Ocular or oculo-cephalic buds 665 The antennal buds 665 The buds of the buccal appendages 665 The buds of the ovipositor 665 DEVELOPMENT OF THE IMAGO IN THE DIPTERA 666 _a._ Development of the outer body-form 668 Formation of the imago in Corethra 668 Formation of the imago in Culex 670 Formation of the imago in Chironomus 671 Formation of the imago in Muscidæ 673 _b._ Development of the internal organs of the imago 678 The hypodermis 678 The muscles 680 The digestive canal 681 The tracheal system 683 The nervous system 684 The fat-body 685 Definitive fate of the leucocytes 685 The post-embryonic changes and imaginal buds in the Pupipara (Melophagus) 686 _c._ General summary 687 HYPERMETAMORPHISM 688 SUMMARY OF THE FACTS AND SUGGESTIONS AS TO THE CAUSES OF METAMORPHISM 705 Theoretical conclusions; causes of metamorphosis 708 TEXT-BOOK OF ENTOMOLOGY [Illustration] PART I.—MORPHOLOGY AND PHYSIOLOGY [Illustration] POSITION OF INSECTS IN THE ANIMAL KINGDOM Although the insects form but a single class of the animal kingdom, they are yet so numerous in orders, families, genera, and species, their habits and transformations are so full of instruction to the biologist, and they affect human interests in such a variety of ways, that they have always attracted more attention from students than any other class of animals, the number of entomologists greatly surpassing that of ornithologists, ichthyologists, or the special students of any other class, while the literature has assumed immense proportions. Insects form about four-fifths of the animal kingdom. There are about 250,000 species already named and contained in our museums, while the number of living and fossil species in all is estimated to amount to between one and two millions.