Another question remains to be solved before we go further. What time-limit shall I allow for this census of the Bees that return to the nest? Let me explain what I mean. The dot which I have made in the middle of the thorax with a touch of my sticky straw is not very permanent: it merely adheres to the hairs. At the same time, it would have been no more lasting if I had held the insect in my fingers. Now the Bee often brushes her back: she dusts it each time she leaves the galleries; besides, she is always rubbing her coat against the walls of the cell, which she has to enter and to leave each time that she brings honey. A Mason-bee, so smartly dressed at the start, at the end of her work is in rags; her fur is all worn bare and as tattered as a mechanic's overall. Furthermore, in bad weather, the Mason-bee of the Walls spends the days and nights in one of the cells of her dome, suspended head downwards. The Mason-bee of the Sheds, as long as there are vacant galleries, does very nearly the same: she takes shelter in the galleries, but with her head at the entrance. Once those old habitations are in use, however, and the building of new cells begun, she selects another retreat. In the harmas (The piece of enclosed waste ground on which the author studies his insects in their natural state. Cf. "The Life of the Fly": chapter 1.--Translator's Note.), as I have said elsewhere, are stone heaps, intended for building the surrounding wall. This is where my Chalicodomae pass the night. Piled up promiscuously, both sexes together, they sleep in numerous companies, in crevices between two stones laid closely one on top of the other. Some of these companies number as many as a couple of hundred. The most common dormitory is a narrow groove. Here they all huddle, as far forward as possible, with their backs in the groove. I see some lying flat on their backs, like people asleep. Should bad weather come on, should the sky cloud over, should the north-wind whistle, they do not stir out. With all these things to take into consideration, I cannot expect my dot on the Bee's thorax to last any length of time. By day, the constant brushing and the rubbing against the partitions of the galleries soon wipe it off; at night, things are worse still, in the narrow sleeping-room where the Mason-bees take refuge by the hundred. After a night spent in the crevice between two stones, it is not advisable to trust to the mark made yesterday. Therefore, the counting of the number of Bees that return to the nest must be taken in hand at once; tomorrow would be too late. And so, as it would be impossible for me to recognize those of my subjects whose dots had disappeared during the night, I will take into account only the Bees that return on the same day.