120 The Naturalist in Nicaragua 
would pass through a band perhaps fifty yards in width, 
where hundreds were always in sight, and all travelling one 
way. I took the direction several times with a pocket com- 
pass, and it was always south-east. Amongst them were a 
few yellow butterflies, but these were not so numerous as in 
former years. In some seasons these migratory swarms of 
butterflies continue passing over to the south-east for three 
to five weeks, and must consist of millions upon millions of 
individuals, comprising many different species and genera. 
The beautiful tailed green and gilded day-flying moth 
(Urania leilus) also joins in this annual movement. When 
in Brazil, I observed similar flights of butterflies at Per- 
nambuco and Maranham, all travelling south-east. Mr. R. 
Spruce describes a migration which he witnessed on the 
Amazon, in November 1849, of the common white and yellow 
butterflies. They were all passing to the south-south-east.1 
Darwin mentions that several times when off the shores of 
Northern Patagonia, and at other times when some miles off 
the mouth of the Plata, the ship was surrounded by butter- 
flies; so numerous were they on one occasion, that it was not 
possible to see a space free from them, and the seamen cried 
out that it was ‘“ snowing butterflies.” * These butterflies 
must also come from the westward. I know of no satis- 
factory explanation of these immense migrations. They 
occurred every year whilst I was in] Chontales, and always 
in the same direction. I thought that some of the earlier 
flights in April might be caused by the vegetation of the 
Pacific side of the continent being still parched up, whilst on 
the Atlantic slope the forests were green and moist. But in 
June there had been abundant rains on the Pacific side, and 
vegetation was everywhere growing luxuriantly. Neither 
would their direction from the north-west bring them from 
the Pacific, but from the interior of Honduras and Guate- 
mala. The difficulty is that there are no return swarms. If 
they travelled in one direction at one season of the year, and 
in an opposite at another, we might suppose that the vegeta- 
tion on which the caterpillars feed was at one time more 
abundant in the north-west, at another in the south-east; 
but during the five years I was in Central America, I was 
1 Journal of the Linnean Society, vol. ix. 
2 Naturalist’s Voyage, p. 158. 
