When the caterpillars approach their full size they are too heavy to crawl in search of their food with the back upwards, as is usual with most caterpillars, but traverse the small branches suspended by the feet, as is shown in the figure.
COCOON OF THE TUSSEH SILKWORM.
When the caterpillar is ready to spin its cocoon, it connects by means of the silk, (which is always glutinous when newly spun,) two or three new leaves into an outer envelope, which serves as a basis to spin the complete cocoon in, besides the cocoon being suspended from a branch of the tree in a wonderful manner, by a thick, strong, consolidated cord, spun of the same material by this persevering creature.
The cocoon is of an exact oval shape, and very firm in texture: in it the animal remains dormant, and perfectly protected, for about nine months, namely from October until July; so that it makes its appearance in time for the caterpillars to come into existence, when Providence has furnished them with the greatest plenty of proper food. When the insect is prepared to make its escape and be changed into its perfect state, it discharges from its mouth a large quantity of liquid, with which the upper end of the case is so perfectly softened as to enable the moth to work its way out in a very short time; an operation which is always performed during the night. In their perfect state these insects do not exist many days: the female deposits her eggs in the branches of the tree she may be resting on, to which they adhere firmly by means of the gluten they are covered with when newly laid.
The eggs are white, round, and compressed, with a depression in the centre on each side. They hatch in from two to four weeks, according to the state of the weather. The larvæ or caterpillars acquire their full size, which is about four inches in length, and three in circumference, in about six weeks: they are nearly the colour of the leaves they feed on, with a light yellowish stripe on each side; under these stripes the middle segments are marked with an oblong gold-coloured speck. The back is also marked with a few round darker coloured spots, from which issue a few long, coarse, distinct hairs, while others of smaller size are scattered over the insect.
The Tusseh silkworm is found in such abundance over many parts of Bengal and the adjoining provinces, as to afford to the natives a large supply of the durable, coarse, dark-coloured silk, already mentioned, commonly called Tusseh silk, which is woven into a kind of cloth called Tusseh doot’hies, much worn by Brahmins and other sects of Hindoos. This substance would no doubt be highly useful to the inhabitants of many parts of America and the south of Europe, where a cheap, light, cool, durable dress, such as this silk makes, is much wanted. Millions of cocoons of the Tusseh silkworm are annually collected in the jungle, and brought to the silk factories near Calcutta. In other parts the people gather and transplant them to the trees near their own dwellings, that they may watch over the safety of the caterpillars, which are very liable to be devoured by crows during the day time, and by bats at night.
The Arindy silkworm feeds only on the leaves of the Palmi Christi; it produces so delicate and flossy a silk that it cannot be wound from the cocoons; it is therefore spun like cotton, and the thread woven into a coarse kind of white cloth, of a loose texture, but so durable that a person can scarcely in his lifetime wear out a garment made of it.
Eleven different species of silkworm have been enumerated as natives of India, which has thus the internal means of providing the whole of Europe with a material which would rival cotton and woollen cloth, and would often be preferred to both, could it be obtained at a low price. The produce of the Arindy silkworm, when sent to this country was much admired, and some manufacturers to whom it was shown seemed to think that they had been hitherto deceived in the account of the shawls of India being made from the wool of a goat, and that this silk, if sent home, could be made into shawls equal to any manufactured in India.
Many of the larvæ of the European moths afford a very strong silk, and it is said, that a manufacture of silk from the cocoons of the emperor moth was at one time established in Germany. There is no doubt, however, that silk might be collected in abundance from many native silkworms in America. Cocoons have been described eight inches long, made of grey silk, which the inhabitants of Chilpancingo, Tixtala, and other places in South America, manufacture into stockings and handkerchiefs. Humboldt also observed similar nests in the provinces of Mechoacan, and the mountains of Santa Rosa; they were of dense tissue, resembling Chinese paper, of a brilliant whiteness, and formed of distinct and separate layers. The interior layers, which are the thinnest, and of extraordinary transparency, were used by the ancient Mexicans as writing tablets.