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With both these Osmiae, the division of the gallery is the same as with the Three-horned Osmia. At the back are large cells with plentiful provisions and widely-spaced partitions; in front, small cells, with scanty provisions and partitions close together. Also, the larger cells supplied me with big cocoons and females; the smaller cells gave me little cocoons and males.
Is the Osmia discreet enough not to put upon the good-natured Mason and to utilize only abandoned passages and waste cells? Or does she take possession of the home of which the real owners could themselves have made use? I lean in favour of usurpation, for it is not rare to see the Chalicodoma of the Sheds clearing out old cells and using them as does her sister of the Pebbles.
When confronted with tubes too small to receive all her family, the Osmia is in the same plight as the Mason-bee in the presence of an old nest. She thereupon acts exactly as the Chalicodoma does. She breaks up her laying, divides it into series as short as the room at her disposal demands; and each series begins with females and ends with males.
When studying the emergence of the Three-pronged Osmia from the bramble-stems shifted from their natural position by my wiles, I recognized the uncertainty in which the evidence of physical science leaves us; and, in the impossibility of finding any other reply, I suggested a special sense, the sense of open space.
Let us in the first place consult the old nests of the Mason-bee of the Shrubs. I have said that these mortar spheroids, pierced all over with little cylindrical cavities, are adopted pretty eagerly by the Three-horned Osmia, who colonizes them before my eyes with females in the deep cells and males in the shallow cells. That is how things go when the old nest remains in its natural state.
More discerning than I in her estimate of what was strictly necessary, better-versed in the economy of space, the Osmia had found a way of lodging two females where I had only seen room for one female and a male. This experiment speaks volumes. When confronted with tubes too small to receive all her family, she is in the same plight as the Mason-bee in the presence of an old nest.
The males of the Osmia make their appearance even before those of the Anthophora and at so early a season that the young Sitaris-larvæ are perhaps not yet aroused by the instinctive impulse which urges them to activity.
That is the sum-total of the building done. For this plasterer's rather than mason's work, the Horned and the Three-horned Osmia employ soft earth. This material is a sort of dried mud, which turns to pap on the addition of a drop of water.
Fitting as best she can the projecting angles of the new cell into the recessed corners of the cell already built, the Osmia runs up walls more or less curved, upright or slanting, which intersect one another at various points, so that each compartment requires a new and complicated plan of construction, which is very different from the circular-partition style of architecture, with its row of parallel dividing-disks.
The large cells, those at the back, receive the bulky cocoons; the small ones, those in front, have cocoons only a half or a third as big. Before opening them and ascertaining the sex of the Osmia inside, let us wait for the transformation into the perfect insect, which will take place towards the end of summer. If impatience gets the better of us, we can open them at the end of July or in August.
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