Notwithstanding the resistance of the substance traversed, the Leucopsis perseveres, certain of succeeding; and she does succeed, although I am still unable to understand her success. The material through which the probe has to penetrate is not a porous substance; it is homogeneous and compact, like our hardened cement. In vain do I direct my attention to the exact point where the instrument is at work; I see no fissure, no opening that can facilitate access. A miner's drill penetrates the rock only by pulverizing it. This method is not admissible here; the extreme delicacy of the implement is opposed to it. The frail stem requires, so it seems to me, a ready-made way, a crevice through which it can slip; but this crevice I have never been able to discover. What about a dissolving fluid which would soften the mortar under the point of the ovipositor? No, for I see not a trace of humidity around the point where the thread is at work. I fall back upon a fissure, a lack of continuity somewhere, although my examination fails to discover any on the Mason-bee's nest. I was better served in another case. Leucopsis dorsigera, FAB., settles her eggs on the larva of the Diadem Anthidium, who sometimes makes her nest in reed-stumps. I have repeatedly seen her insert her auger through a slight rupture in the side of the reed. As the wall was different, wood in the latter case and mortar in the former, perhaps it will be best to look upon the matter as a mystery. My sedulous attendance, during the best part of July, in front of the tiles hanging from the walls of the arch, allowed me to reckon the inoculations. Each time that the insect, on finishing the operation, removed its probe, I marked in pencil the exact point at which the instrument was withdrawn; and I wrote down the date beside it. This information was to be utilized when the Leucopsis finished her labours. When the perforators are gone, I proceed with my examination of the nests, covered with my hieroglyphics, the pencilled notes. One result, one which I fully expected, compensates me straightway for all my weary waitings. Under each spot marked in black, under each spot whence I saw the ovipositor withdrawn, I always find a cell, with not a single exception. And yet there are intervals of solid stone between the cells: the partition-walls alone would account for some. Moreover, the compartments, which are very irregularly disposed by a swarm of toilers who all work in their own sweet way, have great irregular cavities between them, which end by being filled up with the general plastering of the nest. The result of this arrangement is that the massive portions cover almost the same space as the hollow portions. There is nothing outside to show whether the underlying regions are full or empty. It is quite impossible for me to decide if, by digging straight down, I shall come to a hollow cell or to a solid wall.