The Insects That Disappear Have Not Their Places Instantly Supplied By Their Successors.

From half-past-six in the morning till five in the afternoon, the air is filled with mosquitos; which have not, as some travellers have stated, the form of our gnats,* (* Culex pipiens.

This difference between mosquito (little fly, simulium) and zancudo (gnat, culex) exists in all the Spanish colonies. The word zancudo signifies long legs, qui tiene las zancas largas. The mosquitos of the Orinoco are the moustiques; the zancudos are the maringouins of French travellers.) but that of a small fly. They are simuliums of the family Nemocera of the system of Latreille. Their sting is as painful as that of the genus Stomox. It leaves a little reddish brown spot, which is extravased and coagulated blood, where their proboscis has pierced the skin. An hour before sunset a species of small gnats, called tempraneros,* because they appear also at sunrise, take the place of the mosquitos. (* Which appear at an early hour (temprano). Some persons say, that the zancudo is the same as the tempranero, which returns at night, after hiding itself for some time. I have doubts of this identity of the species; the pain caused by the sting of the two insects appeared to me different.) Their presence scarcely lasts an hour and a half; they disappear between six and seven in the evening, or, as they say here, after the Angelus (a la oracion). After a few minutes' repose, you feel yourself stung by zancudos, another species of gnat with very long legs. The zancudo, the proboscis of which contains a sharp-pointed sucker, causes the most acute pain, and a swelling that remains several weeks. Its hum resembles that of the European gnat, but is louder and more prolonged. The Indians pretend to distinguish the zancudos and the tempraneros by their song; the latter are real twilight insects, while the zancudos are most frequently nocturnal insects, and disappear toward sunrise.

In our way from Carthagena to Santa Fe de Bogota, we observed that between Mompox and Honda, in the valley of the Rio Magdalena, the zancudos darkened the air from eight in the evening till midnight; that towards midnight they diminished in number, and were hidden for three or four hours; and lastly that they returned in crowds, about four in the morning. What is the cause of these alternations of motion and rest? Are these animals fatigued by long flight? It is rare on the Orinoco to see real gnats by day; while at the Rio Magdalena we were stung night and day, except from noon till about two o'clock. The zancudos of the two rivers are no doubt of different species.

We have seen that the insects of the tropics everywhere follow a certain standard in the periods at which they alternately arrive and disappear. At fixed and invariable hours, in the same season, and the same latitude, the air is peopled with new inhabitants, and in a zone where the barometer becomes a clock,* (* By the extreme regularity of the horary variations of the atmospheric pressure.) where everything proceeds with such admirable regularity, we might guess blindfold the hour of the day or night, by the hum of the insects, and by their stings, the pain of which differs according to the nature of the poison that each species deposits in the wound.

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