Insects and us 



Bloodsucking Insects 

 Any insect that sucks the blood of man or of domestic animals is both 

 a nuisance and a danger. The bite itself is often painful, though not 

 always so. It used to be thought that the 'gadding' of cattle (when they 

 run wildly about with tail in the air) was caused by the pain of being 

 bitten by 'gad-flies', and though it is now believed that gadding is 

 caused by non-biting inseas, it is still true that cattle and horses in the 

 fields may be kept in a restless state that lowers their condition and 

 renders them more Hable to other infections. 



People, too, are tormented by large numbers of biting flies, and in the 

 brief summer of the Arctic it may be impossible to move about out of 

 doors without wearing special protective clothing against the attacks of 

 hundreds of flies. 



The immediate bite may not be painful, but it is often followed by a 

 painful itching and swelling that lasts for several days. This is generally 

 the result of the injection of saUva into the wound. The saliva of blood- 

 sucking insects contains a powerful anti-coagulin, which prevents the 

 blood from clotting, and so blocking the mouthparts of the insect. It is 

 this that produces the delayed symptoms after a bite. 



The reaction that follows an insect-bite varies in diff"erent people, 

 and in the same person under different conditions. Generally the first 

 bite has little effect, but after a short interval the person becomes 

 'sensitised', and any further bites after that cause violent symptoms of 

 swelling and pain. If the victim stays in the same area and goes on being 

 bitten frequently, the effect gradually diminishes, until complete 

 immunity is reached. 



This is why people who have just arrived in an area often complain 

 bitterly about the biting insects, whereas those who have been there 

 some time say that they are never bitten. In fact they are, but they no 

 longer show any reaction to the bites. 



Transmission of Disease 

 Any insect that bites one person for a time and then flies off and 

 immediately bites another is liable to carry the organisms of disease 

 from one to the other. This is called mechanical transmission, because 

 the insect's part is purely mechanical. You could get the same effect by 

 sticking a hollow needle, first into one person and then into the other. 

 Mechanical transmission of disease is very probable in theory, but 

 difficult to prove in practice. Usually we can only guess which inseas 

 probably carry the infection, because they are the most common on the 



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