The experiment is of paramount importance; and it will perhaps make the result clearer if I quote one instance from among a multitude of similar cases. I give the preference to this particular instance because of the rather exceptional fertility of the laying. An Osmia marked on the thorax is watched, day by day, from the commencement to the end of her work. From the 1st to the 10th of May, she occupies a glass tube in which she lodges seven females followed by a male, which ends the series. From the 10th to the 17th of May, she colonizes a second tube, in which she lodges first three females and then three males. From the 17th to the 25th of May, a third tube, with three females and then two males. On the 26th of May, a fourth tube, which she abandons, probably because of its excessive width, after laying one female in it. Lastly, from the 26th to the 30th of May, a fifth tube, which she colonizes with two females and three males. Total: twenty-five Osmiae, including seventeen females and eight males. And it will not be superfluous to observe that these unfinished series do not in any way correspond with periods separated by intervals of rest. The laying is continuous, in so far as the variable condition of the atmosphere allows. As soon as one tube is full and closed, another is occupied by the Osmia without delay.
The tubes reduced to the exact length of two cells fulfilled my expectation in the great majority of cases: the lower cell was occupied by a female and the upper by a male. There were a few exceptions. 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. 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. This breaking up, on the one hand, into sections in all of which both sexes are represented and the division, on the other hand, of the entire laying into just two groups, one female, the other male, when the length of the tube permits, surely provide us with ample evidence of the insect's power to regulate the sex of the egg according to the exigencies of space.
And besides the exigencies of space one might perhaps venture to add those connected with the earlier development of the males. These burst their cocoons a couple of weeks or more before the females;they are the first who hasten to the sweets of the almond-tree. In order to release themselves and emerge into the glad sunlight without disturbing the string of cocoons wherein their sisters are still sleeping, they must occupy the upper end of the row; and this, no doubt, is the reason that makes the Osmia end each of her broken layings with males. Being next to the door, these impatient ones will leave the home without upsetting the shells that are slower in hatching.
I experimented on Latreille's Osmia, using short and even very short stumps of reed. All that I had to do was to lay them just beside the nests of the Mason-bee of the Sheds, nests beloved by this particular Osmia. Old, disused hurdles supplied me with reeds inhabited from end to end by the Horned Osmia. In both cases I obtained the same results and the same conclusions as with the Three-horned Osmia.
I return to the latter, nidifying under my eyes in some old nests of the Mason-bee of the Walls, which I had placed within her reach, mixed up with the tubes. Outside my study, I had never yet seen the Three-horned Osmia adopt that domicile. This may be due to the fact that these nests are isolated one by one in the fields; and the Osmia, who loves to feel herself surrounded by her kin and to work in plenty of company, refuses them because of this isolation. But on my table, finding them close to the tubes in which the others are working, she adopts them without hesitation.
The chambers presented by those old nests are more or less spacious according to the thickness of the coat of mortar which the Chalicodoma has laid over the assembled chambers. 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. It is this vestibule which is sometimes longer and sometimes shorter, whereas the corresponding chamber is of almost constant dimensions, in the case of the same sex, of course.
We will first consider the short vestibule, at the most large enough to receive the plug with which the Osmia will close up the lodging.
There is then nothing at her disposal except the cell proper, a spacious apartment in which one of the Osmia's females will find ample accommodation, for she is much smaller than the original occupant of the chamber, no matter the sex; but there is not room for two cocoons at a time, especially in view of the space taken up by the intervening partition. Well, in those large, well-built chambers, formerly the homes of Chalicodomae, the Osmia settles females and none but females.
Let us now consider the long vestibule. Here, a partition is constructed, encroaching slightly on the cell proper, and the residence is divided into two unequal storeys, a large room below, housing a female, and a narrow cabin above, containing a male.
When the length of the vestibule permits, allowing for the space required by the outer stopper, a third storey is built, smaller than the second; and another male is lodged in this cramped corner. In this way the old nest of the Mason-bee of the Pebbles is colonized, cell after cell, by a single mother.
