I78 SOME SOUTH INDIAN INSECTS. ETC. [CHAP. XX. 



agency of insects has thrown an entirely new light on the stud 

 Tropical Medicine which bids fair to become to a very large extent 

 merely one branch of Applied Entomology. The modern systems 

 of treatment of Tropical Diseases tend more and more to resolve 

 themselves into the control of the insects which act as transmitters 

 of these diseases. In most cases {e.g., Malaria. Yellow Fever. 

 Filar - tna, Surra, etc.) the problem becomes an entomo- 



logical one. In the first stages of the investigation of such a 



, in considering the species of insect which is likely to prove the 

 transmitting agent, it is primarily the entomologist who. from his 

 knowledge of the insects found under the necessary conditions, is 

 likely to be best able to indicate the species to be suspected and 

 investigated. Later on, when this investigation is being carried 

 out, it is again the entomologist who knows where to look for such 

 an insect and how to capture it and who has the best knowledge of 

 methods of breeding it and what are its natural enemies and the 

 conditions favourable or otherwise to it- occurrence. Finally, when 

 the investigations have been crowned with success and a definite 

 relationship established between a particular insect and a disease, 

 the control of the disease is often attained most simply bv tin- 

 control of the insect and this again falls primarily into the pro- 

 vince of Entomology. 



To cite a specific instance, the case of Malaria may be con- 

 sidered briefly. The term malaria is used to cover three distinct 

 specific fevers, quartan, tertian and subtertian malaria, caused by 

 invasion of the blood of man by three different protozoan parasites 

 which are called respectively Plasmodium malaria, PI. vivax, and 

 rania malaria, the attack of the (ir>t two causing a compara- 

 tively mild type of malaria, that of the last-named usually giving 

 rise to a severe form of infection. These parasites, which are not 

 known to occur in any other vertebrate animal, are introduced into 

 the blood of man by the bite of an infected Anopheline mosquito 

 and may either (I) be killed off by the blood on entry into the bodj 

 or (2) remain dormant in the spleen and develop later on when 

 Opportunity is offered by any diminution of vitality on the part of 

 the subject attacked, as by a chill, or (3) develop forthwith and 

 give rise to an attack of malaria. In the last case, it generally 

 take- about nine to twelve daj s for the parasites to increase suffi- 

 iduce sufficient toxic (poisonous) substances 

 to give rise to fevei. Plasmodium malaria (the parasite of quartan 

 malaria) and P. vivax (the cause ol tertian malaria) go through their 

 developmental stages in the circulating blood, whilst Laverania 

 malaria (the parasite causing subtertian malaria) sporulates to a 



