Insects and their world 



animals concerned. Certain diseases of the eyes and skin in the tropics 

 must clearly be spread by the flies that are seen settUng on the affeaed 

 areas. 



Some of the most dangerous organisms of disease, however, spend a 

 definite part of their life-cycle within the insect vector, or carrier, and 

 indeed are unable to complete their growth and development without 

 this alternative host. This sequence is called cycHcal transmission, and 

 the insect veaor is an essential part of it. 



By far the most important insect vectors of disease by cyclical trans- 

 mission are the mosquitoes. The diseases they carry include malaria in 

 all its forms ; infections by parasitic worms called filariasis, of which 

 elephantiasis (a grotesque swelling of the body) may be a result; yellow 

 fever, and a number of other virus diseases (encephaUtis and encephalo- 

 myelitis), of which human 'sleepy sickness' is one. 'Sleeping sickness', 

 on the other hand, is a different disease, caused by a protozoan, or 

 single-celled animal, which is cyclically transmitted in tropical Africa 

 by the tsetse fly. 



In each of these diseases the organism concerned is present in the 

 blood of the person bitten, and is picked up by the biting insect. Inside 

 the insect, the disease organism moves about through various organs, 

 and finally reaches either the salivary glands or the cavities of the head 

 near the base of the proboscis. During this migration, the organism 

 changes shape, and develops into what is called an 'infective form', and 

 when the insect bites another person, some of the infective forms are 

 passed on to him, thus giving him the disease. 



Thus an insect that has bitten an infected person is not able to pass 

 on the disease to someone else until there has been time for the disease 

 organism to develop into an infective form. This may take eight days 

 for the malaria organism in a mosquito, or twenty days for the organism 

 of sleeping sickness in a tsetse fly. From then onwards the fly may 

 remain infective for most of its life. 



In a similar way, the insects that suck the sap of growing plants play 

 their part in spreading many serious diseases. 



Parasitic Insects 

 These are insects that do not merely attack man or other animals, but 

 which remain attached to their 'host' for long periods. Fleas, Hce and 

 bed-bugs are external parasites; internal parasites include the maggots 

 of various flies, such as the warble fly of cattle, the bot-flies of horses 

 and sheep, and the maggots of many other flies that may get into the 

 himian body more or less by accident. 



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