Relatively long microfilaments made from a low heat conductivity glass composition. The microfilaments are used to make insulation and/or filler materials. The microfilaments are made by forming a liquid film of molten glass across a coaxial blowing nozzle (5), applying a blowing gas (10) at a positive pressure on the inner surface of the glass film to blow the film and form an elongated hollow tube or cylinder (12) of molten glass. Means are provided for forming a thinned wall or weakened portion of the forming elongated tube or cylinder. A transverse jet (13) is used to direct an entraining fluid (14) over and around the blowing nozzle (5) at an angle to the axis of the blowing nozzle. The entraining fluid (14) as it passes over and around the blowing nozzle (5) fluid dynamically induces a pulsating or fluctuating pressure field at the opposite or lee side of the blowing nozzle in the wake or shadow of the coaxial blowing nozzle and produces a laminar flow of entraining fluid in the vicinity of the forming elongated tube or cylinder. The continued movement of the entraining fluid (14) over the elongated cylinder (12) produces asymmetric fluid drag forces on the cylinder, and at the thinned walled or weakened portion longitudinally breaks the cylinder to form a multiplicity of microfilaments, and detaches the microfilaments from the elongated cylinder and from the coaxial blowing nozzle and the detached filaments are carried away from the blowing nozzle.