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THE EVOLUTION THEORY 



one and the same animal have always seemed to me very remarkable; 

 for instance, the change of the food-instinct in the caterpillar and the 

 butterfly, where the food-instinct is liberated in the caterpillar by the 

 leaf of a particular plant, but in the butterfly by the sighf and 

 fragrance of a flower, the nectar of which it sucks. In this case 

 everything is different in the two stages of development, the whole 

 apparatus for seeking and taking food, as well as the nerve-mechanism 

 which determines these modes of action. ' And how far apart often 

 are the stimuli which liberate the instinct ! The larva of the flower- 

 visiting, honey-sucking Eristalis tenax is the ugly, white, so-called 

 rat-tailed larva, well described by Edaumur, which lives swimming in 

 liquid manure, and feeds on that ! What complete and far-reaching 

 changes, not only in the visible structure, but also in the finer 



Fig. 32. Metamorphosis of Siiaris humeraiis, an oil-beetle, after Fabre. 

 a, first larval form, much enlarged. 6, second larval form, c, resting stage of 

 this larva (so-called ' pseudo-pupa '). d, third larval form, e, pupa. 



nervous mechanisms, which we cannot yet verify, must have taken 

 place in the vicissitudes of time and circumstance during the life- 

 history of this insect ! 



• Not the food-instinct alone, but the instinct of self-preservation, 

 of mode of motion, in short, every kind of instinct, may vary in the 

 course of an individual life. Let us follow the somewhat complex 

 life-history of a beetle of the family of the Blister-beetles or 

 Cantharides, as we learnt it first from Fabre. The female of the 

 red-shouldered bee-beetle (Sitaris humeraiis) lays its eggs on the 

 ground in the neighbourhood of the underground nest of a honey- 

 gathering burrowing-bee {Anthophora). The larvse, when they emerge, 

 are agile, six-legged, and furnished with a horny head and biting 

 mouth-parts, as well as with a tail-fork for springing (Fig. 32, a). 



