In Central America some day you might look up into the blue and see what looks like a great black snake stretching from one end of the sky to the other. You might even see the “snake” slithering from side to side like a real snake dose on the ground, but if you looked at it through your field glasses or a telescope you would see that it was made up of thousands upon thousands of green, tailed moths all determinedly flying from northwest to southeast. Sometimes the great snake appears to drop parts of itself down from the sky and then, if you are in the right place, you see what looks like a river of dark green moths flowing over the surface of the land. Swiftly they rush by you – day flying moths driven on and on by a frenzy of migration.
The true story of these great and beautiful moths has not yet been written. We can only say that they apparently migrate in order to find new feeding places for their caterpillar children. But why do they almost always seem to migrate from northwest to southeast and very rarely the other way? One scientist has suggested that ages ago they migrated because of being driven by cold, even as birds are driven by cold to migrate from Canada and the United States south to the tropics. Having once been forced to migrate for this reason, they then continued year after year even when the cold no longer was there to drive them. Probably, though they migrate in one direction as a great mass, they migrate back the other way later as individuals. But no one truly knows.