the University of Chicago, has kindly loaned for reproduction several of his original drawings published in the Journal of Morphology. A number are reproduced from figures in the reports of the United States Entomological Commission. PROVIDENCE, R. I., March 4, 1898. TABLE OF CONTENTS [Illustration] PART I. MORPHOLOGY AND PHYSIOLOGY PAGE POSITION OF INSECTS IN THE ANIMAL KINGDOM 1 RELATIONS OF INSECTS TO OTHER ARTHROPODA 2 The Crustacea 4 The Merostomata 5 The Trilobita 5 The Arachnida 6 Relations of Peripatus to insects 9 Relation of Myriopods to insects 11 Relations of the Symphyla to insects 18 Diagnostic or essential characters of Symphyla 22 INSECTA (HEXAPODA) 26 Diagnostic characters of insects 26 1. EXTERNAL ANATOMY _a._ Regions of the body 27 _b._ The integument (exoskeleton) 28 Chitin 29 _c._ Mechanical origin and structure of the segments (somites, arthromeres, etc.) 30 _d._ Mechanical origin of the limbs and of their jointed structure 35 THE HEAD AND ITS APPENDAGES 42 _a._ The head 42