Insects and their world 



Fig. 77. Beneficial insects. An Arum is pollinated by small flies of the 

 family Psychodidae. Photo by the late J. B. Bradbury 



Fortunately, parasitic insects do not often carry human disease. Out- 

 standing exceptions are plague, which is carried by a rat-flea and typhus 

 carried by lice. Ticks carry a number of infections, but we can exclude 

 these as not being insects, though in appearance and habits they con- 

 form more with the insects than with any other group of animals. The 

 irritation set up by parasitic insects can be very debilitating, and the 

 wounds and sores that they cause may easily become infected with 

 bacteria. The maggots of some flies can be dangerous, because they are 

 able to destroy so much of the living tissue of their host. 



Beneficial Insects 

 It is easy to forget that there are 'good' insects as well as 'bad' ones. 

 The honey-bee, the silkworm moth and the lac insect are obvious 

 examples, where the insea produces something that we eagerly collect 

 and use. 



Perhaps the greatest benefit that we get from insects is their help in 

 polUnating flowers. Not only honey-bees, but a great many wild bees, 

 flies and other inseas, go from blossom to blossom in search of nectar, 

 and in doing so carry pollen from one plant to another. This is a more 

 precise alternative to the wasteful method of relying on haphazard 

 pollination by wind, and is the reason for the evolution of flowers. 

 Everyone takes the beauty of flowers so much for granted that it is hard 

 to reaUse that if there had never been any pollinating insects there 

 would be no flowers. The flowering plants and the higher insects — 

 especially Lepidoptera, Hymenoptera and Diptera — evolved together. 



130 



