The Anthophoræ are the actual pioneers. The work of boring the galleries is wholly theirs, and their cells are right at the end. If they, for any reason, leave the outer cells, the Osmia comes in and takes possession of them. She divides the corridors into unequal and inartistic cells by means of rough earthen partitions, her only idea of masonry. The cells of the Anthophora are faultlessly regular and perfectly finished. They are works of art, cut out of the very substance of the earth, well out of reach of all ordinary enemies; and for this reason the larva of this Bee has no means of spinning a cocoon. It lies naked in the cell, whose inner surface is polished like stucco. In the Osmia’s cells, however, means of defence are required, because they are at the surface of the soil, are roughly made, and are badly protected by their thin partitions. So the Osmia’s grubs enclose themselves in a very strong cocoon, which preserves them both from the rough sides of their shapeless cells and from the jaws of various enemies who prowl about the galleries. It is easy, then, in a bank inhabited by these two Bees, to recognise the cells belonging to each. The Anthophora’s cells contain a naked grub: those of the Osmia contain a grub enclosed in a cocoon. Now each of these two Bees has its own especial parasite, or uninvited guest. The parasite of the Osmia is the black-and-white Fly who is to be seen so often at the entrance to the galleries, intent on laying her eggs within them. The parasite of the Anthophora is the Sitaris, the Beetle whose corpses appear in such quantities on the surface of the bank. If the layer of Osmia-cells be removed from the nest we can observe the cells of the Anthophora. Some will be occupied by larvæ, some by the perfect insect, and some—indeed many—will contain a singular egg-shaped shell, divided into segments with projecting breathing-pores. This shell is extremely thin and fragile; it is amber-coloured, and so transparent that one can distinguish quite plainly through its sides a full-grown Sitaris, struggling as though to set herself at liberty. What is this curious shell, which does not appear to be a Beetle’s shell at all? And how can this parasite reach a cell which seems to be inaccessible because of its position, and in which the most careful examination under the magnifying-glass reveals no sign of violence? Three years of close observation enabled me to answer these questions, and to add one of its most astonishing chapters to the story of insect life. Here is the result of my inquiries.