Might not this intercourse tend to cheer them and encourage them to patience? Meanwhile, slowly, doors are opening here and there through the separating walls; the road is cleared by sections; and a moment arrives when the leader of the file walks out. The others follow, if ready; but there are always laggards who keep the rear-ranks waiting until they are gone.
To sum up, first, the hatching of the larvae takes place without any order; secondly, the exodus proceeds regularly from summit to base, but only in consequence of the insect's inability to move forward so long as the upper cells are not vacated. We have here not an exceptional evolution, in the inverse ratio to age, but the simple impossibility of emerging otherwise. Should a chance occur of going out before its turn, the insect does not fail to seize it, as we can see by the lateral movements which send the impatient ones a few ranks ahead and even release the more favoured altogether. The only remarkable thing that I perceive is the scrupulous respect shown to the as yet unopened neighbouring cocoon. However eager to come out, the Osmia is most careful not to touch it with his mandibles: it is taboo. He will demolish the partition, he will gnaw the side-wall fiercely, even though there be nothing left but wood, he will reduce everything around him to dust; but touch a cocoon that obstructs his way? Never! He will not make himself an outlet by breaking up his sisters' cradles.
It may happen that the Osmia's patience is in vain and that the barricade that blocks the way never disappears at all. Sometimes, the egg in a cell does not mature; and the unconsumed provisions dry up and become a compact, sticky, mildewed plug, through which the occupants of the floors below could never clear themselves a passage.
Sometimes, again, a grub dies in its cocoon; and the cradle of the deceased, now turned into a coffin, forms an everlasting obstacle.
How shall the insect cope with such grave circumstances?
Among the many bramble-stumps which I have collected, some few have presented a remarkable peculiarity. In addition to the orifice at the top, they had at the side one and sometimes two round apertures that looked as though they had been punched out with an instrument. On opening these stalks, which were old, deserted nests, I discovered the cause of these very exceptional windows. Above each of them was a cell full of mouldy honey. The egg had perished and the provisions remained untouched: hence the impossibility of getting out by the ordinary road. Walled in by the unsurmountable obstacle, the Osmia on the floor below had contrived an outlet through the side of the shaft; and those in the lower storeys had benefited by this ingenious innovation. The usual door being inaccessible, a side-window had been opened by means of the insect's jaws. The cocoons, torn, but still in position in the lower rooms, left no doubt as to this eccentric mode of exit. The same fact, moreover, was repeated, in several bramble-stumps, in the case of Osmia tridentata; it was likewise repeated in the case of Anthidium scapulare. The observation was worth confirming by experiment.
I select a bramble-stem with the thinnest rind possible, so as to facilitate the Osmiae's work. I split it in half, thus obtaining a smooth-sided trough which will enable me to judge better of future exits. The cocoons are next laid out in one of the troughs. Iseparate them with disks of sorghum, covering both surfaces of the disk with a generous layer of sealing-wax, a material which the Osmia's mandibles are not able to attack. The two troughs are then placed together and fastened. A little putty does away with the joint and prevents the least ray of light from penetrating. Lastly, the apparatus is hung up perpendicularly, with the cocoons' heads up. We have now only to wait. None of the Osmiae can get out in the usual manner, because each of them is confined between two partitions coated with sealing-wax. There is but one resource left to them if they would emerge into the light of day, that is, for each of them to open a side-window, provided always that they possess the instinct and the power to do so.
In July, the result is as follows: of twenty Osmiae thus immured, six succeed in boring a round hole through the wall and making their way out; the others perish in their cells, without managing to release themselves. But, when I open the cylinder, when I separate the two wooden troughs, I realize that all have attempted to escape through the side, for the wall of each cell bears traces of gnawing concentrated upon one spot. All, therefore, have acted in the same way as their more fortunate sisters; they did not succeed, because their strength failed them. Lastly, in my glass tubes, part-lined with a thick piece of packing-paper, I often see attempts at making a window in the side of the cell: the paper is pierced right through with a round hole.
This then is yet another result which I am glad to record in the history of the bramble-dwellers. When the Osmia, the Anthidium and probably others are unable to emerge through the customary outlet, they take an heroic decision and perforate the side of the shaft. It is the last resource, resolved upon after other methods have been tried in vain. The brave, the strong succeed; the weak perish in the attempt.
