The larvae possess a pair of specially modified salivary glands, called silk glands. These glands secrete a clear, viscous fluid that is forced through openings, called spinnerets on the mouthparts of the larva. The fluid hardens as it comes into contact with air to become the silk thread. The diameter of the spinneret determines the thickness of the silk thread produced. Silkworm moths live only in captivity at present and are grown in commercial operations around the world.
All wild populations are extinct, although presumably there are still related species in Asia.
Silkworms have been domesticated so long that they have lost the ability to fly and can no longer survive independently in nature. Semi-transparent specimens are difficult to image using unaided brightfield optical microscopy. The images above were recorded using the Intel Play QX3 microscope in transmitted brightfield mode with the assistance of Rheinberg illumination . Dyed acetate gel filters were strategically placed below the aperture diaphragm of an aftermarket Abbe-style condenser to generate the Rheinberg effect. A 15-millimeter central filter was surrounded by a larger annular filter to combine the effects of oblique specimen illumination by light filtered through the annular filter superimposed over a background of light filtered through the central filter.