216 STATES OF INSECTS. 



it lays the foundation of the interior cocoon. Fixing it- 

 self by its prolegs to some of the surrounding threads, 

 it bends its body, and by successive motions of its head 

 from side to side spins a layer of silk on the side oppo- 

 site to it : when this is of the requisite thickness, the larva 

 shifts its position, and repeats the same process in another 

 quarter, covering each layer in turn with a new one 

 until the interior cavity is reduced to the size desired. 

 Thus, the silken thread which forms this new cocoon is 

 not, as might have been supposed, wound circularly as 

 we wind the thread of a ball of cotton; but backwards and 

 forwards in a series of zigzags, so as to compose a num- 

 ber of distinct layers. Malpighi could distinguish sjx of 

 these layers a , and Reaumur suspects there is often a 

 greater number b . The former found the length of the 

 ■thread -of silk composing them when wound off, without 

 including the exterior case, to be not less than 930 feet c ; 

 but others have computed it at more than a thousand d : 

 consequently the threads of five cocoons united would be 

 a mile in length. Estimating by the weight, — the thread 

 of a pound of cocoons, each of which weighs about two 

 grains and a half, would extend more than 600 miles e , 

 and such is its tenuity, that the threads of five or six co- 

 coons require to be joined to form one of the thickness 

 requisite in the silk manufacture. It is the continuous 

 thread of the inner cocoon which is most valuable ; the 

 outer loose coating from its irregularity cannot be 



a Be Bombyc. 24. >> i. 498. 



c Be Bombyc. 43. a N. Bid. d'Hist. Nat. vi. 294. 



e Lesser. L. ii. 150, note 22. Boyle says an English lady found that 

 the silk of a single cocoon would extend 300 English leagues or 900 

 miles. But this must be a mistake. 



