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The deep central cells receive only the females of the Osmia; sometimes even the two sexes together, with a partition in the middle, the female occupying the lower and the male the upper storey. True, in such cases economy of space is strained to the utmost, the apartments provided by the Mason-bee of the Shrubs being very small as it is, despite their entrance-halls.
If he is a trifle smaller, it is scarcely noticeable, whereas, in the Horned Osmia and the Three-horned Osmia, the male is only half or a third the size of the female, as we have seen from the respective bulk of their cocoons. In the Mason-bee of the Walls there is also a difference in size, though less pronounced.
The Mason-bee has before her, let me suppose, only five vacant cells: two larger and three smaller. The total space at her disposal would do for about a third of the laying. Well, in the two large cells, she puts females; in the three small cells she puts males.
We must, therefore, abandon Darwin's idea when trying to explain the homing of the Cat as well as of the Mason-bee. The Pigeon transported for hundreds of miles is able to find his way back to his Dove-cot; the Swallow, returning from his winter quarters in Africa, crosses the sea and once more takes possession of the old nest. What guides them on these long journeys? Is it sight?
She has in her favour a memory for places and nothing more. A deviation amounting to two or three of our strides is enough to make her lose her way and to keep her from returning to her people, whereas miles across unknown country will not foil the Mason-bee. I expressed my surprise, just now, that man was deprived of a wonderful sense wherewith certain animals are endowed.
I am therefore very apprehensive as to the success of my experiment when I return to Orange, after first trying to steal some fresh secret from the Aygues Mason-bee of the Pebbles. I have scarcely reached the house before Aglae greets me, her cheeks flushed with excitement: 'Two! she cries. 'Two came back at twenty minutes to three, with a load of pollen under their bellies!
To leave her cell, the Mason-bee has to perforate not only the plug, the lid built at the mouth of the cell, but also the thick plaster wherewith the dome is strengthened at the end of the work. The perforation results in a vestibule which gives access to the chamber itself.
Here we have instinct, the unalterable characteristic of the worker. She has always built in this way and always will. The passing ages will never teach her, neither the struggle for life nor the law of selection will ever induce her to imitate the Mason-bee and collect dry dust for her mortar. This mud nest needs a shelter against the rain. The hiding-place under a stone suffices at first.
The walnut-sized nests are more trustworthy, for everything seems to indicate that they were built by a single Bee. Here females are found in the centre of the group and males at the circumference, in somewhat smaller cells, thus repeating what the Mason-bee of the Pebbles has told us. One clear and simple rule stands out from this collection of facts.
As a matter of fact, I notice, with extreme surprise, that the great majority of the cells visited by the Leucopsis' probe do not contain the one thing which the insect is seeking, namely, the young larva of the Mason-bee enclosed in its cocoon.
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