The acus was a pin or needle used in dressing the hair or fastening a robe. Typically made of gold or silver and ornamented with figures, the acus could also be made of bone or ivory. The length of these hair-pins varied from 6 to 9 inches. The image of an acus being used to fasten plaited hair (shown below) is derived from a marble group found in the south of France. The acus could also be used to curl, dye, or perfume the hair.
SOURCE: A Dictionary of Greek and Roman Antiquities, Volume 1 edited by Sir William Smith, William Wayte, George Eden Marindin