146 THOMPSON YATES AND JOHNSTON LABORATORIES REPORT 
length with a slight diminution in width, and structures which could be made out 
previously are not so obvious, the young filaria being now rather opaque ; still the 
genital papilla, the anus, and alimentary canal can be made out. The anal opening 
is not so widely open as in the previous stages. No further development of the 
reproductive system has been observed. 
The worm has now lost the fine sheath enclosing it, this sheath is probably cast 
off" when the filaria issues from the fat body. 1 Situated at the centre of the rounded 
posterior end of the parasite is a small thorn-like projection, and around it, some 
little distance away, are three or four similar spines, probably corresponding to the 
papilla at the tail of the F. bancrofti in the last stages of its development in the 
mosquito. 
The following are measurements of two worms about this stage : — 
Total length 
Length to 
Length to 
Central width 
Width at 
genital papilla 
368-6 fx 
60-3 fX 
26 08 fx 
26-08 fx 
612-88 
Not seen 
22-82 
The young F, cypseii has thus been traced to a very advanced stage in its 
development ; indeed, it is very probable that after Stage V no further development 
takes place in the body of the louse. The worm now resembles the parent parasite 
in shape, its movements are very active, and it corresponds in its development to 
the last stage seen in the metamorphosis of other filariae, viz., F. bancrofti and 
F. immitis. For these reasons also I think it is justifiable to conclude that the louse 
(sp. inceri) is the intermediate host for this filaria, though actual proof, i.e., by infecting 
birds by means of lice, is wanting. Granting that Stage V is the last in the meta- 
morphosis of F. cypseii before its transference to its definite host, the swift, it has yet 
to be ascertained how this transference takes place. Two possible ways suggest 
themselves. 
A. The birds may eat the infected lice ; the young filaria escaping from the 
bodies of the lice in the process of digestion reach the tissues of their future host, 
via the stomach. 
B. The now active filaria may travel through the tissues of the louse and, 
reaching the proboscis, be transferred to its definitive host when the louse pierces the 
skin of the swift. Fiuiria immitis is transferred from dog to dog in a similar manner 
1. Manson believes that while the young F. bancrofti are in the muscles of the mosquito they are surrounded by a fine 
sheath, which is shed before the worm reaches its final stage of metamorphosis in that insect. That such a sheath is present 
can be beautifully demonstrated in specimens of the mosquito in F. bancrofti, about the thirteenth to fifteenth day of development, 
if torn across in the process of dissection. 
