opus vermiculatum---taken from the latin word 'worm.' It refers to lines of tesserae that snake around a feature in the mosaic. Often 2-3 rows of opus vermiculatum appear like a halo around something in a mosaic picture, helping it stand out from the background.
Mosaico_pavimentale_–_Grotte_Celloni_–_Pal._Massimo. Floor mosaic depicting birds, fish and fruit basket. Opus vermiculatum, Roman artwork of the end of the Ist century AD/begin of the Ist century BC. National Museum of Rome.
Mosaic floor depicting a dog and a knocked-over gold vessel. Alexandria. approx. 200-100 BC. This scene formed the center piece of a large mosaic floor. The quality is fantastic, and this period represents a high point in the mosaic craft in antiquity. Many of the tesserae (the little pieces of stone/glass that make up the floor) are only 1-2mm across, which allows the mosaicist to achieve a painting-like effect. This technique was known in antiquity as opus vermiculatum, or ‘wormy work’.
Central panel of a floor mosaic with a cat and two ducks. Opus vermiculatum, Roman artwork of the late Republican era, first quarter of the 1st century BC. From the triclinium of a suburban villa in the Cecchignola area.