Mason: Nests have been described in detail by Ferton (1908, 1920); the biology is somewhat different from others in the sub-genus. Females build 2–4 cells in holes in rocks; these cells have the general appearance of those built by M. parietina, consisting of mud mixed with “salivary secretions” (Ferton 1908: 545), without pebbles. Once several cells are built, they are covered with a thin (1 mm), concave layer of hardened mud; this layer is located inside the hole of the rock and its outer surface is a few millimeters beneath the external surface of the rock. Subsequently, the female fills the space above the thin mud layer with a mix of pebbles and masticated plant material. According to Ferton, the masticated plant material contains salivary secretions (but no resin), and it hardens quickly. Ferton (1920) reports a nest of M. lefebvrei from southern France; the nest structure and the material used were similar but the nest had been built in an empty snail shell (Praz, 2017).