THE SILKWORM ITS FORM AND LIFE HISTORY. 25 



single cocoon is frequently upwards of half a mile in length. The 

 fineness of the thread will easily be understood when we learn 

 that the cocoon containing this great length weighs on an average, 

 including the enclosed chrysalis, only twenty-five grains. 



The caterpillar takes about three days to complete its task, 

 and very exhausted, one would imagine, it must be when it has 

 finished. A calculation has been made that it moves its head 

 backwards and forwards about 300,000 times during the formation 

 of the cocoon, and yet, of course, it takes no food all the while, 

 to recruit its energies or refresh itself after its severe labour. 



The cocoon is usually oval in shape, with rounded ends and 

 parallel sides, but frequently it is more or less constricted in the 

 middle, or, as the French say, "strangulated' 1 (Fig. 10). The 

 colour is white in the best breeds, but it is often more or less of 

 a yellowish tinge, and not unfrequently of a deep golden yellow. 

 Different breeds 'form cocoons of different colours, some being 

 greenish and others even roseate. It is a curious fact that the 

 colour of the prolegs of the caterpillar during its last age always 

 corresponds to that of the cocoon it will spin ; so that, by 

 observing these, we may tell what coloured cocoons we shall get. 



When the cocoon is finished, the caterpillar may rest in peace. 

 Its active labours are over, and it has now but to resign itself to 

 its destiny, and await its transformation into a chrysalis. It 

 shrinks very considerably in length, but expands in breadth in 

 the middle of the body ; its prolegs shrivel up, its true legs are 

 curled inwards, and it becomes stiff and helpless, lying against 

 the side of the silken bed it has just made for itself. Thus it 

 remains for three or four days more, while its skin is gradually 

 parting company with the portions of its body that lie immediately 

 beneath it, and the covering of the pupa, or chrysalis, is being 

 formed between the two. 



When this is ready for use, the old skin splits near the head as 

 on former occasions, but this time discloses, not another cater- 

 pillar similar to the present one, but a pale object rounded in 

 front and slightly tapering behind, which, though totally devoid 

 of limbs, manages gradually to push its enveloping shroud back- 

 wards, till it has quite worked it off behind. The pale object 

 thus revealed is the pupa, or chrysalis. The cast skin must, of 

 course, now lie by its owner during all the next stage of its life, 

 as there is no means of getting rid of it : but as it shrinks to very 

 small dimensions, it does not get in the way, and remains at the 

 hinder end of the chrysalis. 



The chrysalis itself is a curious mummy-like thing. It is both 

 somewhat stouter and far shorter than the caterpillar from which 



