CHAPTER VI 



TRACHEATES 



To the tracheates belong spiders and insects. How insects grow. 

 Explanation of the metamorphoses of insects. Protective 

 colouring on the wings of butterflies. The Lamarckian principle 

 refuted by protective colours. Insects that resemble objects. 

 Mimicry. Exhalation from male butterflies. Sexual selection. 

 Origin of flowers due to insects. Parts of the insect's mouth. 

 Refutation of the Lamarckian principle. The coat of insects 

 cannot have arisen by use. Harmonious adaptations, co- 

 adaptations. Co-adaptations that Lamarck cannot explain. 

 Explanation of co-adaptations. Are instincts inherited habits? 

 Instincts that can never be affected by the will. Spiders' webs. 

 Care of the young. Instincts that are only used once. Are 

 mutilations inherited ? Protective marks, mildew marks, fore- 

 sight. Infection of embryo. Structure of the embryo. The 

 inheritance of acquired characters is difficult to conceive. 

 Untenability of the Lamarckian principle. 



WE now turn to the animals that we meet most fre- 

 quently at every turn. These are all alike in having 

 the body covered externally with hard parts and jointed, 

 like the legs of the "articulates," as we call the stem to 

 which they belong. Some of these animals breathe by 

 means of gills ; these are the crustaceans. The others 

 breathe by means of what are called tracheae ; to this 

 group belong the spiders and insects, which will engage 

 our attention in this chapter. The tracheae are a system 

 of greatly ramified tubes, which end in a network of 

 very fine microscopic branchlets, and pervade the whole 



animal. The air enters into the tracheae by external 



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