Abstract: This invention relates to a device for detecting the presence of an individual yarn in an advancing lap of parallel yarns, constituted by: detector disposed on one side and in the vicinity of this advancing lap, adapted to detect the presence of a yarn moved away from the lap by a blower means disposed on the other side of said lap; an arrangement, controlled by the detector, for stopping the advancing lap or for triggering off an alarm system, wherein: it is composed, in known manner, of a plurality of parallel comb teeth, between which the advancing yarns pass; and the blower is placed at the base of said teeth and directs the flow of air towards the tip of these teeth. The invention is more particularly applicable to warp knitting machines.