150 



THE EVOLUTION THEORY 



Diie aiul tlu' sauif aiiiiiial have always seemed to me very remarkable ; 

 for instaiu'i', tlu' clian<jjo of the food-instinct in the caterpillar and the 

 huttcrth", wluTi' iW food-instinct is liberated in the caterpillar by the 

 leaf of a particular plant, Imt in the butterfly by the sight and 

 frao-rance of a flower, the nectar of which it sucks. In this case 

 evervthin*'- is difterent in the two stages of development, the whole 

 a})paratus for seeking and taking food, as well as the nerve-mechanism 

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

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

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

 rat-tailed larva, well described by Reaumur, 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 humemlis, an oil-beetle, after Fabre. 

 ((, first larval form, much enlarged, b, 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 humemlis) lays its eggs on the 

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

 gathering burrowing-bee {A'nthoiyhora). The larvae, 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). 



