IXFLUKNCK OF C'Ll.MATE OX ANIM.VL lJlSi;ASr.. 49 1 



ni)t present under the conditions of a lon_i^- i)eriiul of (lr\- weather. 

 and it is important that moderate rains are more favoiu"able to 

 their increase than very heavy torrential rains, whicli latter may 

 even be harmful through washing out small ])ools and vessels, 

 thus destroying large numl)ers of larv;e and i)U]);e. 



Finally, we ma}- (|Uote his remarks th.at although not all of 

 the most[uitoes ie^cl at night, and manv Anoy^heles commence to 

 bite before sunset, yet the rnajoritv of tlie Culicidre are known 

 to be of noctiu'nal feeding habits. 



These facts, then, are sufficient to indicate that the climatic 

 conditions which are favourable to the i)resence of horse-sick- 

 ness and blue tongue are those which at the same time are fav- 

 ourable to the development of the moscphto, and that these latter 

 insects are ver\- ])revalent in areas wiiere the diseases occiu" has 

 been a matter of observation. Even the remark made above as 

 to the harmful eilect of torrential rains on tlie develoi^ment of 

 the moscpiito also has an im])or;ance to us, since it has been 

 observed that following extremel)- heavy rains the pre\-alen_ce of 

 the diseases may be rem]^orarih- checked. 



Thus we see, then, that through accepting these diseases as 

 being mosc(uito-borne. we can explain the points noted ])reviously 

 in regard to their distribution and i^revalence. The fact remains, 

 however, that ex])erin.iental verihcation of this theoretical ex- 

 planation is still lacking, and that some insect other than the mos- 

 quito may eventually be ])roved to be the culprit in transmitting 

 the disease. Even if such should eventuall}-. however, jjrove to 

 be the case, it will be seen that this will most probablv prove to 

 be an insect with habits ver\' similar to that of the mosquito, as 

 have been described above, auvd with the conditions influencing" 

 its distribution ])ractically the same as those aj^plving to the 

 members of the Culicidae grou]). 



There seems little doubt that time and further ex])eriments 

 \y\\\ furnish us with the direct jn'oof that both horse-sickness 

 and blue tongue are diseases which are insect-borne, and 

 that is the reason that they have been included in this section 

 of this present paper, and there would also seem to be little doubt 

 that, in a similar way, we shall find that other of om* South 

 African diseases are transmitted in a like manner. In the case, 

 for instance, of ephemeral fever, a disease due to a filterable 

 virus, there is already strong presumi)tive evidence for believing 

 it to be insect-borne in this countrv. and in the case of another 

 disease due to a virus of the filterable group, namely, equine 

 infectious anaemia, recently shown by Theiler and the writer 

 to be present in South Africa, the evidence coming from Japan 

 points most strongly to the disease being transmitted chieiiy 

 through insect agency, the particular flies on which suspicion 

 rests in that country being included within the group of the 

 Tabani(Ue. It is therefore ]:)ossible that in .South Africa insects or 

 ArachnidcC ma>' be found capable of transmitting this disease. 

 in addition to the mode of transmission, through the ineestion 

 of the urine or material contanu'nated bv the urine of aft'ected 



