It is a well-known fact that the various species of gnats or flies comprehended under this general name, do not associate, but appear at different times of the day. As often as the scene changes or as ‘other insects mount guard,’ one enjoys a few minutes’ rest; for after the retreat of one host, its successors are not immediately on the spot. From half-past six in the morning till five in the afternoon the air is filled with mosquitoes of the genus Simulium. An hour before sunset, these are replaced by a small species of Culex called the Tempraneros or early-risers, as they also show themselves at sunrise. Their stay in the afternoon scarcely lasts an hour and a half, and then one feels soon after the painful sting of a larger Culex--the Zancudo-- who, plunging his blood-thirsty proboscis into the skin, causes an excruciating pain. The Zancudo, a ‘child of the night,’ disappears at sunrise, and then makes place for the matutinal Tempranero. As all these winged tormentors spend the greater part of their lives in the water, we cannot wonder that their numbers diminish as the distance from the banks of the rivers intersecting the forests increases. Their favourite resorts are the places where their transformation takes place, and where they on their part are soon about to lay their eggs. The mosquito clouds hover only above or near the waters, and it would be a great error to suppose that the vast forests extending between the river valleys are all equally infested with this insect plague. From time to time the mosquitoes migrate like the social stentor monkeys. Formerly no other Culex was known at Simiti on the Magdalen river but the small species called Zejen. The people slept unmolested, for the Zejen is a diurnal insect. But in 1801 the great blue-winged Zancudo made his appearance in such numbers that the poor inhabitants of Simiti could find no rest at night. Slight differences of climate or food seem to have an influence on the intensity of the poison which the same species discharges through its serrated proboscis. One cannot refrain from smiling at the disputes of the missionaries about the size and voracity of the mosquitoes in different parts of the same river. In a land so completely severed from the rest of the world, this forms the favourite subject for conversation. ‘How much I pity you,’ said the missionary from the falls as he took leave of his colleague at the Cassiquiare; ‘you are like me, alone in this land of jaguars and monkeys, but as to my mosquitoes I can boast that one of mine is a match for three of yours.’