A stridulatory organ is here described for the first time in the family Argiopidae. The file is on the surface of the lungbook covers, and the scraper is on the base of the femora. Altogether seven different kinds of stridulatory organs have been distinguished in spiders according to the positions of the files and scrapers. The organ now described in the spiny orb-weavers of the genus Micrathena was previously known only in a few genera of the family Erigonidae. In one of the species of Micrathena the ridges of the stridulatory file are sufficiently close together to give bright diffraction spectra at incidences near grazing. The primitive polygonal pattern on the surface of the cuticle of spiders is illustrated, and the mode of formation of the stridulatory file is suggested.