The Bombyx mori, commonly known as the silkworm, is an insect recognized for its role in producing silk. Its life cycle involves a complete metamorphosis, undergoing significant transformations in appearance across different stages. Understanding these changes provides insight into the insect’s biology.
Appearance of the Silkworm Caterpillar
The silkworm caterpillar, or larva, consumes mulberry leaves and produces silk. Newly hatched larvae are tiny, 2 to 3 millimeters long, and may appear dark brown or black with a hairy texture. As they grow, molting four times through five instars, they become smoother and lighter, reaching a maximum length of about 75 millimeters (3 inches). During this growth period, they can increase their weight by up to 10,000 times.
The mature silkworm caterpillar has a cylindrical, segmented body, usually creamy white or pale yellowish. Its body consists of 13 segments: a small head, three thoracic segments, and nine abdominal segments. The head features small, dark ocelli and strong mandibles for chewing mulberry leaves. Small breathing holes called spiracles are visible along its sides.
The caterpillar possesses distinct types of legs. Three pairs of true, jointed legs are on its thoracic segments, used for holding onto leaves. Additionally, five pairs of fleshy prolegs are on its abdominal segments, aiding in gripping surfaces.
A small, pointed caudal horn is present on its eighth abdominal segment. Near its mouth, the larva has a spinneret, an organ through which it extrudes liquid silk that hardens upon exposure to air.
Silkworm Metamorphosis: Changes in Form
The life cycle begins as an egg. These tiny, oval eggs measure about 1 to 1.3 millimeters. When first laid, they are often lemon-yellow but may turn white if infertile or black if fertile. Female moths lay several hundred eggs, usually in clusters.
Following the larval stage, the silkworm enters the pupal stage, which occurs inside a protective cocoon spun from silk. The pupa itself is a brownish, oval-shaped form, segmented and immobile, often described as resembling a mummy. Initially, the pupa is soft and white, gradually turning brown to dark brown as its skin hardens. This stage represents a transitional phase where the larval organs dissolve and adult structures develop.
The final stage is the adult silkworm moth (Bombyx mori). The moth has a stout, hairy body, typically creamy white or blond, sometimes with faint brown markings. Adult moths have a wingspan of about 3 to 5 centimeters (1 to 2 inches), though their wings are large, they are not capable of functional flight due to millennia of domestication.
The head is small, featuring feathery antennae, which are more pronounced in males and sensitive to pheromones. The adult moth possesses reduced or absent mouthparts and does not feed during its brief lifespan, which typically lasts only two to ten days. Females are generally larger and bulkier than males due to carrying eggs.