CHAPTER X. 
MOSQUITOES AND PARASITE PROBLEMS. 
Tuere cannot be a doubt that some animals 
possess an instinctive knowledge of their enemies 
—or, at all events, of some of their enemies— 
though I do not believe that this faculty is so com- 
mon as many naturalists imagine. The most striking 
example I am acquainted with is seen in gnats or 
mosquitoes, and in the minute South American 
sandflies (Simulia), when a dragon-fly appears in a 
place where they are holding their aérial pastimes. 
The sudden appearance of a ghost among human 
revellers could not produce a greater panic. I 
have spoken in the last chapter of periodical 
storms or waves of dragon-flies in the Plata region, 
and mentioned incidentally that the appearance of 
these insects is most welcome in oppressively hot 
weather, since they are known to come just in 
advance of arush of cool wind. In ha Plata we 
also look for the dragon-fly, and rejoice at its 
coming, for another reason. We know that the 
presence of this noble insect will cause the clouds 
of stinging gnats and flies, which make life a 
burden, to vanish like smoke. 
When a flight of dragon-flies passes over the 
country many remain along the route, as I have 
