4 TROPICAL MEDICINE AND HYGIENE 



Where arthropoda are essential transmitting agents, the 

 parasites, whether protozoa or metazoa, must develop to 

 a certain extent in them before being capable of further 

 development in man. 



In other cases insects are important as mechanical 

 carriers, " porters" of disease germs, usually bacteria. 

 Then they are but the mechanical means of conveying 

 organisms present in excreta to food used for human 

 consumption. Typhoid fever, cholera, &c., may be con- 

 veyed in this way. In other cases, such as plague, the 

 insects in this instance fleas feed on an infected animal, 

 then on others, which they infect with the germs derived 

 from the first. In these insects, though multiplication of 

 the germs may take place, they are not essential for in- 

 oculation, and the germs can be conveyed from man to 

 man in other ways under natural conditions. 



Aquatic larvae may acquire parasites, and in their adult 

 stage, as imagines distribute them. 



Venomous Metazoa. Many of the insects form definite 

 poisons or venoms, and these are then described as 

 venomous. The term is more usually applied to larger 

 animals, such as scorpions and certain snakes, in which 

 a considerable quantity of venom is formed, and this, 

 injected into man, may produce severe and even fatal 

 symptoms. The venom differs in character in different 

 species of these animals, and the effects in man are 

 diverse. 



