THE BEHAVIOUR OF INSECTS 123 



twists the severed portion into a compact screw or funnel, 

 within which the eggs are deposited, finally closing the 

 orifice by tucking in the tip of the leaf. All these opera- 

 tions are performed in a characteristic manner, with little 

 or no variation, just as if the weevil had served a long 

 apprenticeship to leaf-rolling and had acquired unerring 

 proficiency in the art. Yet the eggs, which hatch in the 

 rolled-up leaf, produce blind, legless grubs, which when 

 full-grown fall to the ground, where they change to pupae 

 and lie hidden throughout the winter ; so that the perfect 

 weevils, which come forth in the early summer and creep 

 up the birch stems, cannot possibly have seen a rolled-up 

 leaf, much less have witnessed the method of construc- 

 tion. Experience and memory manifestly play no part 

 in the insect's behaviour. It is purely instinctive. 



The nervous mechanism whereby a complicated in- 

 stinct is performed has been slowly built up from small 

 beginnings through the agency of natural selection, exactly 

 as the outward form and coloration of the insect have 

 been adapted to its needs and environment. In other 

 words, an insect is not only equipped at birth with an 

 elaborate system of nerves and sense-organs, but it in- 

 herits also a complete set of stereotyped habits. These 

 are often performed with such unerring precision that 

 they appear rational in a high degree. But in face of 

 unaccustomed circumstances most insects are com- 

 pletely nonplussed. A species of digger-wasp (Sphecc) 

 habitually drags its victim — a long-horn grasshopper — by 

 one antenna. The great French naturalist, J. H. Fabre, 

 cut off both antenna, with the result that the wasp, after 

 vain efforts to secure its customary hold, abandoned its 

 prey. Yet it might easily have dragged the grasshopper 

 by one of its legs. Another instance which illustrates 

 this inflexibility of instinct may be quoted from the writ- 



