356 ME. B. SPRUCE OS INSECT-MIGRATIONS IN SOUTH AMERICA. 



sailors no eggs at all. As we returned to tlie brig we saw a vast 

 multitude of Butterflies flying across the Amazon, from the 

 northern to the southern side, in a direction about from N.N.W. 

 to S.S.E. They were evidently in the last stage of fatigue : some 

 of them attained the shore ; but a large proportion fell exhausted 

 into the water, and we caught several in our hands as they passed 

 over the canoe. They were all of common white and orange- 

 yellow species, such as are bred in cultivated and waste grounds, 

 and, having found no matrix whereon to deposit their eggs to the 

 northward of the river (the leaves proper for their purpose having 

 probably been already destroyed, or at least occupied, by cater- 

 pillars), were going in quest of it elsewhere." 



The very little wind there was blew from between E. and N.E.; 

 therefore the hutterflies steered their course at right angles to it; 

 and this was the case in subsequent flights I saw across the Ama- 

 zon, although when the wind was strong the weaker-winged in- 

 sects made considerable leeway, and would doubtless most of them 

 succumb before reaching land. But the most notable circum- 

 stance is that the movement is always southward, like the human 

 waves which from the earliest times seem to have surged one after 

 the other over the whole length of America, generating after a 

 time a reflux northwards, as in the case of the empire of the Incas. 

 Is this tendency southwards the continuation of an impulse 

 given in the remote past by the influx at the north-eastern and 

 north-western corners of America of races of insects as well as of 

 men to people the vast continent, or to dispute its possession with 

 beings already existing there ? For, allowing their due weight 

 to such motives as hunger and desire, they seem insufiicient to 

 explain a movement invariably directed towards the same point of 

 the compass ; and if, as I suppose, butterflies steer their flight at 

 right angles to the wind, because they thus make most headway, 

 why do they not sometimes cross from south to north, which would 

 be quite as advantageous with an easterly vnnd, unless they in- 

 herit some instinct which constantly impels them southward ? 



Since my return to England I have read Mr. Bates's graphic 

 description of a flight of butterflies across the Amazon below 

 Obidos, lasting for two days without intermission during daylight. 

 These also all crossed in one direction, from north to south. 

 Nearly all were species of Callidryas, the males of which genus 

 are wont to resort to beaches, while the females hover on the 

 borders of the forest and deposit their eggs on low-growing, shade- 

 loivng Mimosce. He adds, " The migrating hordes, so far as I 



