VIII. INSECTS AND US 



Entomologists are often asked : ' What good are insects ? ' It is dis- 

 concerting to give an enthusiastic account of the appearance or the 

 behaviour of some exotic insect, and then to be met with a puzzled 

 frown and the comment : ' Yes, that is remarkable, but what good does 

 it do?' 



There is no foundation for the common behef that inseas must be 

 here for some purpose. They just exist, and their existence is a challenge 

 to us to find out more about them, in the same way that people want to 

 climb Mount Everest 'because it is there'. 



All the same, the lives of inseas cross with ours at many points, 

 and when they do so, their activities may be a help or a hindrance to 

 us. In our self-centred way we then classify them as beneficial or harm- 

 ful insects, and call them 'Insect friends and foes'. 



Harmful Insects 

 The damage caused by insects is generally an outcome of their feeding 

 habits. Like other groups of animals, a very large number of inseas 

 feed on plants or on vegetable juices; another large group feed upon 

 rotting materials of either vegetable or animal origin; a third group 

 are carnivorous, feeding upon the herbivorous insects; and a fourth 

 group are parasites, feeding at the expense of bigger animals, which 

 they do not normally kill, but suck their blood, or destroy their tissues. 



Cultivated crops 



Plant-feeding insects do a tremendous amount of damage to cultivated 

 crops (Figs. 13, 20, 66, 67, 73). The principal leaf-eaters are the grass- 

 hoppers (especially the gregarious locusts), and the caterpillars of the 

 Lepidoptera and of the sawflies (Hymenoptera). These insects have sur- 

 prisingly large appetites, and they are especially destructive because they 

 are liable suddenly to appear in very large numbers, and so to devour 

 enough of the foliage at the same time to set back the growth of the 

 plant, or even to kill it altogether. In Chapter VI we have seen how 

 locusts suddenly break out in migrating swarms of milhons of indi- 

 viduals, and can strip the vegetation in one place overnight. 



Less is known about the cause of 'plagues' of caterpillars, though 

 every gardener has experienced one at some time or other. It seems 



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