88 E. H. STRICKLAND 
so that it is probable that only those species of Simulium, whose 
life history coincides with parastised species, could be infected 
with the parasites of the latter. Since, as previously stated, I 
am led to believe that infection is possible only during the early 
stages of larval life, it would probably be necessary to collect 
a number of infected larvae, and place them in streams where 
the other species is always abundant, some little while before 
the eggs of the latter hatch. If the parasite is present in the 
larvae of the following generation it will be readily detected dur- 
ing the later stages of larval development. 
It will be seen from the list of Myxosporidia, and the seasons 
in which they were found, that those occurring in the spring 
brood parasitise a much higher percentage of larvae than those 
occurring in the fall brood. This is due, it seems, to the much 
greater numbers of larvae present in the streams during the 
former season, for at this time they are living a gregarious life, 
and all stages of development are present together. This results 
in the newly hatched larvae always being in close proximity to 
some recently dead larva, from which innumerable spores have 
escaped. In the fall, however, larvae are comparatively rare, 
and are more solitary in their habits, so that the infection of 
young larvae is less certain. 
The Gregarine species, however, which occurred in two streams, 
not examined in the spring, was found heavily to parasitise larvae 
which were not by any means abundant ia the streams, and this 
is probably due to the motility of the bodies liberated by the 
host upon its death. It would thus seem that, were the parasites 
liberated in streams where Simulium larvae are very numerous, 
the spread of infection would be rapid, and the disease should 
soon be firmly established. 
It is not easy to understand how it is that the parasites do 
not slowly travel down-stream, from year to year, so that in 
time the stream becomes clear of infection. That this is not 
the case, is indicated by the fact that where the source of a 
stream has been found as a spring, Simulium larvae occurring 
near this region were found to be parasitised. As before stated, 
I do not believe that the adults are capable of spreading the 
