Insects and Disease 627 



wonderful as that interpretation is. The blast must evidently pass down 

 the ducts of the salivary gland into the wound made by the proboscis of 

 the insect, and thus cause infection in a fresh vertebrate host. 



'■ That this actually happens could, fortunately, be proved without difficulty. 

 As I had now been studying the parasites of birds for some months, I possess 

 a number of birds of different species, the blood of which I had examined 

 from time to time (by pricking the toes with a fine needle). Some of them 

 were infected, and some, of course, were not. Out of iii wild sparrows 

 examined by me in Calcutta, I had found H. relicta — the parasite which I 

 had just cultivated in Culex jatigans — in 15, or 13.5 per cent. As a rule, 

 non-infected birds were released; but I generally kept a few for the control 

 experiments mentioned above, and the blood of these birds had consequently 

 been examined on several occasions, and had always been found free from 

 parasites. At the end of June I possessed five of these healthy control birds — 

 four sparrows and one weaver-bird. All of them were now carefully examined 

 again and found healthy. They were placed in their cages within mosquito- 

 nets, and at the same time a large stock of old infected mosquitoes were 

 released within the same nets. By 'old infected mosquitoes' I mean 

 mosquitoes which had been previously repeatedly fed on infected birds, 

 and many of which on dissection had been shown to have a very large number 

 of blasts in their salivar)' glands. Next morning numbers of these infected 

 gnats were found gorged with blood, proving that they had indeed bitten 

 the healthy birds during the night. The operation was repeated on several 

 succeeding nights, until each bird had probably been bitten by at least a 

 dozen of the mosquitoes. On July 9 the blood of the birds was examined 

 again. I scarcely expected any result so complete and decisive. Every 

 one of the five birds was now found to contain parasites — and not merely 

 to contain them, but to possess such immense numbers of them as I had 

 never before seen in any bird (with H. relicta) in India. \Miile wild sparrows 

 in Calcutta seldom contain more than one parasite in every field of the micro- 

 scope, those which I had just succeeded in infecting contained ten, fifteen, 

 and even more in each field — a fact due probably to the infecting gnats 

 having been previously fed over and over again on infected birds, a thing 

 which can rarely happen in nature. 



'•The experiment was repeated many times — generally on two or three 

 healthy birds put together. But now I improved on the original experiment 

 by also employing controls in the following manner: A stock of wild sparrows 

 would be examined, and the infected birds eliminated. The remainder 

 would then be kept apart, and at night would be carefully excluded from 

 the bites of gnats by being placed within mosquito-nets. These constituted 

 my stock of healthy birds. From time to time two or three of these would 

 be separated, examined again to insure their being absolutely free from 



