46 DARWINISM AND THE PROBLEMS OF LIFE 



would be a mistake to imagine that every insectivorous 

 bird is useful. We have a large number of useful 

 insects. I need only mention the bee and the silk-worm, 

 and the insects that do us a service by destroying their 

 neighbours. Many a caterpillar succumbs to the large 

 running beetle, many a plant-louse is eaten up by the 

 larvae of the lady-bird, and it has been observed that 

 trees visited by ants do not suffer from caterpillar-blight. 

 But it is especially the ichneumon-flies, slender animals 

 with long antennae, darting constantly here and there, 

 that protect us from this blight. These insects have 

 what is called an "ovipositor" on the hind part of the 

 body, with which they stab the caterpillar, and deposit 

 their eggs inside it. The larvae of the flies develop from 

 the eggs in the flesh of their unfortunate host, and 

 gradually feed on its body from within. The process 

 goes on for a long time, the caterpillar continuing to 

 live and eat, and it is only when it reaches the chrysalis 

 stage that the larvae creep out, and enter on that stage 

 themselves near, or on, the dead covering of their 

 former host. Many a butterfly - collector has been 

 disagreeably surprised when he caught an apparently 

 sound caterpillar of a cabbage-butterfly in his cage, and 

 found one day an empty shell instead of the expected 

 chrysalis, with the yellow, oval cocoons of the deadly 

 enemy of the caterpillar beside it. 



The ichneumon-wasp is so thorough in its activity 

 that it must be put higher than the birds as a destroyer 

 of caterpillars. We must not, for instance, have too great 

 an idea of the work of the cuckoo, which was formerly 

 regarded as the chief agent of destruction of the 



