The silkworm, Bombyx mori, is an insect famous for producing the natural fiber used in silk textiles. This species is entirely domesticated, relying on human care for survival and reproduction, having lost the ability to live in the wild over thousands of years of selective breeding. The silkworm undergoes complete metamorphosis, transitioning through four distinct stages: egg, larva, pupa, and adult moth.
Appearance of the Silkworm Larva
The larval stage is the period of intense feeding and growth. The newly hatched caterpillar is small, measuring 4 to 6 millimeters long, and initially appears dark brown or black with a hairy body. As the larva progresses through five instars, or growth stages, it sheds its hairy coat and becomes smoother and lighter in color.
A fully grown larva reaches 6 to 8 centimeters (up to 3 inches) and has a distinct, segmented, cylindrical body. Its coloration is typically creamy white or pale yellow, though diet and genetic background can sometimes introduce a slight greenish tinge. The body includes three pairs of true legs on the thoracic segments, followed by five pairs of fleshy prolegs on the abdomen, which help it grip surfaces.
The most distinguishing feature is the small, pointed caudal horn, located on the dorsal side of the eighth abdominal segment near the tail end. The larva constantly eats mulberry leaves, storing the energy and protein needed to fuel the next two stages of its life.
The Pupal Stage and Cocoon Structure
As the larva matures, it stops feeding and begins to spin a protective shelter called a cocoon, marking the transition to the pupal stage. The cocoon is an oval, dense, and tightly woven structure made from a single, continuous strand of silk thread, which can measure up to 1,500 meters long.
Cocoon colors vary widely depending on the silkworm’s breed, ranging from pure white and cream to shades of yellow and green. Inside this silky shell, the larva transforms into the pupa, an immobile, transitional form. The pupa is a rigid, mummy-like structure that is initially soft and white but quickly hardens and turns light to dark brown.
During this resting phase, the pupa undergoes a complete reorganization to develop the wings, antennae, and legs of the adult moth. This stage typically lasts between 8 and 14 days, with the cocoon providing a secure, insulated environment for metamorphosis.
The Adult Silkworm Moth
The final stage is the adult moth, which emerges by secreting an alkaline fluid to soften and pierce the end of the silk cocoon. The adult Bombyx mori possesses a thick, stout body covered in creamy-white to pale-yellow fuzz.
The moth is small, with an average body length of about 2.5 centimeters and a wingspan between 3 and 5 centimeters. The most notable feature is its small, underdeveloped wings, a result of domestication, which render the moth flightless. Females are typically larger, with a heavier abdomen for carrying eggs, and are less active than males.
The male moth possesses larger, feathery, bi-pectinate antennae used to detect female pheromones. Lacking functional mouthparts, the adult moth’s sole purpose is reproduction before it dies, as it cannot feed.