This explanation seemed clear enough.
“Now I have heard,” said Hendrik, “of farmers kindling fires around their crops to keep off the locusts. I can't see how fires would keep them off—not even if a regular fence of fire were made all round a field. These creatures have wings, and could easily fly over the fires.”
“The fires,” replied Hans, “are kindled, in order that the smoke may prevent them from alighting; but the locusts to which these accounts usually refer are without wings, called voetgangers (foot-goers). They are, in fact, the larvæ of these locusts, before they have obtained their wings. These have also their migrations, that are often more destructive than those of the perfect insects, such as we see here. They proceed over the ground by crawling and leaping like grasshoppers; for, indeed, they are grasshoppers—a species of them. They keep on in one direction, as if they were guided by instinct to follow a particular course. Nothing can interrupt them in their onward march unless the sea or some broad and rapid river. Small streams they can swim across; and large ones, too, where they run sluggishly; walls and houses they can climb—even the chimneys—going straight over them; and the moment they have reached the other side of any obstacle, they continue straight onward in the old direction.
“In attempting to cross broad rapid rivers, they are drowned in countless myriads, and swept off to the sea. When it is only a small migration, the farmers sometimes keep them off by means of fires, as you have heard. On the contrary, when large numbers appear, even the fires are of no avail.”
“But how is that, brother?” inquired Hendrik. “I can understand how fires would stop the kind you speak of, since you say they are without wings. But since they are so, how do they get through the fires? Jump them?”
“No, not so,” replied Hans. “The fires are built too wide and large for that.”
“How then, brother?” asked Hendrik. “I'm puzzled.”
“So am I,” said little Jan.
“And I,” added Trüey.
“Well, then,” continued Hans, “millions of the insects crawl into the fires and put them out!”