They are just over an inch long and about the width of a pencil, beautifully crafted from leaves and appear to have a door at one end. I would love to know what is going to emerge.
These are the brood cells of a Leafcutter bee (Hymenoptera: Megachilidae) and probably those of Megachile willughbiella as this species is well known to unilise the soil in plant pots and hanging baskets as a substrate for their brood cells.
Here is more info.:
Leaf-cutter bees – Megachile spp.
Eight species of leaf-cutter bees have been found in the British Isles. They are so-called because the females cut from plants sections of green leaves (and sometimes petals) to use in building their nests. Leaf-cutter bees are the size of the common honeybee. Females have a broad, dense brush of hairs (frequently golden or red in colour) on the underside of their abdomens. This is the “pollen brush” and it is amongst its hairs that pollen is stored for the journey back to the nest by the foraging female bee.
Three leaf-cutter species are commonly seen in gardens, particularly in southern Britain. These are summarised as follows (none has a specific common name):
Megachile centuncularis. The fore legs of both sexes are entirely black and the pollen
brush of the female is red. Body length 9-12mm. The nests are built in holes in walls and dead wood; occasionally in spaces under roof boards. Common in England and Wales, but scarce in Scotland and Ireland.
M. versicolor. This species is very similar in appearance and size to the above and it is virtually impossible to distinguish the two with the naked eye. It nests in holes in wood, including tree trunks and old roofing timbers, and in light soil; it will also excavate nest burrows in the exposed pith of dead stems of suitable diameter (e.g. bramble and rose). Widely distributed in the British Isles.
M. willughbiella. The fore legs of both sexes are black but those of the male have the end segments creamy-white and conspicuously dilated; those of the female are black and simple. The pollen brush of the female is pale golden. Body length 10-15 mm. The bee nests in borings in rotten tree stumps, logs and posts; under bark and in the soil. In gardens and greenhouses it will often nest in soil in plant pots. Common in Britain and southern Ireland.
Leaf-cutter bees are non-social bees, each female building her own nest independently of other individuals of the same species. For nesting sites all leaf-cutter bees either use existing cavities (e.g. in wood or masonry) or excavate burrows in the soil. The thimble-shaped cells have a flat lid and a rounded base. A typical nest contains six or more cells. The sidewalls and lids of each cell consist of several layers of leaf portions: about 12 oval sections are used to form the side walls and a further 6 or so circular sections are used to form the lid. These multi-layered sections of leaf are gummed together by the sap which seeps out of the crushed leaf margins, and possibly the bee’s salivary secretion. The female’s mandibles are used as scissors to cut out the leaf sections. A bee may often use the same plant (e.g. rose) consistently, so that leaf margins become disfigured by many scalloped indentations. The three species of leafcutter bees described above are active from late May or early June to the end of August and thus it is during this period that the damage will occur.
Cells are provisioned with a semi-liquid mixture of pollen and nectar. An egg is laid on this and the cell sealed. Development from egg to full grown larva occupies only a short time. The fully-fed larva spins an oval, reddish brown, silken cocoon in which it over-winters. Pupation occurs in the late spring and the adult emerges from the nest from the end of May onwards. All British leaf-cutters are single-brooded but usually long-lived (especially females); some are still flying in September. These bees are harmless; females have a sting but will only use it if picked up roughly between a finger and thumb.