UNHEALTHINESS OF BRANCH RIVERS 299 



The weather, during the month of July, was uninter- 

 ruptedly fine ; not a drop of rain fell, and the river sank 

 rapidly. The mornings, for two hours after sunrise, were 

 very cold ; we were glad to wrap ourselves in blankets on 

 turning out of our hammocks, and walk about at a quick 

 pace in the early sunshine. But in the afternoons the heat 

 was sickening ; for the glowing sun then shone full on the 

 front of the row of whitewashed houses, and there was 

 seldom any wind to moderate its effects. I began now 

 to understand why the branch rivers of the Amazons were 

 so unhealthy, whilst the main stream was pretty nearly 

 free from diseases arising from malaria. The cause lies, 

 without doubt, in the slack currents of the tributaries in 

 the dry season, and the absence of the cooling Amazonian 

 trade-wind, which purifies the air along the banks of the 

 main river. The trade-wind does not deviate from its 

 nearly straight westerly course, so that the branch 

 streams, which run generally at right angles to the Ama- 

 zons, and have a slack current for a long distance from 

 their mouths, are left to the horrors of nearly stagnant 

 air and water. 



Aveyros may be called the headquarters of the fire-ant, 

 which might be fittingly termed the scourge of this fine 

 river. The Tapajos is nearly free from the insect pests 

 of other parts, mosquitoes, sand-flies, Motucas and piums ; 

 but the formiga de fogo is perhaps a greater plague than 

 all the others put together. It is found only on sandy 

 soils in open places, and seems to thrive most in the neigh- 

 bourhood of houses and weedy villages, such as Aveyros : 

 it does not occur at all in the shades of the forest. I noticed 

 it in most places on the banks of the Amazons, but the 

 species is not very common on the main river, and its 

 presence is there scarcely noticed, because it does not 

 attack man, and the sting is not so virulent as it is in the 

 same species on the banks of the Tapajos. Aveyros was 

 deserted a few years before my visit on account of this 

 little tormentor, and the inhabitants had only recently 

 returned to their houses, thinking its numbers had de- 

 creased. It is a small species, of a shining reddish colour, 

 not greatly differing from the common red stinging ant 

 of our own country (Myrmica rubra), except that the pain 

 and irritation caused by its sting are much greater. The 

 soil of the whole village is undermined by it : the ground 

 is perforated with the entrances to their subterranean 



