I caught them with the net and released them as soon as captured in a closed room, where they passed the night.

On the next day they were marked, by means of a slight tonsure on the thorax.
The total number of butterflies attracted on these eight nights amounted to a hundred and fifty; a stupendous number when I consider what searches I had to undertake during the two following years in order to collect the specimens necessary to the continuation of my investigation.
Without being absolutely undiscoverable, in my immediate neighbourhood the cocoons of the Great Peacock are at least extremely rare, as the trees on which they are found are not common.

For two winters I visited all the decrepit almond-trees at hand, inspected them all at the base of the trunk, under the jungle of stubborn grasses and undergrowth that surrounded them; and how often I returned with empty hands! Thus my hundred and fifty butterflies had come from some little distance; perhaps from a radius of a mile and a quarter or more.

How did they learn of what was happening in my study?
Three agents of information affect the senses at a distance: sight, sound, and smell.

Can we speak of vision in this connection?
Sight could very well guide the arrivals once they had entered the open window; but how could it help them out of doors, among unfamiliar surroundings?
Even the fabulous eye of the lynx, which could see through walls, would not be sufficient; we should have to imagine a keenness of vision capable of annihilating leagues of space.