_c._ Morphology and homologies of the tracheal system 442 _d._ The spiral threads or tænidia 444 _e._ Origin of the tracheæ and of the “spiral thread” 447 Internal, hair-like bodies 451 _f._ The mechanism of respiration and the respiratory movements of insects 451 _g._ The air-sacs 456 The use of the air-sacs 457 _h._ The closed or partly closed tracheal system 459 _i._ The rectal, tracheal gills, and rectal respiration of larval Odonata and other insects 463 _j._ Tracheal gills of the larvæ of insects 466 Blood-gills 475 _k._ Tracheal gills of adult insects 476 THE ORGANS OF REPRODUCTION 485 _a._ The male organs of reproduction 494 The testes 495 The seminal ducts 496 The ejaculatory duct 497 The accessory glands 497 The spermatozoa 497 Formation of the spermatozoön 498 _b._ The female organs of reproduction 500 The ovaries and the ovarian tubes 500 Origin of incipient eggs in the germ of the testes 504 The bursa copulatrix 505 The spermatheca 506 The colleterial glands 506 The vagina or uterus 507 Signs of copulation in insects 507 PART II. EMBRYOLOGY OF INSECTS _a._ The egg 515 Mode of deposition 518 Vitality of eggs 520 Appearance and structure of the ripe egg 520 The egg-shell and yolk-membrane 520 The micropyle 522 Internal structure of the egg 524 _b._ Maturation or ripening of the egg 525 _c._ Fertilization of the egg 525 _d._ Division and formation of the blastoderm 526