548 TEE POPULAR SCIENCE MONTHLY 



sible for the transmission of a parasitic disease of the tropics known as 

 filariasis. The direct cause is a nematode worm belonging to the genus 

 Filaria which is present in the circulation and lymphatics of the infected 

 person. In the late stages of the disease the microscopic larval worms 

 occur abundantly in the blood. For some unexplained reason they 

 remain in the deep-seated blood vessels during the day, but usually 

 appear more abundantly in the peripheral circulation during the night. 

 Here they are readily obtained by mosquitoes with their meal of blood. 

 In the alimentary canal of the mosquito the larval Filaria discards a 

 sheath-like envelope which has previously invested it, and works its 

 way through the wall of the stomach into the thoracic muscles where it 

 increases greatly in size and finally migrates to the base of the proboscis. 

 From two to three weeks are necessary for this metamorphosis, and for 

 some time longer the Filaria may remain in the proboscis awaiting its 

 opportunity to enter another person through the wound occasioned by 

 the mosquito's beak. Once they have been transferred to their human 

 host, the parasites enter the lymphatics where they attain sexual 

 maturity and give rise to the abundant microscopic larval Filarias that 

 reenter the circulation to await ingestion by another mosquito. 



Filariasis is most common in equatorial regions, but extends less 

 commonly into the subtropics. The parasites themselves do not ordi- 

 narily cause great inconvenience, but their presence in the lymphatics 

 may clog these vessels to such an extent that secondary swellings may 

 be developed in the limbs or other parts of the body. 



Several insects have been associated with a peculiar tropical disease 

 of the old world, variously known as kala-azar, dum-dum fever and 

 leishmaniosis. In this case the organism is a flagellate protozoan, 

 Leishmania donovani, of which there are possibly two forms, one pro- 

 ducing a children's disease termed infantile kala-azar and the other the 

 true leishmaniosis of adults. It has been shown that among domestic 

 animals the dog at least is susceptible, and other animals may be also. 

 In 1907 Patton discovered various stages of the Leishmania parasites 

 in bed-bugs that had fed on persons suffering from kala-azar and this 

 insect has been considered to be one, if not the exclusive, carrier. Very 

 recently, however, some doubt has been expressed regarding the role of 

 the bed-bug and a certain reduviid bug has been suspected. There seems 

 to be no doubt, however, that kala-azar is insect-borne. 



The diseases which we have enumerated are the more important 

 insect-borne ones that affect man. A number of others of greater rarity 

 or of minor nature are known to be carried regularly or occasionally by 

 various insects, and many others are now being investigated in the light 

 of present knowledge to ascertain if they, too, may not be spread by 

 insects. It seems probable that flies may take some part in the dis- 

 semination of the bacilli of leprosy, although to how great an extent 



