While the boys listened to the chorus their efforts to attract the attention of the aeroplane had produced, there came into the discord another sound--the hissing of a monster serpent.  Heretofore the boys had little to do with Brazilian forms of animal life, for they had kept near the middle of the main stream of the Amazon, and also about in the center of the Madeira and the much smaller Beni, which was only a creek when compared with the other rivers.