Like other caterpillars, the silk-worm is not a warm-blooded animal, and its temperature is therefore always equal to that of the atmosphere in which it is placed. In the silk-producing countries, where modes of artificial heating have not been studied practically and scientifically, the difficulty and expense that must attend the prosecution of this heating system, form abundant reasons why it cannot be generally adopted. The great susceptibility of the insect to atmospheric influences would also in a great degree render unsuitable the more common arrangements for the purpose. The plan of warming apartments by means of stoves, in its passage through which the air becomes highly heated before it mixes with and raises the general temperature of the air in the chamber, is liable to this inconvenience,--that the portion so introduced, having its vital property impaired by the burning heat through which it has passed, injures, proportionably, the respirable quality of the whole atmosphere; an effect which is easily perceptible by those who inhale it. A better plan of heating has lately been suggested, and is rapidly coming into practice, viz., of warming buildings by a current of hot water (an American invention), which is, by a very simple process, kept constantly flowing in close channels through the apartment, where it continually gives off its heat by radiation; and the degree of this being far below the point which is injurious to the vital quality of air, the evil before alluded to is avoided. If the expense of fuel be not too great, as compared with that of the labor which would be saved by this invention, the adoption in silk countries of such a mode of raising and regulating the temperature might, probably, prove advantageous. The silk-worm remains in the form of a chrysalis, for periods which, according to the climate or the temperature wherein it may be placed, vary from fifteen to thirty days. In India, the time is much shorter (See Chapter VIII.); in Spain and Italy, eighteen to twenty days. In France three weeks; and in the climate of England, when unaccelerated by artificial means, thirty days will elapse from the time the insect begins to spin until it emerges in its last and perfect form. It then throws off the shroud which had confined it in _seeming lifelessness_, and appears as a large moth of a grayish-white color, furnished with four wings, two eyes, and two black horns or antlers which present a feathery appearance (See Figure 6. plate III.).