436 THE BIOLOGY OF INSECTS 



The vital connection of mosquitoes with yellow fever 

 and malaria has become known only during the present 

 century, but it is nearly fifty years since P. Manson (1878) 

 demonstrated that Culex qimiquefasctatus and other allied 

 mosquitoes harbour and transmit the larva of a threadworm, 

 Filaria bancrofti^ which is often very abundant in human 

 blood, and may become fully grown and mature in man's 

 body. Young larval Filaria are sucked into the stomach of 

 the mosquito when it feeds on blood, and migrate thence 

 into the insect's muscles and on to the base of the proboscis, 

 so that, after some weeks' development in the Culex acting 

 as an alternative host, they can be injected into the body of 

 a human being wherein they may attain maturity. As the 

 full-grown Filaria are several inches long, and produce large 

 numbers of eggs, their presence in numbers may " obstruct 

 the lymphatic canals and cause enormous swellings of feet, 

 legs, and arms or other parts," symptoms of the horrible 

 tropical disease known as '' elephantiasis." 



One of the most dreaded diseases that has ever affected 

 mankind, the Oriental '' Plague," which made its first 

 appearance in Europe in the sixth century a.d., became 

 notorious in the Middle Ages as the *' Black Death," and 

 carried off thousands of London citizens in the great 

 epidemic of 1665, is now known to be spread largely through 

 the agency of blood-sucking insects. Long ago Indian 

 writers noticed that human plague was accompanied or 

 preceded by a heavy mortahty among rats, and the mention 

 of '' golden mice " as part of the offering made by the 

 Philistines after an outbreak of pestilence recorded in the 

 Old Testament (i Sam. vi. 4), has been regarded by some 

 students as indicating similar observations. The causal 

 organism of plague. Bacillus pestis^ a member of the bacterial 

 group which contains many deadly germs, was not detected 

 until the year 1894, and this bacillus was found to cause 

 fatal *' bubonic " disease in rats as well as in human beings. 

 Very soon investigators reaHsed the probabiHty that blood- 

 sucking insects were instrumental in carrying the plague 

 organism from rats to men, and it was detected in bugs, 



