324 A BOOK OF INSECTS 



equally minute unicellular animals of the class Protozoa — 

 these parasites, by living and multiplying in the blood or 

 tissues of men and animals, giving rise to characteristic 

 symptoms. Still more recently it has been proved that 

 disease "germs," or microbes, may be carried from one 

 host to another by blood-sucking creatures of various 

 kinds ; and that in some instances the microbe actually 

 requires two different hosts for the completion of its life- 

 cycle. The sexual generation usually exists in an insect or 

 other invertebrate, and the asexual generation in a verte- 

 brate. The enormous importance of these facts in regard 

 to mankind was first established by Major Ronald Ross, 

 who (in 1897) definitely traced in the stomach of a mos- 

 quito the malaria parasite (discovered in human blood 

 by the French army surgeon Laveran in 1880), and two 

 years later worked out its complete life-history. Ross's 

 discoveries were subsequently confirmed by others, notably 

 by Professor Grassi, of Rome. 



Briefly stated, the life-cycle of the malaria parasite is 

 as follows : Excessively minute needle-shaped bodies are 

 introduced into human blood with the saliva of a mos- 

 quito. Each of these penetrates a red corpuscle, upon 

 the contents of which it feeds, and wherein it undergoes 

 a definite development — the original needle-shaped body 

 multiplying into a number of separate cells, or spores, as 

 they are called. Eventually the wall of the corpuscle 

 bursts, and all these spores, together with the waste pro- 

 ducts that have accumulated as a result of the digestive 

 process which has been proceeding, are liberated into the 

 blood-plasm — i.e. the colourless liquid in which the cor- 

 puscles float. It is this destruction of the corpuscles, and 

 the production of waste and poisonous substances, which 

 are the actual cause of malarial fever ; and as each newly 

 formed spore attacks a new red corpuscle, and rapidly 



