336 Insect Architecture. 



thick web on the upper surface of a leaf, to which it adheres 

 till the change is effected. 



The most important operation, however, of silk-spinning is 

 performed before the caterpillar is transformed into a chrysa- 

 lis, and is most remarkable in the caterpillars of moths and 

 other four -winged flies, with the exception of those of butter- 

 flies ; for though these exhibit, perhaps, greater ingenuity, 

 they seldom spin more than a few threads to secure the 

 chrysalis from falling, whereas the others spin for it a complete 

 envelope or shroud. We have already seen, in the preceding 

 pages, several striking instances of this operation, when, pro- 

 bably for the purpose of husbanding a scanty supply of silk, 

 extraneous substances are worked into the texture. In the 

 case of other caterpillars, 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 manufac- 

 tures and commerce, and on that account will demand from 

 us somewhat minute details, though it would require volumes 

 to incorporate all the information which has been published 

 on the subject. 



SlLK-WORM. 



The silk-worm, like 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 Fahren- 

 heit, 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 fifth day after 

 hatching, the second begins on the eighth day, the third 



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



