THE STRUCTURE AND LIFE OF INSECTS 253 



tropical countries and our native clothes moths are 

 notorious for the mischief which they do. Certain insects 

 cause great damage by their attacks on cattle, sheep and 

 horses, while a few are harmful or even deadly to man 

 himself. It has been discovered of late years that malarial 

 fevers are due to the bite of a gnat. When it pierces the 

 skin to draw blood, the gnat introduces a microscopic 

 parasite from its own salivary gland. In human blood 

 the parasite multiplies prodigiously, and by penetrating 

 the blood-corpuscles sets up the fever. Other gnats in 

 turn become infected by drawing blood from malarious 

 patients, and so the round is kept up. It is probable 

 that several formidable diseases are propagated by different 

 insects. 



BENEFITS RECEIVED FROM INSECTS. 



The list of benefits conferred by insects is not so long, 

 but it includes some that we could ilf spare. Insects are 

 one great agent for the destruction of corrupting sub- 

 stances of many kinds. In visits to sewage-works I have 

 been struck by remarking how much putrid matter is 

 turned into small flies, and scattered harmlessly over the 

 face of the country. Insects yield the favourite food of 

 many birds and fishes which we prize as useful or agree- 

 able. Insects yield honey, wax, cochineal, lac and silk. 

 But I suppose that the chief benefit which we draw from 

 the existence of insects springs from their activity in the 

 fertilisation of flowers. Many useful and beautiful plants 

 would cease to ripen seed at all, if it were not for the 

 visits of insects. 



THE NUMBERS OF INSECTS. 



More insects have been described by naturalists than 

 animals of all other kinds put together, and many sorts 

 of insects are extremely plentiful, so that it is not unlikely 

 that a majority of the animals now living on the surface 

 of the globe are insects. The only doubt relates to 

 microscopic creatures, far smaller even than insects, and 



