FILAEIA SANGUINIS HOMINES IN THE MOSQUITO. 379 



I believe the Filaria, after it has attained this final stage, has hut a very short time 

 longer to pass in the mosquito. I am inclined to think it preys on the tissues of the 

 insect, and thereby contributes to its death. For, in examining mosquitoes found dead 

 on the surface of the water, and which I know could have died only a very short time 

 before, I have been struck with a singular absence of viscera and muscular tissue — the 

 thorax seemed but a hollow shell. Again, some mosquitoes, even after death, cling or 

 stick to the sides of the vessel containing them ; in such specimens, also, I have remarked 

 an absence of viscera and muscular tissue. My impression is, that these have been con- 

 sumed by the Filaria, that the death of the insect had been thereby hurried, and that 

 when this occurred the parasite bored its way out of the body, and thence sped to the 

 water. I have never found a Filaria in the last stage of the metamorphosis in a dead 

 mosquito ; often, however, I have found them at all stages but this, even up to the time 

 when they measure the fortieth or the thirtieth of an inch in length. The final stretching 

 process must be rapidly effected, lasting probably an hour or two ; and, as during it the 

 Filaria becomes more than double in size, there must be expenditure of the tissues of 

 the intermediary host to provide pabulum for this rapid growth. Hence, in some 

 measure, it comes about that one rarely catches the Filaria in the last stage of growth, 

 and that the dead mosquito has an empty thorax. 



Such, I believe, is a true description of the life of Filaria sanguinis hominis in the 

 mosquito. Some of my interpretations may be wrong, but all the principal facts I have 

 stated are true. I have verified them over and over again. 



I can understand how, if fig. 1 and fig. 34 are compared, some doubts may arise as to 

 their connexion. The contrast between fig. 1 and fig. 23, or between fig. 23 and fig. 33, 

 is great — so great that unless one had carefully traced the connecting links step by step, 

 one would positively declare them organisms of entirely different species. Over and 

 over again, when working at the Filaria of the fourth or fifth day, I have hesitated to 

 believe it was really the outcome of the Filaria I had seen in the abdomen of the 

 mosquito an hour or two after feeding; and I have turned to mosquitoes of the first, 

 second, and third days to reassure myself of the migration to the thorax, the differen- 

 tiation of the tail, the swelling of the body, the formation of the mouth, anus, and 

 alimentary lines before I was at my ease as to the genesis of the particular organism 

 that I was specially engaged in studying. But now, to my mind, there does not exist the 

 slightest doubt in the matter. 



Besides being able to trace the gradation in mosquitoes of different ages, we often 

 encounter specimens of the Filaria, at all stages of development from fig. 3 to fig. 14, in 

 one and the same mosquito. Sometimes in an insect in which a Filaria like fig. 14 is 

 found, many others, at all stages of development, from fig. 14 to fig. 24, may be encoun- 

 tered. And so with the latest stages : fig. 23 may be found in a slide in which fig. 33 is 

 moving. This gradation is very striking, and clearly indicates the connection between 

 the Filaria ingested by the mosquito and the Filaria about to leave it. 



The most difficult step to follow, and the one over which Lewis apparently hesitated, 

 is that represented from figs. 4-12. If, however, insects of from twenty-four to forty- 

 eight hours after feeding are studied, transition-forms are found in abundance. Eigs. 40, 



