SPINNING CATERPILLARS, 313 



with the exception of those of butterflies; for though 

 these exhibit, perhaps, greater ingenuity, they seldom 

 spin more than a few threads to secure the chrysalis 

 from falling, wliereas the others spin for it a com- 

 plete envelope or shroud. We have already seen, in 

 the preceding pages, several striking instances of this 

 operation, when probably for the purpose of husband- 

 ing a scanty supply of silk, extraneous substances are 

 worked into the texture. In the case of other cater- 

 pillars, silk is the only material employed. Of this 

 the cocoon of the silk-worm is the most prominent 

 example, in consequence of its importance in our 

 manufactures and commerce, and on that account 

 will demand from us somewhat minute details, though 

 it would require volumes to incorporate all the in- 

 formation which has been published on the subject. 



SlLK-WoRM. 



The silk-worm, hke most other caterpillars, changes 

 its skin four times during its growth. The intervals 

 at which the four moultings follow each other depend 

 much on climate or temperature, as well as on the 

 quality and quantity of food. It is thence found, 

 that if they are exposed to a high temperature, say 

 from 81° to 100° Fahrenheit, the moultings will be 

 hastened; and only five days will be consumed in 

 moulting the third or fourth time, whilst those worms 

 that have not been hastened take seven or eight 

 days.* 



The period of the moultings is also influenced by 

 the temperature in which the eggs have been kept 

 during the winter. When the heat of the apartment 

 has been regulated, the first moulting takes place on 

 the fourth or filth day after hatching, the second 

 begins on the eighth day, the third takes up the 

 thirteenth and fourteenth days, and the last occurs 



* Cours d'Agriculture, par M. Rozier. Paris, 1801. 

 VOL. IV. 27 



