At night there was always a heavy dew, and it rained intermittently each day. On dark nights, and often after a shower, the banks of the river where there was forest glowed with twinkling phosphorescence. Examination showed that the decaying vegetation was filled with myriads of small, wriggling insects, greatly resembling our well-known cellar-bug (_Isopod_), and one day we paddled for many hours through a mass of flying ants which had come to grief in the river. The water was covered with them and the waves had tossed them up on the banks to a depth of several inches. Another thing that attracted our attention was the large number of bats. On one occasion we heard a dull rumbling among the granite ledges near camp, and not long after a stream of bats began to emerge from the cracks; from a distance they resembled a cloud of smoke. There must have been many thousands, for the black masses continued to rise until darkness obscured them from our view. Spruce records that on one occasion he saw not less than a million under similar circumstances. This brings up an interesting problem. The individual range of these bats is probably not very great, the result of which is that immense numbers of them are distributed over a comparatively small area. Now, if the struggle for existence is as keen as is often supposed, how can the female, encumbered with her offspring fully three-fourths as large as herself, compete successfully with the unhampered males, and secure enough food not only for herself but also for her young? The fruit-eating varieties might not suffer seriously from this handicap, but it does seem as if the agility of the insectivorous kinds catching their food on the wing would be greatly affected.
There are numbers of curious formations along the river which cannot fail to attract the interest of the traveller, no matter what his particular mission might be. One of these is the Cerro Yapacana, a square block of granite not over one thousand five hundred feet high; it is a very conspicuous landmark as it towers above the forest like a giant monument, and can be seen many miles away. We did not come abreast of it until eight days after first sighting it.
There are few rubber-camps along this part of the river, but several Indian families had come to spend some weeks collecting turtles and eggs on the sand-banks. At night absolute quiet reigned on the _playas_ so long as the moon shone; but no sooner had the brilliant orb disappeared below the horizon than the water was broken with ripples as numbers of turtles emerged to deposit their eggs in the loose, warm sand, and jaguars came from the dark forest to feast on the defenseless creatures and rend the still night air with ugly coughs and grunts.