490 IXFLL'KNCE Ui" CLIMATE ON ANIMAL DISEASE. 



tions wliere the natural mode of transmission was known. Imt in 

 the case of both horse-sickness and blue tongue the manner in 

 which the disease is transmitted in nature still remains to be 

 definitely determined. We can. however, find an explanation 

 in accepting the hypothesis which, amongst those acquainted 

 with the problems that both of these diseases ofifer. is regarded 

 as being so extremely feasible as to be very ])robably 

 correct, though still lacking in direct experimental verification. 

 This hypothesis is one which I am siu'e that most of you have 

 heard of before, and is that both diseases are transmitted by 

 winged insects of noctinmal habits, and most probably n^.embers 

 of the group of Culicidas or mosquitoes. 



Now in accepting this theory as we do here it is n_ot neces- 

 sary to go into the evidence in support of it, although this would 

 be extremely interesting in itself. Such a consideration would 

 occupy much time, and it does not seem quite necessary to do 

 so. For our purposes it is necessary to see whether, having 

 accepted the theory, we can ex])la]n the influence of the climatic 

 and tellurical factors on the distribution of the disease. This 

 ultimately, then, really resolves itself into a consideration of how 

 these factors influence the transmitting agent, which in this 

 case is sup])osed to be one of the CulicidcC. Let us. then, 

 enquire into the bionomics of this grouj) in order to see what 

 information Ave gain in this resiiect. 



NoA\', if we follow Theobald on this i)oint, we find that the 

 mosquitoes are of wide-world distribution, the majority Ijeing 

 vegetable feeding (the blood-sucking varieties being in the 

 minority), and that in the life-cycle four stages are recognised, 

 namely, egg, larva, pupa, and adult, or imago. That water or 

 dam]) mud is absolutely essential to their development he shows 

 in ])ointing out that it is only in ])laces oft'ering such that the 

 larval and ptipal stages can be ])assed, and that water influ- 

 ences the distribution of the adult form is seen from the fact 

 that the adults occur in greatest abtmdance in damp marshy 

 places, along river coiu'ses, and the borders of large lakes, and 

 althougli they may be met with in some bare, rocky spots, yet 

 even here water determines their i)re valence, since the_\' must 

 for breeding purposes use the small collections of water such as 

 are formed in the hollows of rocks or boulders. He points otit 

 also that heat imi^ortantb' influences the rate of their develop- 

 ment, acting more marked^ on the puj^al than on the larval 

 stages, and he remarks that in warm climates mosquitoes may 

 continue to breed all the year round in small niunbers, although 

 during the dry seasons they rest, and that in temperate and cold 

 regions they pass the winter as either adults. larv;e. or ova. 

 Altitude he refers to as offering no protection against their pre- 

 sence, and instances the fact that they swarm at a height of 

 13,000 feet in the Himalayas. Discussing the effects of weather 

 on mosquitoes, he points out that they are very susceptible to 

 weather changes, and that a certain rainfall is necessary for their 

 development, this giving them the breeding facilities whicli are 



