In constructing the body of the net, the most remarkable circumstance is her using her limbs as a measure, to regulate the distances of her _radii_ or wheel-spokes, and the circular meshes interweaved into them. These are consequently always proportional to the size of the spider. She often takes up her station in the centre, but not always, though it is so said by inaccurate writers; for she as frequently lurks in a little chamber constructed under a leaf or other shelter at the corner of her web, ready to dart down upon whatever prey may be entangled in her net. The centre of the net is said also to be composed of more viscid materials than its suspensory lines,--a circumstance alleged to be proved by the former appearing under the microscope studded with globules of gum.[FN] We have not been able to verify this distinction, having seen the suspensory lines as often studded in this manner as those in the centre. (J. R.) Mason-Spiders. A no less wonderful structure is composed by a sort of spiders, natives of the tropics and the south of Europe, which have been justly called mason-spiders by M. Latreille. One of these (_Mygale nidulans_, Walckn.), found in the West Indies, "digs a hole in the earth obliquely downwards about three inches in length, and one in diameter. This cavity she lines with a tough thick web, which, when taken out, resembles a leathern purse; but, what is most curious, this house has a door with hinges, like the operculum of some sea-shells, and herself and family, who tenant this nest, open and shut the door whenever they pass and repass. This history was told me," says Darwin, "and the nest, with its door, shown me by the late Dr. Butt, of Bath, who was some years physician in Jamaica."[FO] [Illustration: Nest of the Mason-Spider. A. The nest shut. B. The nest open. C. The spider, _Mygale cæmentaria_. D. The eyes magnified. E, F. Parts of the foot and claw magnified.]