These Rivers, Like The Orinoco, Cross Thick Forests, But The Tipulary Insects, As Well As The Crocodiles, Shun The Proximity Of The Black Waters.

Possibly these waters, which are a little colder, and chemically different from the white waters, are adverse to the larvae of tipulary insects and gnats, which may be considered as real aquatic animals.

Some small rivers, the colour of which is deep blue, or yellowish-brown (as the Toparo, the Mataveni, and the Zama), are exceptions to the almost general rule of the absence of mosquitos over the black waters. These three rivers swarm with them; and the Indians themselves fixed our attention on the problematic causes of this phenomenon. In going down the Rio Negro, we breathed freely at Maroa, Daripe, and San Carlos, villages situated on the boundaries of Brazil. But this improvement of our situation was of short continuance; our sufferings recommenced as soon as we entered the Cassiquiare. At Esmeralda, at the eastern extremity of the Upper Orinoco, where ends the known world of the Spaniards, the clouds of mosquitos are almost as thick as at the Great Cataracts. At Mandavaca we found an old missionary, who told us with an air of sadness, that he had had his twenty years of mosquitos in America*. (* "Yo tengo mis veinte anos de mosquitos.") He desired us to look at his legs, that we might be able to tell one day, beyond sea (por alla), what the poor monks suffer in the forests of Cassiquiare. Every sting leaving a small darkish brown point, his legs were so speckled that it was difficult to recognize the whiteness of his skin through the spots of coagulated blood. If the insects of the genus Simulium abound in the Cassiquiare, which has white waters, the culices or zancudos are so much the more rare; you scarcely find any there; while on the rivers of black waters, in the Atabapo and the Rio, there are generally some zancudos and no mosquitos.

I have just shown, from my own observations, how much the geographical distribution of venomous insects varies in this labyrinth of rivers with white and black waters. It were to be wished that a learned entomologist could study on the spot the specific differences of these noxious insects,* which in the torrid zone, in spite of their minute size, act an important point in the economy of nature. (* The mosquito bovo or tenbiguai; the melero, which always settles upon the eyes; the tempranero, or putchiki; the jejen; the gnat rivau, the great zancudo, or matchaki; the cafafi, etc.) What appeared to us very remarkable, and is a fact known to all the missionaries, is, that the different species do not associate together, and that at different hours of the day you are stung by distinct species. Every time that the scene changes, and, to use the simple expression of the missionaries, other insects mount guard, you have a few minutes, often a quarter of an hour, of repose.

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