Essential Compendium of Sericulture

...eat, they will eventually perish.

Silkworms grow with remarkable speed; with every feeding, they grow larger. Consequently, the accumulation of leaf scraps and sand-mats (沙茵); the bedding layer consisting of silkworm excrement and leaf remnants also increases with every feeding. As the silkworms grow, they become crowded and overlap. When the layer of waste and refuse becomes thick, it generates a "steaming" heat likely referring to the heat of fermentation from damp waste. Silkworms crowded together in such steaming refuse rarely escape falling ill. The technique of thinning and spreading original: "fen li" (分躒); the process of reducing density and redistributing the worms is the means by which one thins out the crowded and removes the filth.

There is no fixed schedule for thinning; it should be done according to the density of the silkworms. Spreading is also not fixed; it may be done once at midday or once at dusk, depending on whether the weather is cold or warm. The Wuben Xinshu original: "New Book of Essential Agricultural Labors," a classic agricultural treatise from the 14th century states: "When lifting silkworms, one needs many hands and speed; when spreading silkworms, one must use a light touch." These two words—Light and Fast—are the finest methods for thinning and spreading. If one is reckless or slow, the silkworms will suffer harm. Those who rear them must be cautious.

Leaving Space for Thinning

When placing silkworms in the baskets, do not allow them to become too crowded. This is especially critical for young silkworms, as they have little strength and cannot withstand being squeezed together. If they are placed so that their heads cluster and tails overlap, the stronger ones will compete for food, while the weaker ones will inevitably be pushed down into the refuse and suffer from hunger. Therefore, when spreading the silkworms, one must not crowd them against the edges of the basket. A margin of two or three inches approximately 6 to 10 centimeters should be left around the four sides to serve as space for the silkworms to spread out during feeding.