372 DR. P. MANSON ON THE METAMORPHOSIS OF 



movements of the little animals tend to entangle them still further, and bring them 

 under the influence of the suction-force exercised by the insect. 



Arrived in the abdomen of the mosquito the Filaria for a short time retains the appear- 

 ances and movements that it exhibited while in its human host, and which have 

 frequently been described (fig. 1). Presently a delicate, closely set, transverse striatiou, 

 as if from general longitudinal shrinking, gradually becomes very evident. The sheath 

 also in many cases can be more readily seen, and oral pouting is very distinct. Within 

 an hour of the time of ingestion, the Filaria usually casts its sheath, which, by careful 

 searching, may be seen either trailing after it, or lying across it, or somewhere near. 

 After the casting of the sheath transverse striatiou and oral pouting become still more 

 distinct, and one can almost fancy the parasite has a true mouth surrounded by lips. 

 It is impossible to determine this, however. 



Until the casting of the sheath, the peculiar lashing movements of the animal continue ; 

 but when this has been effected a change in the character of these movements is in many 

 instances observable. Hitherto they have been free, lashing, purposeless apparently; 

 and although the little animal has kept in perpetual and vigorous motion, it has never 

 changed, materially, the spot it moved in. There has been no locomotion. But when 

 the sheatli has been got rid of, the nature of the motion changes to a suake-likc wriggle 

 of regular undulations, which cause a definite forward movement (fig. 2). In many 

 specimens this is very striking. If motion is prevented by some insuperable obstacle 

 in one direction, the Filaria retreats and tries in another. It is evidently endeavouring 

 to change its locality. 



The object of this singular change in the character of the movements exhibited by 

 the Filaria is explained if we examine the thorax in a batch of insects at short intervals 

 after they have fed. We tease it up with needles in a little water, or sulphate-of-soda 

 solution, tearing up the muscles of the wings thoroughly, before applying the cover glass. 

 Examined, thus prepared, within a short time, say half an hour, of feeding, all the Filarice 

 are to be found in the abdomen, none in the thorax ; after an hour, two or three may 

 be found moving among the muscles of the thorax ; after another hour, many more may 

 be detected here, till at the end of 12 or 18 hours, the thorax is found to be full of 

 parasites. Therefore, the movement of progression we see in the Filaria that has cast 

 its sheath in the blood in the abdomen, has for its object the migration of the parasite. 



Not all, but a very large proportion of the injected Filaria do thus migrate. My 

 former observations were made entirely ou Filaria found in the abdomen, or believed 

 to be in the abdomen. Not suspecting this migration, and finding metamorphosis 

 going on in the abdomen, I may have, unwittingly, included some of the viscera of the 

 former in my examinations of the latter. On that occasion I traced out the metamor- 

 phosis to its conclusion, and entirely in what I thought, at the time, were abdominal 

 tissues. It is likely, therefore, that migration to the thorax is not a necessary step 

 indispensable for the welfare of the parasite. Eut it is certainly the usual first step for 

 the animal to take, and it is a fortunate one in the interests of the observer, as in tracing 

 the subsequent steps of the metamorphosis, the ova, which in the abdomen are so 

 annoying from their obscuring the field when ruptured, are not encountered. Lewis was 



