378 DR. P. MANSON ON THE METAMORPHOSIS OP 



the anus backwards, and its very extremity is covered with the three or four papilla just 

 described. Faint indications of an alimentary canal are at times discernible, but further 

 than this no trace of internal structure can be clearly made out. The parasite has now 

 arrived at the sixth and last stage of its metamorphosis, and its stay in the mosquito is 

 about concluded. 



Such specimens as I have been describing as belonging to the fifth stage are to be 

 found only about the end of the sixth, and beginning and middle of the seventh day, 

 from 130 to 156 hours after ingestion. The Filaria of the sixth stage I have only seen in 

 mosquitoes from 156 to 160 hours after capture, probably about seven days after feeding. 

 Most mosquitoes lay their eggs and die during the fifth or sixth day after feeding, 

 consequently it is a rare thing to find an insect of the age requisite for the maturation of 

 the Filaria. Probably of twenty mosquitoes one or two may attain this. I have seen 

 many Filarial in the fourth and fifth stages, but very few in the sixth and last. 



During this stage the swaying movements that had commenced during the fifth 

 gradually become more active. The animal is now no longer burst or killed by immersion 

 in water. On the contrary, even as we look at it, the water we have just immersed it in 

 seems to have a vivifying effect on the parasite ; its tissues appear to become more 

 consolidated, its motions more active, it seems to feel that it has got into its proper 

 element. It rapidly acquires great muscular power, it wriggles, twists, bends, extends, 

 and lashes about in all directions (figs. 23, 33). As seen with an inch objective it resembles 

 nothing so much as the embryo F'daria in the blood looked at through a quarter-inch ; 

 the tail, of course, is different, but the character of the movements is exactly the same. 

 One peculiar motion is strikingly like one that is often seen in the blood- Filaria. Often it 

 pauses in the midst of its contortions, and suddenly extends itself, remaining outstretched 

 for a moment, its body quivering in a sort of tetanic spasm. Those who have watched 

 the movements of the Filaria in the blood will recognize what I mean. In other examples 

 the wriggling movement is not seen, the activity of the animal being expended in frantic 

 rushes forwards and backwards, and in every direction. I presume this description of 

 movement belongs to a slightly later stage than does the wriggling motion ; probably it- 

 is the motion suited to the time when the mosquito has died and fallen into water, and 

 the parasite has obtained its liberty. 



Frequently, during the contortions of the animal, it turns its mouth towards the 

 observer, and seems to be endeavouring to uplift the cover-glass. During this movement 

 we can see that the mouth is pursed up into a cone, the lips being firmly approximated ; 

 and, around the spot where the mouth must be, a number of exceedingly minute horny- 

 like papillae are arranged. This may be the boring-apparatus. But the motions of any 

 animal I have examined at this stage have been too incessant to permit a satisfactory 

 view, and these boring-papillae may be, to some extent, matters of imagination. So 

 incessant are the movements that it is quite impossible to obtain satisfactory measure- 

 ments. After watching one for over an hour, and despairing at length of getting it to 

 slow down, I killed it by instilling a little osmic-acid solution below the cover-glass. It 

 then gradually slowed down, stretched out, and died. I found it to measure -fe" x &W- 

 In a good light this particular parasite was perfectly visible to the naked eye. 



