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CHAPTERS ON EVOLUTION. 



times within the larval skin, or it may be (as in butterflies and moths) 

 within a special case or cocoon (b\ the chrysalis passes its existence, 

 which, however quiefand apparently unimportant, externally viewed, 

 is nevertheless marked by a wonderful activity inside. There the 

 elements and nutrient parts of the larva, accumulated during its 

 season of epicurean enjoyment, may be practically broken down, 

 and rebuilt to form the body of the perfect insect, as in some flies, 

 or more gradually changed into the adult organs, as in the butterflies. 

 As Sir John Lubbock succinctly puts it, "the change from the 

 caterpillar to the chrysalis, and from this to the butterfly, is in reality 

 less rapid than might at first sight be supposed ; the internal organs 

 are metamorphosed very gradually, and even the sudden and striking 

 change in external form (from the chrysalis to the perfect insect) is 



FIG. 172. METAMORPHOSIS OF SWALLOW-TAILED BUTTERFLY. 

 a, larva ; b, chrysalis ; c, imago, or perfect insect. 



very deceptive, consisting merely of a throwing-off of the outer skin 

 the drawing aside, as it were, of a curtain, and the revelation of a 

 form which, far from being new, has been in preparation for days, 

 sometimes even for months." 



In the metamorphosis of certain of the flies c.g. the flesh-flies 

 the changes are in reality much more sweeping than in the butterflies, 

 although perhaps less apparent than in these brilliant members 

 of the class. The body of the maggot or larval fly contains, 

 when it leaves the egg, a number of curious rounded structures 

 named imaginal discs. Some twelve of these are placed in the 

 young insect's chest-region four in each segment and two are 



