It is needless to discuss the matter further; sight cannot be the guiding sense.
Sound is equally out of the question.

The big-bodied creature capable of calling her mates from such a distance is absolutely mute, even to the most sensitive ear.

Does she perhaps emit vibrations of such delicacy or rapidity that only the most sensitive microphone could appreciate them?
The idea is barely possible; but let us remember that the visitors must have been warned at distances of some thousands of yards.

Under these conditions it is useless to think of acoustics.
Smell remains.

Scent, better than any other impression in the domain of our senses, would explain the invasion of butterflies, and their difficulty at the very last in immediately finding the object of their search.