Ch. 10: Mosquitos and Parasite Problems

There 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 common 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 aerial 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 a rush of cool wind. In La 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 said, sheltering themselves wherever trees occur;
and, after the storm blows over, these strangers and stragglers remain
for some days hawking for prey in the neighbourhood. It is curious to
note that they do not show any disposition to seek for watercourses. It
may be that they feel lost in a strange region, or that the panic they
have suffered, in their long flight before the wind, has unsettled their
instincts; for it is certain that they do not, like the dragon-fly in
Mrs. Browning's poem, "return to dream upon the river." They lead
instead a kind of vagabond existence, hanging about the plantations, and
roaming over the surrounding plains. It is then remarked that gnats and
sand-flies apparently cease to exist, even in places where they have
been most abundant. They have not been devoured by the dragon-flies,
which are perhaps very few in number; they have simply got out of the
way, and will remain in close concealment until their enemies take their
departure, or have all been devoured by martins, tyrant birds, and the
big robber-flies or devil's dykes--no name is bad enough for them--of
the family Asilidaa. During these peaceful gnatless days, if a person
thrusts himself into the bushes or herbage in some dark sheltered place,
he will soon begin to hear the thin familiar sounds, as of "horns of
elf-land faintly blowing"; and presently, from the ground and the under
surface of every leaf, the ghost-like withered little starvelings will
appear in scores and in hundreds to settle on him, fear not having
blunted their keen appetites.

When riding over the pampas